Temperature and solubility: usually up, but not always

The problem

Sugar dissolves faster and in larger amounts in hot tea. Yet a cold bottle of soda water stays fizzy longer than a warm one, which means warmth makes carbon dioxide less soluble. So does heat help solutes dissolve or not?

The solubility of a solute is the amount of it that can dissolve in 100 g of a solvent at a particular temperature. Changing the temperature affects different compounds differently. Solubility usually increases with temperature, but your book warns that this cannot be taken as a general rule.

Rises with temperature: potassium nitrate (KNO₃), silver nitrate (AgNO₃), potassium chloride (KCl), copper sulphate and sodium nitrate. Hardly changes: sodium chloride. Falls with temperature: lithium carbonate (Li₂CO₃), calcium chromate (CaCrO₄), calcium hydroxide and gases.

Solubility is measured per 100 g of solvent at a stated temperature. For most solids it rises with temperature, for sodium chloride it hardly changes, and for gases and a few solids it falls.

Key terms

Solubility

How much solute dissolves in 100 g of solvent at a particular temperature.

Solubility curve

A graph showing whether a solute's solubility rises, stays level or falls as it gets hotter.

Crystals

Pure solid that comes out when a hot saturated solution cools.

Dissolved gas

A gas held in a liquid; its solubility falls as the temperature rises.

Short questions with model answers

1. Q1. A saturated sugar solution is heated and more sugar is added. What happens, and what does it show?

   • At room temperature the solution held the maximum sugar, so extra sugar would have settled.
   • After heating, the added sugar dissolves.

   The extra sugar dissolves, showing that the solubility of sugar increases with temperature.

2. Q2. Why are soda water bottles stored in the refrigerator?

   • The fizz is carbon dioxide gas dissolved in water.
   • The solubility of gases falls as temperature rises, so CO₂ is more soluble in cold water.

   Cold keeps more carbon dioxide dissolved, so the drink stays fizzy longer.

3. Q3. Why can potassium nitrate be purified by crystallising it from water, while sodium chloride cannot be crystallised the same way?

   • KNO₃ is much more soluble in hot water than in cold water.
   • On cooling, the extra KNO₃ comes out as pure crystals.
   • NaCl's solubility hardly changes with temperature, so cooling releases almost no crystals.

   KNO₃'s solubility rises steeply with temperature; NaCl's hardly changes, so cooling gives it few crystals.

Common mistakes

• ✗ Writing that solubility always increases with temperature.

  ✓ Usually, not always: NaCl hardly changes, and Li₂CO₃, CaCrO₄, calcium hydroxide and gases fall.

• ✗ Defining solubility as the amount that dissolves, with no amount of solvent or temperature.

  ✓ State both: per 100 g of solvent, at a particular temperature.

• ✗ Saying warm water holds more dissolved carbon dioxide, like sugar.

  ✓ Gases are the opposite: their solubility falls as temperature rises.

MCQs

1. 1. KClO₃ has a solubility of about 13.2 g per 100 cm³ of water at 40 °C. If you decrease the temperature, its solubility will:

   1. (a) Increase
   2. (b) Decrease
   3. (c) Remain the same
   4. (d) First increase, then decrease
   Show answer

   (b) KClO₃ is a solid whose curve in Fig 1.6 rises with temperature, so cooling lowers its solubility (MCQ ix, p. 15).

2. 2. The solubility of which compound decreases as temperature increases?

   1. (a) KNO₃
   2. (b) AgNO₃
   3. (c) Li₂CO₃
   4. (d) KCl
   Show answer

   (c) Lithium carbonate is on the book's falling list; the other three rise with temperature.

3. 3. Solubility is the amount of solute that dissolves in:

   1. (a) 100 g of solvent at a particular temperature
   2. (b) 1 litre of solution at any temperature
   3. (c) 100 g of solution at room temperature
   4. (d) Any amount of solvent
   Show answer

   (a) That is the book's definition (p. 11): per 100 g of solvent, not solution, at a stated temperature.

4. 4. Soda water bottles are kept in the refrigerator because carbon dioxide:

   1. (a) Reacts with the bottle when warm
   2. (b) Freezes into a solid
   3. (c) Turns into a liquid
   4. (d) Is more soluble at low temperature
   Show answer

   (d) Gas solubility falls with rising temperature, so the cold keeps more CO₂ dissolved (p. 13).

5. 5. Whose solubility in water hardly changes as temperature increases?

   1. (a) KNO₃
   2. (b) NaCl
   3. (c) Copper sulphate
   4. (d) Sodium nitrate
   Show answer

   (b) The book: the solubility of sodium chloride does not increase appreciably with temperature. The other three rise.

Quick revision

• Solubility: grams of solute that dissolve in 100 g of solvent at a particular temperature.
• Rises: KNO₃, AgNO₃, KCl, copper sulphate, sodium nitrate, sugar. Hardly changes: NaCl. Falls: Li₂CO₃, CaCrO₄, calcium hydroxide, gases.
• Uses: purifying solids such as KNO₃ as crystals, and keeping soda water cold to hold its CO₂.