Introduction

Everything around us is made of matter, but not all matter is the same. Some materials consist of only one type of atom, while others contain atoms that are chemically bonded together or simply mixed. Scientists classify matter into three broad categories: elements, compounds, and mixtures.

Understanding the differences between these three types of matter is one of the foundations of chemistry. It helps us explain why air can be separated into its gases, why water can be broken down into hydrogen and oxygen, and why gold cannot be chemically broken down into anything simpler.

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Three Types of Matter

Matter can be classified into three main categories:

• Elements
• Compounds
• Mixtures

Each has a different particle structure and different properties.

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Figure 1. Elements, compounds, and mixtures have different particle arrangements.

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Elements

An element is a pure substance made of only one type of atom.

All atoms in an element contain the same number of protons.

Examples include:

• Oxygen (O)
• Iron (Fe)
• Gold (Au)
• Carbon (C)
• Helium (He)

Elements:

• Contain only one type of atom.
• Cannot be broken down into simpler substances by ordinary chemical reactions.
• Are the building blocks of all matter.

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Compounds

A compound is a pure substance made when two or more different elements are chemically bonded together in fixed proportions.

Examples include:

• Water (H₂O)
• Carbon dioxide (CO₂)
• Sodium chloride (NaCl)
• Ammonia (NH₃)

Compounds:

• Contain more than one type of atom.
• Have a fixed chemical formula.
• Can be broken down into their elements by chemical reactions.

The atoms are permanently joined by chemical bonds.

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Figure 2. Compounds consist of different atoms chemically bonded together in fixed ratios.

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Mixtures

A mixture contains two or more substances that are physically combined but not chemically bonded.

Examples include:

• Air
• Seawater
• Soil
• Salad
• Trail mix

In a mixture:

• The substances are mixed together.
• No new substance is formed.
• Each substance keeps its own identity and properties.

The proportions of the substances can vary.

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Comparing Particle Structures

The arrangement of particles explains the differences between elements, compounds, and mixtures.

Element

• One type of atom only.

Example:

O O O O O

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Compound

• Different atoms chemically bonded.

Example:

H—O—H

H—O—H

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Mixture

• Different particles mixed together.
• No chemical bonds between different substances.

Example:

O₂ molecules mixed with N₂ molecules in air.

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Figure 3. Particle diagrams clearly show the differences between elements, compounds, and mixtures.

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How Mixtures Differ from Compounds

Although both contain more than one substance, mixtures and compounds are very different.

Understanding this difference is one of the most important ideas in chemistry.

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Why Mixtures Retain Their Properties

Because the substances in a mixture are not chemically bonded, each substance keeps its original properties.

For example:

Air contains:

• Oxygen
• Nitrogen
• Carbon dioxide
• Water vapour

Each gas behaves as it normally would because no chemical reaction has taken place.

Similarly, in a fruit salad:

• Apples still taste like apples.
• Grapes still taste like grapes.

The substances remain unchanged.

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Everyday Examples

Elements

• Copper wire
• Aluminium foil
• Gold jewellery
• Helium in balloons

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Compounds

• Water
• Table salt
• Baking soda
• Sugar

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Mixtures

• Air
• Soft drinks
• Milk
• Soil
• Concrete

Many everyday materials are mixtures rather than pure substances.

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Figure 4. Elements, compounds, and mixtures are found throughout everyday life.

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Classifying Substances

Scientists classify matter by examining:

• The types of particles present.
• Whether chemical bonds exist.
• Whether the composition is fixed.
• Whether the substance can be separated physically or chemically.

Example:

Evidence is always used when classifying substances.

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Why This Classification Is Important

Classifying matter helps scientists:

• Understand chemical reactions.
• Predict properties.
• Separate materials.
• Develop new substances.
• Explain natural processes.

This classification system is one of the foundations of chemistry.

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Figure 5. Matter can be classified into elements, compounds, and mixtures based on its composition and particle structure.

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Worked Example

Question

Classify each substance.

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Real-World Connection

Water treatment plants separate mixtures every day. Before drinking water reaches homes, it is filtered to remove solid particles, disinfected to kill microorganisms, and treated to remove unwanted dissolved substances. These processes work because many impurities are physically mixed with the water rather than chemically bonded to it. In contrast, compounds such as water itself cannot be separated by simple filtration because the hydrogen and oxygen atoms are chemically bonded together.

