Mixtures and Separation
2. Solutions and Solubility
Learning outcomes
- I can define solutes, solvents, and solutions.
- I can explain how solutions form using the particle model.
- I can distinguish between soluble and insoluble substances.
- I can describe factors that affect solubility.
- I can interpret solubility information to predict whether substances will dissolve.
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.
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
Figure 1. A solution is formed when solute particles are evenly dispersed among solvent particles.
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.
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.
Figure 2. The solvent surrounds and separates the solute particles to form a solution.
How Solutions Form
The particle model helps explain how a solution forms.
When a solid such as salt is added to water:
- Water particles move continuously.
- They attract particles from the surface of the salt crystal.
- Salt particles separate from one another.
- Water particles surround the dissolved particles.
- The dissolved particles spread evenly throughout the water.
Although the salt appears to disappear, its particles are still present in the solution.
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.
Figure 3. Water particles separate and surround solute particles during the dissolving process.
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.
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.
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.
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.
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.
Figure 4. Temperature, pressure, and the nature of the substances all influence solubility.
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.
Using Solubility Information
Scientists use solubility data to predict whether substances will dissolve.
For example:
| Substance | Solubility in Water |
|---|---|
| Sodium chloride. | Soluble |
| Sugar | Soluble |
| Sand | Insoluble |
| Vegetable oil | Insoluble |
| Copper sulfate | Soluble |
Knowing whether substances are soluble helps chemists design experiments and separate mixtures.
Figure 5. Solubility information helps predict whether substances will dissolve in a particular solvent.
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.
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.
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.
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.
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.
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.