What is solubility in chemistry? It is a chemical property that helps to explain how and why certain substances mix, while others remain separate.

1. Introduction

Let us make lemonade at home.

• Take a lemon and squeeze it into a glass to get lemon juice.
• Add some water and sugar to it.
• Stir it properly with a spoon.
• Lemonade is ready!

Every sip of lemonade tastes great. But wait, where did the sugar go? Did it disappear?

Well, it did not vanish anywhere — it dissolved.

When a solid (like sugar) disappears into a liquid (like water), we say it has dissolved to form a solution. The solid is called the solute, the liquid is the solvent, and the final drink you enjoy is called a solution.

This whole process is termed dissolution, and the ability of a substance to dissolve is called solubility.

What is Solubility?

Solubility can be defined as:

“The maximum amount of a solute that can be dissolved in 100 g of a solvent at a specific temperature to form a saturated solution”.

Here, the solution is in dynamic equilibrium.

Dynamic Equilibrium

Dynamic equilibrium occurs in a saturated solution. It is established when the rate at which solute particles dissolve equals the rate at which they recrystallize (precipitate).

Although particles are continuously dissolving and recrystallizing, the overall concentration of the solute remains constant.

3 Factors Affecting Solubility

The solubility of a substance depends on several factors, including:

1. Effect of Temperature on Solubility

Temperature has a profound effect on solubility. As a general trend, two cases are mentioned here.

Solids in Liquids

In most cases, solubility increases with an increase in temperature.

Solubility ∝ Changes in Temperature

This means that more solute can dissolve in hot water than in cold water.

You can try this at home by mixing sugar in a hot glass of water and a cold glass of water. A hotter glass of water will dissolve more sugar than a colder glass of water.

Examples of Solids and Their Solubility Behaviour

Gases in Liquids

The solubility of gases decreases with increasing temperature. This is why cold soda fizzes more when opened.

Note that warmer temperatures make gases escape faster.

2. Nature of Solute & Solvent

There is a simple idea in chemistry that helps us understand this:

“Like dissolves like”.

This means that substances with similar properties tend to mix well.

Examples

Polar Substances

Water is a polar substance, meaning it has tiny electric charges. So, it can only dissolve other polar or ionic substances (e.g., NaCl, K₂CO₃, CuSO₄, sugar).

Non-Polar Substances

Oil is a non-polar substance, meaning it does not have tiny charges. So, it will not mix with water. However, being a non-polar substance, it will dissolve easily with other non-polar substances (e.g., wax, grease, ether, naphthalene, or CCl₄).

This is why oil floats on top of water and does not mix.

3. Solute–Solvent Interactions

Dissolving a solute in a solvent requires overcoming the intermolecular forces within both the solute and the solvent. The extent to which a solute dissolves depends on the strength and compatibility of these interactions.

Factors Influencing the Interactions

There are mainly 3 factors that limit the solubility of a solute in a solvent. The factors are:

• Solute–Solute Attractions
• Solvent–Solvent Attractions
• Solute–Solvent Attractions

So, the solubility rule is:

Solute–Solute & Solvent–Solvent Attraction < Solute–Solvent Attraction            → Soluble

Solute–Solute & Solvent–Solvent Attraction > Solute–Solvent Attraction            → Insoluble

Complete Solubility Requirement

Most solutes are solids, and a major portion of those solids are ionic in nature. These solute ions are held together by strong electrostatic forces in a crystal lattice. For such solids to dissolve:

• Solvent molecules must disrupt the ionic lattice.
• The ions must then be surrounded (hydrated or solvated) by solvent molecules.

For a solution to be completely soluble, the following 3 steps must occur:

i. Separation of solute particles

ii. Separation of solvent particles

iii. Formation of solute–solvent attractions

Example | Ionic Solids and Dissolution

For instance, table salt (NaCl) is dissolved in water (H₂O) as follows:

• The polar water molecules interact with Na⁺ and Cl⁻ ions.
• The positive end of water (H) faces Cl⁻, and the negative end (O) faces Na⁺.
• The strong ion–dipole attractions pull the Na⁺ and Cl⁻ ions away from the crystal.
• The ions become surrounded by water molecules and go into solution.

Endothermic Dissolution vs Exothermic Dissolution vs No Net Heat Change | The Science behind Dissolution

When a solute dissolves in a solvent, energy changes take place. There are 3 types of energy behaviour during dissolution:

This shows, different compounds respond differently to temperature changes, leading to distinct solubility patterns in water at a given temperature.

Solubility Curves

A solubility curve is a graph of solubility (g solute/100 g solvent) versus temperature (°C). It shows how the solubility of substances varies with temperature.

There are mainly 2 types of solubility curves that are discussed below.

Continuous Solubility Curve

The substances that show a continuous change in solubility are termed a continuous solubility curve. These are smooth and monotonic changes, and the curve appears as a single continuous line.

Example

• KNO₃, KClO₃, and Pb(NO₃)₂ show a continuous increase with temperature.
• NaCl shows only a slight increase.
• Ce₂(SO₄)₃ shows a decrease, which then levels off at higher temperatures.

Discontinuous Solubility Curves

The curves that show sudden changes in solubility due to changes in hydration (water of crystallisation) are known as discontinuous solubility curves. These curves appear as a combination of multiple solubility lines.

Example

• Na₂SO₄·10H₂O
• CaCl₂·6H₂O

Conclusion

Here, you saw what solubility is and how it is affected by different factors, especially the effect of temperature on the solubility of solids and gases.

Understanding solubility and heat changes during solute dissolution helps to explain why solubility increases or decreases. Solubility curves and fractional crystallisation are also useful tools for studying and applying solubility behaviour.

Frequently Asked Questions (FAQs)

What is solubility in chemistry?

The maximum amount of solute that dissolves in 100 g of solvent at a specific temperature.

What are 3 factors affecting solubility?

• Temperature
• Nature of solute & solvent
• Solute–solvent interaction

How does NaCl dissolve in water?

Polar molecules of water pull apart Na⁺ and Cl⁻ ions, forming a solution.

Why do different compounds have different solubilities in water at a particular temperature?

Due to differences in bonding, energy changes, and solute–solvent interactions.

Why cannot NaCl be crystallised like KNO₃?

The solubility of NaCl changes very little with temperature, making it hard to recover by cooling. In contrast, the solubility of KNO₃ increases significantly with heat, so cooling a hot, saturated solution leads to visible crystallisation.

What happens if solute–solute forces are stronger?

The solute will not dissolve.

What if solute–solvent forces are stronger?

A solution will form.

Why does iodine dissolve in CCl₄ but not in water?

Because both iodine and CCl₄ are non-polar and water is polar (“like dissolves like”).

Why does KNO₃ make the test tube cold?

Dissolution of KNO₃ is an endothermic process (absorbs heat).

What does “like dissolves like” mean?

Polar dissolves polar (e.g., salt in water); non-polar dissolves non-polar (e.g., oil in hexane).

How do intermolecular forces affect solubility?

Solute dissolves only if solute–solvent attraction is stronger than internal forces.