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Did You Know?

Although air may seem like a single substance, it is actually a mixture of gases. Dry air is made up of about 78% nitrogen, 21% oxygen, and small amounts of argon, carbon dioxide, and other gases. Because these gases are not chemically bonded, they can be separated using industrial processes such as the fractional distillation of liquid air.

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Key Terms

Chemical bond – The force that holds atoms together in a compound.

Compound – A pure substance made of two or more different elements chemically bonded together in fixed proportions.

Element – A pure substance consisting of only one type of atom.

Mixture – Two or more substances physically combined but not chemically bonded.

Particle – A general term for atoms, molecules, or ions that make up matter.

Physical change – A change that does not produce a new substance.

Pure substance – A material with a fixed composition and consistent properties.

Separation – The process of dividing substances in a mixture using physical methods.

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Key Takeaways

• Matter is classified as an element, compound, or mixture based on its particle structure and composition.
• Elements contain only one type of atom.
• Compounds contain different elements that are chemically bonded in fixed proportions.
• Mixtures contain substances that are physically combined but not chemically bonded.
• The substances in a mixture retain their original properties because no new substance is formed.
• Scientists use evidence such as particle structure, composition, and methods of separation to classify substances correctly.

Introduction

Many of the substances we use every day are solutions. Soft drinks, seawater, vinegar, sports drinks, medicines, and even the blood flowing through your body are all examples of solutions. A solution forms when one substance dissolves evenly in another, producing a uniform mixture.

Understanding how solutions form requires us to think about matter at the particle level. Although a solution may appear perfectly clear, billions of tiny particles are moving and interacting continuously. Learning about solutions and solubility helps explain many natural processes and is essential in chemistry, medicine, environmental science, and industry.

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What Is a Solution?

A solution is a homogeneous mixture in which one substance is dissolved evenly throughout another.

A solution contains two parts:

• Solute
• Solvent

Because the particles are evenly distributed, every sample of the solution has the same composition.

Examples include:

• Salt water
• Sugar water
• Vinegar
• Soft drinks

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Figure 1. A solution is formed when solute particles are evenly dispersed among solvent particles.

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Solutes and Solvents

Solute

The solute is the substance that is dissolved.

Examples:

• Salt in salt water.
• Sugar in tea.
• Carbon dioxide in soft drinks.

The solute is usually present in the smaller amount.

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Solvent

The solvent is the substance that does the dissolving.

The solvent is usually present in the larger amount.

The most common solvent is water, often called the universal solvent because it dissolves many different substances.

Examples:

• Water in salt water.
• Water in sugar solution.
• Ethanol in some perfumes.

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Figure 2. The solvent surrounds and separates the solute particles to form a solution.

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How Solutions Form

The particle model helps explain how a solution forms.

When a solid such as salt is added to water:

1. Water particles move continuously.
2. They attract particles from the surface of the salt crystal.
3. Salt particles separate from one another.
4. Water particles surround the dissolved particles.
5. The dissolved particles spread evenly throughout the water.

Although the salt appears to disappear, its particles are still present in the solution.

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The Particle Model of Dissolving

At the particle level:

• Solute particles separate.
• Solvent particles surround them.
• The particles mix uniformly.
• The dissolved particles remain in constant motion.

This is why dissolved substances cannot usually be seen with the naked eye.

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Figure 3. Water particles separate and surround solute particles during the dissolving process.

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Soluble and Insoluble Substances

A substance is soluble if it can dissolve in a particular solvent.

Examples:

• Salt in water.
• Sugar in water.
• Copper sulfate in water.

A substance is insoluble if it does not dissolve significantly.

Examples:

• Sand in water.
• Chalk in water.
• Oil in water.

Whether a substance dissolves depends on both the solute and the solvent.

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Factors Affecting Solubility

Several factors influence how much of a substance dissolves.

Temperature

For many solids:

• Higher temperatures increase solubility.

For gases:

• Higher temperatures usually decrease solubility.

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Pressure

Pressure mainly affects gases.

Higher pressure:

• Increases the solubility of gases.

Example:

Carbon dioxide remains dissolved in unopened soft drinks because of high pressure.

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Nature of the Solute and Solvent

Some substances naturally dissolve well together.

