2.4.8 Effect of Temperature on Solubility

This topic explains how temperature affects the solubility of a solute in a solvent. It shows that temperature can either increase or decrease solubility, depending on the nature of the solute.

Definition

The solubility of solutes depends on temperature. Depending on the nature of the solute, there is either:

a) Increase in solubility with temperature Examples: KCl, NH₄Cl

b) Decrease in solubility with temperature Examples: Na₂SO₄, Ca(OH)₂

Simple explanation: For most solids, heating the solvent lets it dissolve more solute. But for some substances, heating actually makes them less soluble.

Main Idea

Temperature is one of the most important factors affecting solubility:

• For most ionic solids, solubility increases as temperature increases.
• For some solids (like calcium hydroxide), solubility decreases as temperature increases.
• For gases, solubility usually decreases as temperature increases.

This means there is no single rule — it depends on the substance.

Core Concept

Solubility Curves

A solubility curve is a graph showing how the solubility of a substance changes with temperature.

• X-axis: temperature (°C)
• Y-axis: grams of solute per 100 g of water
• Each curve represents the solubility of one substance

• NaNO₃, KNO₃, NH₄Cl, KCl — solubility increases with temperature
• NaCl — solubility increases only slightly with temperature
• Ce₂(SO₄)₃ (and some others) — solubility decreases with temperature
• Gases (like SO₂, HCl) — solubility decreases with temperature

Detailed Explanation

Case 1: Increase in Solubility with Temperature

For most salts, solubility increases with temperature. Common examples from the book:

• KCl (potassium chloride)
• NH₄Cl (ammonium chloride)

Looking at Table 2.2 (page 23):

Both show a clear increase in solubility as temperature rises.

Why? Higher temperature gives solvent molecules more energy to pull apart and surround solute particles, so more solute can dissolve.

Case 2: Decrease in Solubility with Temperature

For some substances, solubility decreases with temperature. Examples:

• Na₂SO₄ (sodium sulfate)
• Ca(OH)₂ (calcium hydroxide)

Looking at Table 2.2:

The solubility decreases from 0.173 to 0.066 g/100 g water as temperature rises from 20 °C to 100 °C.

Book example: An example of a solute whose solubility decreases with increasing temperature is calcium hydroxide, which can be used to treat chemical burns and as an antacid.

Case 3: Gases

For gases, the general rule is:

• Higher temperature → lower gas solubility
• Lower temperature → higher gas solubility

• Warm soda goes "flat" faster than cold soda.
• Fish need cold water because cold water holds more dissolved oxygen.

Intermediate Level

Solubility Curves — How to Read Them

The book shows solubility curves (Figure 2.4). Here is what they tell you:

• Steep upward curve → solubility increases sharply with temperature (e.g., KNO₃, NaNO₃)
• Gentle upward curve → solubility increases only slightly (e.g., NaCl)
• Downward curve → solubility decreases with temperature (e.g., Ce₂(SO₄)₃, gases)
• At any given temperature, the curve tells you the maximum solute that can dissolve (solubility).

Why Different Substances Behave Differently

The effect of temperature on solubility depends on:

• Whether dissolution is endothermic or exothermic
• The nature of the solute–solvent interactions
• The state of the solute (solid, liquid, or gas)

For most ionic solids, dissolving is endothermic, so heating helps. But for substances like Ca(OH)₂, dissolving is exothermic, so heating reduces solubility.

Practical Applications

• Recrystallization — used to purify solids by dissolving them in hot solvent and cooling to crystallize.
• Soft drinks — carbonated at low temperature and high pressure, then sealed, so CO₂ stays dissolved.
• Aquariums — cold water holds more oxygen, so fish survive better in cooler water.

Important Points

• - Solubility of solutes depends on temperature.
• - For most salts (KCl, NH₄Cl), solubility increases with temperature.
• - For some salts (Na₂SO₄, Ca(OH)₂), solubility decreases with temperature.
• - For gases, solubility decreases with temperature.
• - Solubility curves show how solubility varies with temperature.
• - Ca(OH)₂ (limewater) is a classic example of a substance whose solubility decreases with temperature; it is used to treat chemical burns and as an antacid.

Remember

• - Most solids: solubility increases with temperature (KCl, NH₄Cl).
• - Some solids: solubility decreases with temperature (Ca(OH)₂, Na₂SO₄).
• - Gases: solubility decreases with temperature.
• - Solubility curve shows the effect of temperature.
• - Ca(OH)₂ → decreases with temperature; used to treat chemical burns and as an antacid.

Common Mistakes

• 1. Thinking all solids become more soluble when heated. Some do not — Ca(OH)₂ becomes less soluble.
• 2. Forgetting gases. Gases become less soluble when heated.
• 3. Confusing solubility with rate of dissolving. Temperature affects both, but they are not the same.
• 4. Not reading curves carefully. A curve that goes down means solubility decreases with temperature.
• 5. Thinking solubility curves are the same for all substances. Each substance has its own curve.

Quick Revision

• Solubility depends on temperature.
• Most solids → solubility increases with temperature (KCl, NH₄Cl).
• Some solids → solubility decreases with temperature (Ca(OH)₂, Na₂SO₄).
• Gases → solubility decreases with temperature.
• Solubility curves show the variation of solubility with temperature.
• Ca(OH)₂ is an example where solubility decreases with temperature; used to treat chemical burns and as an antacid.
• At 20 °C, Ca(OH)₂ solubility = 0.173 g/100 g H₂O; at 100 °C, it is 0.066 g/100 g H₂O.

60-Second Revision

• Most solids: solubility increases with temperature.
• Ca(OH)₂, Na₂SO₄: solubility decreases with temperature.
• Gases: solubility decreases with temperature.
• Solubility curves show the effect.
• Warning: temperature does not always increase solubility — it depends on the substance.