# 1.1.4 Solubility

### Solubility

• Solubility is a measurement of how much of a substance will dissolve in a given volume of a liquid
• The liquid is called the solvent
• The solubility of a gas depends on pressure and temperature
• Different substances have different solubilities
• Solubility can be expressed in g per 100 g of solvent
• Solubility of solids is affected by temperature
• As temperature increases, solids usually become more soluble
• Solubility of gases is affected by temperature and pressure; in general:
• As pressure increases, gases become more soluble
• As temperature increases, gases become less soluble

### 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 solvent that can be dissolved in 100 g of water before a saturated solution is formed, is determined at a series of different temperatures

Solubility curve for three salts. While the solubility of most salts increases with temperature, sodium chloride, or common salt, hardly changes at all

#### Worked Example

Use the solubility curve to answer these questions:

1. Determine how much potassium nitrate will dissolve in 20 g of water at 40 °C?
2. 200 cm3 of saturated lead(II)nitrate solution was prepared at a temperature of 90 °C. What mass of lead(II)nitrate crystals form if the solution was cooled to 20 °C?

Answers

Problem 1

At 40 °C the solubility is 68 g per 100 g of water

So scaling, 68 x (20 / 100) = 13.6 g of potassium nitrate will dissolve in 20 g of water

Problem 2

Solubility of lead(II)nitrate at 90 oC is 118 g / 100 g water, and 47 g / 100 g water at 20 °C.

Therefore for mass of crystals formed = 118 – 47 = 71 g (for 100 cm3 of solution).

However, 200 cm3 of solution was prepared,

So total mass of copper sulphate crystallised = 2 x 71 = 142 g

#### Exam Tip

As temperature increases, solids usually become more soluble and gases become less soluble.

### Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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