A useful rule is:

"Like dissolves like."

Polar substances often dissolve in polar solvents such as water.

Non-polar substances dissolve better in non-polar solvents.

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Stirring and Particle Size

Stirring and crushing a solid into smaller pieces increase the rate of dissolving, but they do not change the maximum amount that can dissolve.

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Figure 4. Temperature, pressure, and the nature of the substances all influence solubility.

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Saturated and Unsaturated Solutions

An unsaturated solution can still dissolve more solute.

A saturated solution has dissolved as much solute as possible at a given temperature.

If additional solute is added to a saturated solution, it remains undissolved.

Changing the temperature can change the amount of solute that can dissolve.

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Using Solubility Information

Scientists use solubility data to predict whether substances will dissolve.

For example:

Knowing whether substances are soluble helps chemists design experiments and separate mixtures.

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Figure 5. Solubility information helps predict whether substances will dissolve in a particular solvent.

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Why Solutions Are Important

Solutions are essential in many areas of life.

Examples include:

• Medicines dissolved in blood.
• Nutrients transported in plants.
• Oxygen dissolved in lakes for aquatic life.
• Cleaning products.
• Fertilisers.
• Food and beverages.

Many biological and industrial processes depend on solutions.

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Worked Example

Question

Identify the solute and solvent in a salt solution.

Solution

Salt is the substance being dissolved.

Solute: Salt

Water is the substance doing the dissolving.

Solvent: Water

The mixture formed is a solution.

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Real-World Connection

Sports drinks contain solutions of water, sugars, minerals, and flavourings. The dissolved sugars provide energy, while dissolved ions such as sodium and potassium help replace electrolytes lost through sweating. In hospitals, intravenous (IV) fluids are carefully prepared solutions that deliver water, salts, and medicines directly into a patient's bloodstream.

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Did You Know?

Although water is often called the universal solvent, it does not dissolve everything. Substances such as oils, waxes, and many plastics do not dissolve in water because their particles do not interact strongly with water molecules. This is why oil and water separate into distinct layers instead of forming a solution.

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Key Terms

Homogeneous mixture – A mixture with a uniform composition throughout.

Insoluble – Unable to dissolve significantly in a particular solvent.

Particle model – A model describing matter as being made of tiny particles that are constantly moving.

Saturated solution – A solution that contains the maximum amount of dissolved solute at a given temperature.

Solubility – The ability of a substance to dissolve in a particular solvent under specified conditions.

Soluble – Able to dissolve in a particular solvent.

Solute – The substance that is dissolved in a solution.

Solution – A homogeneous mixture formed when a solute dissolves in a solvent.

Solvent – The substance that dissolves the solute.

Unsaturated solution – A solution that can still dissolve more solute.

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Key Takeaways

• A solution is a homogeneous mixture made from a solute dissolved in a solvent.
• The particle model explains that dissolving occurs when solvent particles separate and surround solute particles.
• Soluble substances dissolve readily, while insoluble substances do not.
• Solubility is affected by temperature, pressure (for gases), and the nature of the solute and solvent.
• Stirring and reducing particle size increase the rate of dissolving but do not increase the maximum amount that can dissolve.
• Understanding solutions and solubility is important in chemistry, biology, medicine, environmental science, and many everyday applications.

Introduction

Many of the materials we use every day are mixtures rather than pure substances. Scientists often need to separate the different components of these mixtures so they can study them, recycle them, or use them for other purposes. To do this, they use separation techniques that take advantage of differences in the physical properties of the substances involved.

Two of the simplest and most commonly used separation methods are filtration and evaporation. Filtration separates insoluble solids from liquids, while evaporation recovers dissolved solids by removing the liquid. These methods are widely used in laboratories, industry, water treatment plants, and even in everyday activities such as making coffee or producing sea salt.

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Why Do We Separate Mixtures?

Mixtures are made of substances that are physically combined, not chemically bonded.

Because of this, they can often be separated using physical methods.

Scientists separate mixtures to:

• Obtain pure substances.
• Remove unwanted materials.
• Recover valuable products.
• Prepare samples for experiments.
• Recycle materials.

Different mixtures require different separation techniques.

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Figure 1. Different separation methods are used depending on the properties of the substances in the mixture.

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What Is Filtration?

Filtration is a method used to separate an insoluble solid from a liquid.

