Solutions and Solubility
1. Solutes and Solvents
Learning outcomes
- I can define solute, solvent, and solution.
- I can identify the solute and solvent in a solution.
- I can distinguish between homogeneous and heterogeneous mixtures.
- I can explain how solutions differ from pure substances.
- I can identify examples of solutions in everyday life.
Learning Targets
- I can define solute, solvent, and solution.
- I can identify the solute and solvent in a solution.
- I can distinguish between homogeneous and heterogeneous mixtures.
- I can explain how solutions differ from pure substances.
- I can identify examples of solutions in everyday life.
Introduction
Many of the substances we use every day are not pure substances but mixtures. Seawater, soft drinks, vinegar, air, and even blood are all mixtures made by combining different substances. One of the most common types of mixture is a solution, in which one substance dissolves evenly in another.
Understanding solutes, solvents, and solutions is an important foundation for chemistry. These concepts help explain how medicines are prepared, how nutrients are transported in living organisms, how cleaning products work, and why many chemical reactions take place more easily in solution.
What Is a Solution?
A solution is a homogeneous mixture formed when one substance dissolves completely in another.
A solution contains:
- A solute.
- A solvent.
The particles are evenly distributed throughout the mixture, so every sample of the solution has the same composition.
Figure 1. A solution forms when a solute dissolves uniformly in a solvent.
What Is a Solute?
A solute is the substance that is dissolved.
The solute is usually present in the smaller amount.
Examples:
- Sugar in sugar water.
- Salt in seawater.
- Carbon dioxide in soft drinks.
- Oxygen dissolved in water for fish.
A solute may be a:
- Solid.
- Liquid.
- Gas.
What Is a Solvent?
A solvent is the substance that does the dissolving.
The solvent is usually present in the greater amount.
The most common solvent is water, which is often called the universal solvent because it dissolves many substances.
However, not everything dissolves in water.
Other common solvents include:
- Ethanol.
- Acetone.
- Hexane.
Identifying Solutes and Solvents
| Solution | Solute | Solvent |
|---|---|---|
| Salt water | Salt | Water |
| Sugar solution. | Sugar | Water |
| Vinegar | Acetic acid | Water |
| Soft drink | Carbon dioxide, sugar. | Water |
| Brass | Zinc | Copper |
In most liquid solutions, the solvent is the substance present in the greatest amount.
Figure 2. Many common solutions consist of one or more solutes dissolved in a solvent.
Homogeneous Mixtures
A homogeneous mixture has a uniform composition throughout.
This means:
- The different substances are evenly mixed.
- The individual components cannot be seen.
- Every sample is the same.
Examples:
- Salt water.
- Air.
- Vinegar.
- Brass.
- Soft drinks.
Solutions are always homogeneous mixtures.
Heterogeneous Mixtures
A heterogeneous mixture has a non-uniform composition.
The different substances remain visibly separate.
Examples:
- Sand and water.
- Oil and water.
- Soil.
- Granite.
- Salad.
Different parts of a heterogeneous mixture may have different compositions.
Comparing Homogeneous and Heterogeneous Mixtures
| Homogeneous Mixture | Heterogeneous Mixture |
|---|---|
| Uniform throughout | Non-uniform |
| One visible phase | Two or more visible phases |
| Components evenly mixed. | Components remain separate |
| Example: Salt water | Example: Sand and water |
Understanding this difference helps classify mixtures correctly.
Figure 3. Homogeneous mixtures are uniform throughout, while heterogeneous mixtures are not.
Solutions vs Pure Substances
A pure substance contains only one type of particle.
Examples:
- Pure water (H₂O).
- Oxygen gas (O₂).
- Copper (Cu).
A solution contains two or more substances mixed together.
Although a solution appears to be one substance, it is actually a mixture.
| Pure Substance | Solution |
|---|---|
| One type of particle | Two or more substances |
| Fixed composition | Variable composition |
| Cannot be separated by simple physical methods into identical particles. | Components can often be separated by physical methods such as evaporation or distillation |
Everyday Examples of Solutions
Solutions are found almost everywhere.
Examples include:
- Seawater.
- Tea and coffee.
- Soft drinks.
- Vinegar.
- Cleaning solutions.
- Medicines.
- Perfume.
- Air.
Even many metals are mixed to form alloys, which are solid solutions.
Figure 4. Solutions are common in everyday life, from drinks to medicines and cleaning products.
Why Solutions Are Important
Solutions play an important role in:
Biology
- Blood transports nutrients and oxygen in solution.
- Plant roots absorb mineral ions dissolved in water.
Medicine
- Many medicines are prepared as solutions.
- Saline solution is used in hospitals.
Industry
- Cleaning products.
- Paints.
- Food production.
- Chemical manufacturing.
Environment
- Oceans contain dissolved salts.
- Rivers carry dissolved minerals.
Solutions are essential in many natural and technological processes.
Particle Model of a Solution
When a solute dissolves:
- Solute particles separate.
- Solvent particles surround the solute particles.
- The particles spread evenly throughout the solvent.
Because the particles are so small and evenly distributed, they cannot usually be seen.
This explains why solutions appear uniform.
Figure 5. In a solution, solvent particles surround and evenly distribute solute particles.
Worked Example
Question
Identify the solute and solvent in each mixture.
| Mixture | Solute | Solvent |
|---|---|---|
| Salt water | ? | ? |
| Sugar solution | ? | ? |
| Carbonated water. | ? | ? |
Solution
| Mixture | Solute | Solvent |
|---|---|---|
| Salt water | Salt | Water |
| Sugar solution | Sugar | Water |
| Carbonated water. | Carbon dioxide | Water |
Real-World Connection
Sports drinks are carefully prepared solutions containing water, sugars, and dissolved mineral ions such as sodium and potassium. During exercise, these solutions help replace water and electrolytes lost through sweating while also providing energy. Because the ingredients are dissolved evenly, every sip has the same composition.
Did You Know?
Although water is often called the universal solvent, it cannot dissolve every substance. For example, oil does not dissolve in water because oil molecules are non-polar, while water molecules are polar. Chemists often summarise this idea with the phrase "like dissolves like," meaning substances with similar chemical properties tend to dissolve in one another.
Key Terms
Heterogeneous mixture – A mixture with a non-uniform composition in which different components remain distinguishable.
Homogeneous mixture – A mixture with a uniform composition throughout.
Pure substance – A material made of only one type of particle with a fixed composition.
Solution – A homogeneous mixture formed when a solute dissolves in a solvent.
Solute – The substance that is dissolved in a solution.
Solvent – The substance that dissolves the solute, usually present in the greatest amount.
Key Takeaways
- A solution is a homogeneous mixture made when a solute dissolves in a solvent.
- The solute is the substance being dissolved, while the solvent is the substance that does the dissolving.
- Homogeneous mixtures are uniform throughout, whereas heterogeneous mixtures have a non-uniform composition.
- Solutions differ from pure substances because they contain two or more substances mixed together.
- Many everyday materials, including seawater, soft drinks, vinegar, medicines, and air, are examples of solutions.
- The particle model explains why solutions appear uniform: the solute particles are evenly distributed among the solvent particles.