Unsaturated and saturated: when the water cannot take any more

The problem

Keep stirring sugar into a cup of tea and at some point the last spoonful just sits at the bottom. The tea has not changed, so why does the sugar stop dissolving? Your book turns this into an experiment.

Take about 100 g of water in a beaker, add 5 g of table sugar and stir; it dissolves. Add another 5 g; it dissolves too. A solution that can still dissolve more solute at a particular temperature is called an unsaturated solution.

Keep adding sugar. Dissolving gets harder, and a stage comes when no more will dissolve at that temperature; any extra sugar settles at the bottom of the beaker. A solution in which the maximum amount of solute has been dissolved in a particular amount of solvent at a particular temperature is a saturated solution.

Different solutes reach that limit at very different amounts. At 20 °C, 36 g of sodium chloride saturates 100 g of water, but it takes 203.9 g of table sugar. Your book's reason: sugar molecules are larger than salt ions, so more water molecules can surround each sugar molecule.

Saturated always comes with three conditions: a particular solute, a particular amount of solvent and a particular temperature. Change any one and the limit changes.

Key terms

Solute

The substance added in portions until no more dissolves.

Solvent

The liquid whose amount is fixed in the experiment.

Unsaturated solution

A solution that can still dissolve more solute at that temperature.

Saturated solution

A solution holding the maximum solute for that solvent amount and temperature.

Short questions with model answers

1. Q1. How much sodium chloride is needed to saturate 250 g of water at 20 °C?

   • The book's figure: 36 g NaCl saturates 100 g of water at 20 °C.
   • 250 g of water is 2.5 times 100 g, so 2.5 × 36 g = 90 g.

   90 g of sodium chloride.

2. Q2. At 20 °C, how much more table sugar than sodium chloride can 100 g of water dissolve?

   • Sugar 203.9 g; sodium chloride 36 g.
   • 203.9 g − 36 g = 167.9 g.

   167.9 g more sugar.

3. Q3. You are given a clear sugar solution at room temperature. How can you tell whether it is saturated?

   • Keep the temperature the same and add a little more sugar, then stir.
   • If it dissolves, the solution was unsaturated. If it settles at the bottom, it was saturated.

   Add a little more sugar at the same temperature: it settles if saturated, dissolves if not.

Common mistakes

• ✗ Defining a saturated solution without saying at a particular temperature.

  ✓ The limit depends on temperature, so the definition must name it, along with the amount of solvent.

• ✗ Saying that adding more sugar to a saturated solution makes it more concentrated.

  ✓ At the same temperature the extra sugar settles at the bottom; the dissolved amount does not change.

• ✗ Assuming every solute saturates 100 g of water at the same mass.

  ✓ Different solutes have different solubilities: 36 g for NaCl but 203.9 g for sugar at 20 °C.

MCQs

1. 1. A solution that can dissolve more solute at a particular temperature is called:

   1. (a) Saturated
   2. (b) Unsaturated
   3. (c) Colloidal
   4. (d) Suspension
   Show answer

   (b) That is the book's definition of an unsaturated solution (p. 11).

2. 2. How much sodium chloride saturates 50 g of water at 20 °C?

   1. (a) 18 g
   2. (b) 36 g
   3. (c) 72 g
   4. (d) 9 g
   Show answer

   (a) 36 g per 100 g of water; 50 g is half of that, so 36 g ÷ 2 = 18 g.

3. 3. 20 g of sodium chloride dissolved in 100 g of water at 20 °C gives a solution that is:

   1. (a) Saturated
   2. (b) A suspension
   3. (c) Unsaturated
   4. (d) Colloidal
   Show answer

   (c) The limit is 36 g, and 20 g is below it, so 16 g more could still dissolve.

4. 4. More sugar is added to a saturated sugar solution at the same temperature. The extra sugar:

   1. (a) Dissolves slowly
   2. (b) Raises the concentration
   3. (c) Turns it into a colloid
   4. (d) Settles at the bottom
   Show answer

   (d) The solution already holds the maximum at that temperature, so the book says any more sugar settles.

5. 5. At 20 °C, the solubility of table sugar in water compared with sodium chloride is:

   1. (a) Far greater
   2. (b) About the same
   3. (c) Far less
   4. (d) Zero
   Show answer

   (a) 203.9 g of sugar against 36 g of sodium chloride per 100 g of water.

Quick revision

• Unsaturated: can still dissolve more solute at that temperature.
• Saturated: maximum solute in that amount of solvent at that temperature; extra solute settles.
• Limits differ by solute: 36 g NaCl but 203.9 g sugar per 100 g water at 20 °C.