It works because:

• The liquid can pass through the filter.
• The solid particles are too large to pass through.

Filtration is useful only when the solid is insoluble.

Examples include:

• Sand and water.
• Chalk and water.
• Muddy water.

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How Filtration Works

A typical filtration setup includes:

• Filter funnel.
• Filter paper.
• Beaker or flask.

During filtration:

1. The mixture is poured into the filter paper.
2. The liquid passes through.
3. The solid remains on the paper.

The liquid that passes through is called the filtrate.

The solid left behind is called the residue.

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Figure 2. During filtration, the liquid (filtrate) passes through the filter paper while the insoluble solid (residue) remains behind.

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What Is Evaporation?

Evaporation is used to recover a dissolved solid from a solution.

The solution is heated.

As the solvent evaporates:

• The dissolved solid remains.

Evaporation is commonly used for:

• Salt solutions.
• Copper sulfate solutions.
• Sugar solutions.

Unlike filtration, evaporation separates soluble substances.

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How Evaporation Works

During evaporation:

1. The solution is placed in an evaporating dish.
2. Heat causes the solvent to evaporate.
3. The dissolved solid becomes more concentrated.
4. Eventually, solid crystals remain.

Only the solvent changes into a gas.

The solute remains behind.

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Figure 3. Heating causes the solvent to evaporate, leaving the dissolved solid behind.

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Choosing the Correct Separation Method

Different mixtures require different methods.

The key question is:

Is the solid dissolved or undissolved?

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Physical Properties Used in Separation

Filtration and evaporation work because substances have different physical properties.

Filtration

Uses differences in:

• Particle size.
• Solubility.

Large insoluble particles cannot pass through the filter.

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Evaporation

Uses differences in:

• Boiling point.
• Volatility.

The solvent evaporates while the dissolved solid remains.

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Figure 4. Filtration and evaporation separate mixtures by using different physical properties.

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Advantages and Limitations

Filtration

Advantages

• Simple.
• Quick.
• Inexpensive.
• Effective for insoluble solids.

Limitations

• Cannot remove dissolved substances.
• Does not separate liquids.

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Evaporation

Advantages

• Recovers dissolved solids.
• Produces solid crystals.

Limitations

• Solvent is usually lost.
• Some substances may decompose if heated too strongly.
• Not suitable if the solvent also needs to be collected.

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Evaluating Separation Methods

Scientists choose separation techniques based on:

• The type of mixture.
• The physical properties of the substances.
• The desired product.

For example:

If the goal is to obtain clean drinking water from muddy water:

Filtration is suitable.

If the goal is to recover salt from seawater:

Evaporation is suitable.

Choosing the correct method makes separation more efficient.

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Figure 5. Filtration and evaporation are widely used in water treatment and the production of sea salt.

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Why Separation Techniques Are Important

Separation methods are used in many fields.

Examples include:

• Water purification.
• Food production.
• Chemical laboratories.
• Mining.
• Recycling.
• Medicine.
• Environmental science.

Understanding how mixtures are separated helps scientists recover useful materials and protect the environment.

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Worked Example

Question

Choose the best separation method for each mixture.

 

Explanation

• Insoluble solids → Filtration
• Dissolved solids → Evaporation

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Real-World Connection

Many coastal communities produce sea salt by allowing seawater to evaporate in shallow ponds under the Sun. As the water gradually evaporates, salt crystals are left behind and collected for use in food and industry. Water treatment plants also rely on filtration to remove suspended particles before water is disinfected and supplied to homes.

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Did You Know?

A household coffee maker works using the same principle as laboratory filtration. The coffee filter traps the solid coffee grounds (the residue), while the liquid coffee (the filtrate) passes through into the pot below.

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Key Terms

Evaporation – The process by which a liquid changes into a gas, leaving dissolved solids behind.

Evaporating dish – A shallow dish used to heat solutions during evaporation.

Filtrate – The liquid that passes through the filter during filtration.

Filtration – A method of separating an insoluble solid from a liquid using a filter.

Insoluble – Unable to dissolve in a particular solvent.

Mixture – Two or more substances physically combined but not chemically bonded.

Residue – The solid remaining on the filter paper after filtration.

Separation technique – A method used to separate the components of a mixture based on their physical properties.

Solute – The substance dissolved in a solution.

