Solubility Curves (WJEC GCSE Chemistry)

Producing Solubility Curves

What is solubility?

• Solubility is a measurement of how much of a substance will dissolve in a given volume of a liquid

  • The substance being dissolved is called the solute 

  • The liquid the solute is dissolved in is called the solvent 

  • The mixture of the solute and solvent is called the solution 

• The solubility of a substance depends on: 

  • The solute being dissolved

  • The solvent being used

  • The temperature 

• Solubility can be expressed in g per 100 g of solvent

  • E.g. The solubility of sodium chloride is 36 g / 100 g of water at room temperature (20^oC)

• A saturated solution is one where no more solute can dissolve

How is solubility determined?

Method 1: 

• The following method is used to measure solubility of a solid at room temperature:

  1. Add small amounts of solute e/g ammonium chloride to a known volume of water until no more will dissolve 

  2. Weigh the mass of an evaporating dish

  3. Filter the mixture to remove the undissolved solid 

  4. Place the solution into an evaporating dish and heat the solvent until only the solute is left behind

  5. Reweigh the evaporating dish and calculate the mass of solute that dissolved 

Method 2: 

• To investigate the solubility of a substance at different temperatures:

  1. Prepare two beakers, one as a hot water bath and one as an ice bath

  2. Using a small measuring cylinder, measure 4 cm³ of distilled water into a boiling tube.

  3. On a balance, weigh out a known mass of solute e.g 2.6 g of ammonium chloride and add it to the boiling tube

  4. Place the boiling tube into the hot water bath and stir until the solid dissolves

  5. Transfer the boiling tube to the ice bath and allow it to cool while stirring

  6. Note the temperature at which crystals first appear and record it in a table of results

  7. Repeat steps 2-6 adding 1 cm³ of distilled water for each experiment, until a total of 10cm³ has been added

Diagram to show the apparatus required for investigating the solubility of a salt with temperature

The temperature at which crystals form when the solution is cooled is recorded 

• The results for method 2 can be recorded in a table 

• The solubility in g / 100g is calculated by dividing the mass of the solute by the volume and multiplying by 100 

Example results table 

• The results can be used to plot a solubility curve at different temperatures

  • Solubility is on the y-axis and temperature is on the x-axis

Interpreting Solubility Curves

• Solubility graphs or curves represent solubility in g per 100 g of water plotted against temperature

• To plot a solubility curve, the maximum mass of solute that can be dissolved in 100 g of water before a saturated solution is formed, is determined at a series of different temperatures

Graph to show the solubility curves of different substances

Different substances have different solubilities 

• You must be able to interpret solubility curves

• The conclusions we can draw from these solubility curves are: 

  • As temperature increases, the solubility of all three solutes increases 

  • The solubility of sodium chloride barely increases

  • The solubility of potassium nitrate increases the most (approximately 30 g per 100 g water to over 200 g per 100 g water)

  • At 55^oC, the solubility of lead (II) nitrate and potassium nitrate is the same (approximately 80 g per 100 g of water- this is shown where the solubility curves of these substances cross)

  • At low temperatures, lead(II) nitrate and sodium chloride are more soluble than potassium nitrate

  • At about 33^oC the solubility of sodium chloride and potassium nitrate is the same (approximately 38 g per 100 g water, where the solubility curves of these substances cross)