Solutions and Solubility
2. The Process of Dissolving
Learning outcomes
- I can describe how particles interact during dissolving.
- I can explain dissolution using the particle model.
- I can distinguish between dissolving and chemical reactions.
- I can explain why some substances dissolve more readily than others.
- I can describe factors that affect the dissolving process.
Introduction
When sugar disappears in a cup of tea or salt seems to vanish in water, it may look as though the substance has disappeared completely. In reality, the particles of the substance have simply spread throughout the liquid to form a solution. This process is called dissolving or dissolution.
The particle model of matter helps explain exactly what happens during dissolving. It shows how particles of the solute separate, interact with particles of the solvent, and become evenly distributed throughout the solution. Understanding this process also helps explain why some substances dissolve easily while others do not.
What Is Dissolving?
Dissolving is the process in which the particles of a solute become evenly distributed throughout a solvent to form a solution.
During dissolving:
- The solute breaks into tiny particles.
- Solvent particles surround the solute particles.
- The particles spread evenly throughout the solvent.
The solute has not disappeared—its particles are simply too small to see.
Figure 1. Solute particles separate and become evenly distributed among solvent particles.
Dissolving and the Particle Model
According to the particle model:
- All matter is made of tiny particles.
- Particles are constantly moving.
- There are spaces between particles.
- Particles attract one another.
When a solute is added to a solvent:
- Solvent particles collide with the surface of the solute.
- They pull individual solute particles away.
- The solvent surrounds these particles.
- The solute particles spread throughout the solvent.
Eventually, the solution becomes uniform.
Particle Interactions During Dissolving
During dissolution:
- Attractive forces between solute particles are weakened.
- New attractions form between solute particles and solvent particles.
If the attraction between the solute and solvent is strong enough, the substance dissolves.
If these attractions are too weak, the substance remains undissolved.
Figure 2. Solvent particles surround and separate solute particles during dissolving.
Dissolving Is Not a Chemical Reaction
Dissolving is a physical change, not a chemical reaction.
During dissolving:
- No new substance is formed.
- The chemical identity of the solute remains unchanged.
- The process can often be reversed.
For example:
Salt dissolved in water can be recovered by evaporating the water.
Comparing Dissolving and Chemical Reactions
| Dissolving | Chemical Reaction |
|---|---|
| Physical change | Chemical change |
| No new substance formed | New substances formed |
| Usually reversible | Often difficult to reverse |
| Particles remain chemically unchanged. | Chemical bonds are broken and formed |
Understanding this difference helps identify physical and chemical changes.
Why Do Some Substances Dissolve?
Not every substance dissolves in every solvent.
A substance dissolves when:
- Solute particles are strongly attracted to solvent particles.
A useful rule is:
"Like dissolves like."
This means:
- Polar substances usually dissolve in polar solvents.
- Non-polar substances usually dissolve in non-polar solvents.
For example:
- Salt dissolves well in water.
- Sugar dissolves well in water.
- Oil does not dissolve in water.
Figure 3. Some substances dissolve easily, while others do not because of differences in particle interactions.
Factors That Affect the Rate of Dissolving
Several factors affect how quickly a substance dissolves.
Temperature
Increasing the temperature usually causes solids to dissolve faster because:
- Solvent particles move more quickly.
- More frequent collisions occur.
For gases, increasing temperature usually makes them less soluble.
Stirring
Stirring mixes the solvent.
Fresh solvent particles are continually brought into contact with the solute, increasing the rate of dissolving.
Particle Size
Crushing a solid into smaller pieces increases its surface area.
Greater surface area allows:
- More solvent particles to collide with the solute.
- Faster dissolving.
Powdered sugar dissolves faster than a sugar cube.
Nature of the Solute and Solvent
Some substances naturally dissolve more readily than others because of differences in their particle attractions.
For example:
- Sugar dissolves easily in water.
- Sand does not dissolve in water.
Figure 4. Temperature, stirring, and particle size all affect the rate at which a substance dissolves.
Dissolving vs Melting
Students sometimes confuse dissolving with melting.
| Dissolving | Melting |
|---|---|
| Requires a solvent | Does not require a solvent |
| Forms a solution | Forms a liquid |
| Solute mixes with solvent. | Solid changes to liquid |
| Physical change | Physical change |
For example:
- Ice melting forms liquid water.
- Sugar dissolving forms a sugar solution.
These are different processes.
Everyday Examples of Dissolving
Examples include:
- Sugar dissolving in tea.
- Salt dissolving in cooking water.
- Instant coffee dissolving in hot water.
- Medicines dissolving in the stomach.
- Oxygen dissolving in rivers and lakes for aquatic life.
Dissolving is an essential process in both nature and everyday life.
Figure 5. Dissolving occurs in many everyday situations and natural processes.
Worked Example
Question
A student wants sugar to dissolve more quickly in a cup of tea.
Suggest three ways to increase the rate of dissolving.
Solution
The student could:
- Stir the tea.
- Use hotter tea.
- Crush the sugar into smaller pieces.
Each of these methods increases the rate at which sugar dissolves.
Real-World Connection
Many medicines are designed to dissolve at different rates inside the body. Some tablets dissolve quickly to provide rapid pain relief, while others have special coatings that dissolve slowly over several hours, releasing the medicine gradually. Scientists carefully control particle size and tablet design to achieve the desired rate of dissolution.
Did You Know?
Tiny fish and other aquatic animals depend on oxygen dissolved in water for survival. Although oxygen is a gas, some of its particles dissolve naturally in rivers, lakes, and oceans. Cold water can hold more dissolved oxygen than warm water, which is one reason why many fish thrive in cooler environments.
Key Terms
Dissolving (dissolution) – The process in which a solute becomes evenly distributed throughout a solvent to form a solution.
Particle model – A model describing matter as tiny particles that are constantly moving.
Physical change – A change that does not produce a new substance.
Solution – A homogeneous mixture formed when a solute dissolves in a solvent.
Solubility – The ability of a substance to dissolve in a particular solvent.
Solute – The substance being dissolved.
Solvent – The substance that dissolves the solute.
Surface area – The amount of exposed surface of a material; increasing surface area generally increases the rate of dissolving.
Key Takeaways
- Dissolving is the process in which a solute becomes evenly distributed throughout a solvent to form a solution.
- The particle model explains dissolving as solvent particles separating and surrounding solute particles.
- Dissolving is a physical change, not a chemical reaction, because no new substance is formed.
- A substance dissolves when the attraction between the solute and solvent particles is strong enough.
- The rate of dissolving is affected by temperature, stirring, particle size (surface area), and the nature of the solute and solvent.
- Dissolving is important in everyday life, from preparing food and medicines to supporting life in aquatic ecosystems.