Solvent – The substance that dissolves the solute.

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Key Takeaways

• Filtration separates insoluble solids from liquids using differences in particle size.
• The liquid collected after filtration is the filtrate, while the solid left behind is the residue.
• Evaporation separates a dissolved solid from a solution by removing the solvent.
• Separation techniques rely on physical properties, such as particle size, solubility, and boiling point.
• The best separation method depends on the type of mixture and the desired product.
• Filtration and evaporation are widely used in laboratories, water treatment, food production, and many industrial processes.

Introduction

Some mixtures cannot be separated using filtration or evaporation because the substances are both liquids or because we want to recover the liquid rather than the dissolved solid. In these situations, chemists often use distillation, a separation technique that takes advantage of differences in boiling points.

Distillation is one of the oldest and most important laboratory techniques. It is used to produce clean drinking water, manufacture perfumes and medicines, separate alcoholic drinks, and refine crude oil into fuels such as petrol and diesel. By carefully heating a mixture, scientists can collect pure liquids for many different purposes.

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What Is Distillation?

Distillation is a method of separating substances based on differences in their boiling points.

During distillation:

1. The mixture is heated.
2. The substance with the lower boiling point evaporates first.
3. The vapour is cooled.
4. The vapour condenses back into a liquid.
5. The pure liquid is collected.

This process allows one liquid to be separated from another or recovered from a solution.

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Figure 1. Distillation separates substances by heating, evaporating, cooling, and condensing a liquid.

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The Principle Behind Distillation

Different substances boil at different temperatures.

For example:

If a mixture of ethanol and water is heated:

• Ethanol reaches its boiling point first.
• Ethanol vapour forms before most of the water.
• The vapour is cooled and collected.

This difference in boiling points makes separation possible.

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Simple Distillation Apparatus

A simple distillation setup contains:

• A heating source.
• A distillation flask.
• A thermometer.
• A condenser.
• Cooling water.
• A receiving flask or beaker.

The condenser cools the hot vapour so it changes back into a liquid.

The collected liquid is called the distillate.

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Figure 2. The main parts of a simple distillation apparatus include a flask, thermometer, condenser, and receiving vessel.

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How Simple Distillation Works

Suppose we want to obtain pure water from salt water.

Step 1

Heat the salt solution.

Step 2

Water reaches its boiling point.

Step 3

Water vapour rises into the condenser.

Step 4

Cold water flowing through the condenser cools the vapour.

Step 5

The vapour condenses into pure liquid water.

Step 6

The dissolved salt remains behind in the flask.

This allows the solvent to be recovered instead of being lost through evaporation.

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Recovering Pure Liquids

Simple distillation is commonly used to recover:

• Pure water from seawater.
• Distilled water for laboratories.
• Water from contaminated sources.
• Some solvents used in chemical industries.

Unlike evaporation, distillation allows the liquid to be collected and reused.

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Figure 3. Simple distillation recovers pure water while dissolved salts remain in the flask.

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Simple vs Fractional Distillation

There are two main types of distillation.

Simple Distillation

Used when:

• One liquid is mixed with a dissolved solid.
• Two liquids have very different boiling points.

Examples:

• Salt water.
• Sugar solution.

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Fractional Distillation

Used when:

• Two or more liquids have similar boiling points.

A fractionating column is placed between the flask and the condenser.

The column allows repeated evaporation and condensation, improving the separation.

Examples:

• Ethanol and water.
• Crude oil.

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Figure 4. Fractional distillation uses a fractionating column to separate liquids with similar boiling points.

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Comparing the Two Methods

Choosing the correct method depends on the substances being separated.

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Real-World Applications

Water Purification

Seawater can be distilled to produce fresh drinking water.

This process is called desalination.

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Petroleum Refining

Crude oil is separated by fractional distillation.

Products include:

• Petrol (gasoline)
• Kerosene
• Diesel
• Lubricating oils
• Bitumen

Each fraction is collected at a different temperature.

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Food and Drink

Distillation is used to:

• Produce alcoholic beverages.
• Extract essential oils.
• Manufacture perfumes.

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Laboratories

Scientists use distilled water because it contains very few dissolved impurities.

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Figure 5. Distillation is widely used in water purification, petroleum refining, and many industrial processes.

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Why Distillation Is Important

Distillation is one of the most valuable separation techniques because it allows scientists to:

• Recover pure liquids.
• Purify chemicals.
• Produce clean drinking water.
• Separate useful fuels.
• Manufacture medicines and industrial chemicals.

Many modern industries depend on distillation every day.

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Worked Example

Question

Which separation method should be used?

 

Explanation

• Recovering a liquid from a solution → Simple distillation
• Separating two liquids with similar boiling points → Fractional distillation
• Recovering only the dissolved solid → Evaporation

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Real-World Connection

Many countries with limited freshwater supplies use desalination plants to convert seawater into drinking water. One common method involves distillation, where seawater is heated until the water evaporates, leaving the dissolved salts behind. The water vapour is then condensed to produce clean, fresh water. Fractional distillation is also essential in oil refineries, where crude oil is separated into fuels and other useful products that power vehicles, heat homes, and provide raw materials for making plastics.

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Did You Know?

The International Space Station (ISS) recycles much of its water to reduce the amount that must be launched from Earth. The station uses advanced purification systems that include processes similar to distillation and condensation to recover clean water for astronauts, making long-term space missions possible.

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Key Terms

Boiling point – The temperature at which a liquid changes into a gas.

Condenser – A piece of apparatus that cools vapour so that it condenses back into a liquid.

Condensation – The process by which a gas changes into a liquid.

Distillate – The purified liquid collected during distillation.

Distillation – A separation technique that separates substances based on differences in boiling points.

Fractional distillation – A form of distillation that uses a fractionating column to separate liquids with similar boiling points.

Fractionating column – A column that allows repeated evaporation and condensation during fractional distillation.

Simple distillation – A method used to recover a liquid from a solution or separate liquids with very different boiling points.

Vapour – The gaseous form of a substance that is normally a liquid at room temperature.

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Key Takeaways

• Distillation separates substances using differences in their boiling points.
• In simple distillation, the substance with the lower boiling point evaporates first, then condenses to form the distillate.
• A simple distillation apparatus includes a flask, thermometer, condenser, and receiving vessel.
• Simple distillation is used to recover a liquid from a solution or separate liquids with very different boiling points.
• Fractional distillation uses a fractionating column to separate liquids with similar boiling points.
• Distillation has many important applications, including water purification, petroleum refining, food production, and chemical manufacturing.

Introduction

Many substances that appear to be a single colour or material are actually mixtures of several different substances. For example, black ink may contain blue, purple, and green dyes, while plant leaves contain several different pigments. To separate and identify these substances, scientists often use a technique called chromatography.

Chromatography is one of the most powerful separation methods used in chemistry. It allows scientists to separate tiny amounts of substances, identify unknown chemicals, test the purity of compounds, and analyse complex mixtures. Today, chromatography is widely used in medicine, environmental science, food testing, and forensic investigations.

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What Is Chromatography?

Chromatography is a separation technique used to separate the components of a mixture.

It works because different substances move at different speeds through a material.

As the substances move, they separate into distinct spots or bands.

Chromatography is commonly used to separate:

• Inks
• Food colourings
• Plant pigments
• Medicines
• Biological samples

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Figure 1. Chromatography separates the different substances in a mixture as they move across the paper.

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How Chromatography Works

In paper chromatography:

1. A small spot of the mixture is placed near the bottom of the paper.
2. The paper is placed in a solvent.
3. The solvent rises up the paper.
4. The substances dissolve in the solvent.
5. Different substances travel different distances.
6. The mixture separates into individual spots.

The separated pattern is called a chromatogram.

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The Stationary and Mobile Phases

Chromatography uses two important components.

Stationary Phase

The stationary phase does not move.

In paper chromatography:

• The chromatography paper is the stationary phase.

Some substances stick strongly to the paper and move slowly.

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Mobile Phase

The mobile phase moves through the stationary phase.

In paper chromatography:

• The solvent is the mobile phase.

It carries the dissolved substances up the paper.

Different substances travel at different speeds depending on how strongly they are attracted to the paper and how well they dissolve in the solvent.

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Figure 2. The stationary phase remains fixed while the mobile phase carries substances through it.

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Why Substances Separate

Each substance has different properties.

Some:

• Dissolve well in the solvent.
• Travel further.

Others:

• Stick more strongly to the paper.
• Travel more slowly.

The balance between these two effects causes the mixture to separate.

This allows scientists to identify individual substances within a mixture.

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Interpreting a Chromatogram

A chromatogram is the final pattern of separated spots.

Scientists can use it to determine:

• How many substances are present.
• Whether two samples contain the same components.
• Whether a sample is pure or a mixture.

For example:

A pure substance usually produces:

• One spot.

A mixture usually produces:

• Two or more spots.

If two samples produce spots at the same height under identical conditions, they may contain the same substance.

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Figure 3. A chromatogram can reveal whether a sample is pure or contains multiple substances.

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Rf Values

Scientists often compare substances using their Rf value (Retention factor).

The formula is:

Rf = Distance travelled by the substance ÷ Distance travelled by the solvent front

The Rf value is always between 0 and 1.

For example:

Distance travelled by spot = 6 cm

Distance travelled by solvent front = 8 cm

Rf = 6 ÷ 8

Rf = 0.75

Rf values help scientists compare substances tested under the same conditions.

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Calculating an Rf Value

Example

Distance travelled by substance:

4.5 cm

Distance travelled by solvent front:

7.5 cm

Rf = 4.5 ÷ 7.5

Rf = 0.60

Scientists compare calculated Rf values with known values to help identify unknown substances.

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Figure 4. The Rf value compares the distance travelled by a substance with the distance travelled by the solvent front.

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Applications of Chromatography

Chromatography is widely used in science.

Medicine

Scientists analyse:

• Blood samples.
• Urine samples.
• Medicines.

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Forensic Science

Investigators compare:

• Ink from documents.
• Paint samples.
• Drug samples.
• Poison residues.

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Food Industry

Manufacturers test:

• Food colourings.
• Flavourings.
• Additives.
• Contaminants.

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Environmental Science

Scientists monitor:

• Water pollution.
• Air pollution.
• Soil contamination.

Chromatography helps detect substances even when only tiny amounts are present.

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Figure 5. Chromatography is widely used in forensic science, medicine, environmental monitoring, and food testing.

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Advantages of Chromatography

Chromatography is valuable because it:

• Separates complex mixtures.
• Identifies unknown substances.
• Requires only small samples.
• Is highly accurate.
• Can detect tiny amounts of material.

Modern laboratories use many advanced forms of chromatography for scientific research and quality control.

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Worked Example

Question

A chromatography experiment gives the following results:

• Distance travelled by solvent front = 10 cm
• Distance travelled by a spot = 7 cm

Calculate the Rf value.

Solution

Rf = Distance travelled by substance ÷ Distance travelled by solvent front

Rf = 7 ÷ 10

Rf = 0.70

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Real-World Connection

Chromatography is an essential tool in forensic laboratories. If police recover a handwritten note from a crime scene, scientists can separate the dyes in the ink and compare the chromatogram with inks from suspected pens. Similar techniques are used to identify drugs in blood samples, detect banned substances in athletes, and monitor pollutants in rivers and drinking water.

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Did You Know?

Modern analytical laboratories often use highly advanced forms of chromatography, such as gas chromatography (GC) and high-performance liquid chromatography (HPLC). These techniques can detect substances present in amounts as small as a few billionths of a gram, making them invaluable in medicine, environmental monitoring, and scientific research.

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Key Terms

Chromatogram – The pattern of separated substances produced during chromatography.

Chromatography – A technique used to separate the components of a mixture.

Mobile phase – The moving substance (usually a solvent) that carries the sample through the stationary phase.

Pure substance – A material that produces a single spot under suitable chromatography conditions.

Retention factor (Rf) – The ratio of the distance travelled by a substance to the distance travelled by the solvent front.

Solvent – A liquid used to dissolve and transport substances during chromatography.

Solvent front – The furthest point reached by the solvent during a chromatography experiment.

Stationary phase – The material that remains fixed while the mobile phase moves through it.

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Key Takeaways

• Chromatography separates the components of a mixture because different substances move at different speeds.
• The stationary phase remains fixed, while the mobile phase carries substances through it.
• A chromatogram can show whether a sample is pure or a mixture.
• Rf values are calculated by dividing the distance travelled by the substance by the distance travelled by the solvent front.
• Chromatography is widely used to identify substances and test purity.
• Important applications include medicine, forensic science, environmental monitoring, and food analysis.