☰ Contents · Chemistry (General chemistry)

Solutions and solubility

Lessons 12–13 · 2 lessons · S. Masharipov, A. Mutalibov, E. Murodov, H. Islomova. General chemistry, Grade 11, 1st edition. G‘afur G‘ulom Publishing and Printing House, Tashkent, 2018
13

Solubility

Textbook: pp. 65–70
GoalExplain the solubility coefficient and saturated, unsaturated and supersaturated solutions, and describe how solubility depends on temperature and pressure.
New words
solubility coefficient · eruvchanlik koeffitsiyentisaturated solution · to‘yingan eritmasupersaturated solution · o‘ta to‘yingan eritmacrystallisation · kristallanish
Explanation

The largest mass of a substance that can dissolve in 100 g of solvent at a given temperature is the solubility coefficient S; for example S(NaCl, 20 °C) = 36 means that 36 g of salt dissolve in 100 g of water. By solubility substances are divided into well soluble (more than 10 g per 100 g of water), slightly soluble (less than 1 g) and practically insoluble (0.01 g or less). For most solids dissolving absorbs heat, so solubility rises with temperature (there are exceptions, such as Ca(OH)₂); the solubility of gases falls as temperature rises and rises with pressure — that is why gas bubbles escape when a fizzy drink is opened and why a diver must surface slowly. A solution in which no more solute can dissolve is saturated, one with less than the possible amount is unsaturated and one with more is supersaturated. A supersaturated solution is unstable: on stirring or when a seed crystal is added the excess salt separates. When a hot saturated solution cools, solubility drops and the excess substance crystallises.

Worked examples
Let S(KNO₃, 20 °C) = 32. In 50 g of water 16 g of salt dissolve to give a saturated solution; its mass is 50 + 16 = 66 g. If only 12 g are added the solution is unsaturated.
Cooling: if S = 110 at 60 °C and S = 32 at 20 °C, then for every 100 g of water 110 – 32 = 78 g of salt crystallises.
Class activity

Comparison: with the teacher’s help, open fizzy drinks that were cold and warm at the same time (pointed away from the face and eyes) and link the rate of bubbling to temperature. Never open a container that has been shaken.

Practice
1
At a given temperature 40 g of a salt dissolve in 200 g of water to give a saturated solution. What is S?
2
If S = 36, what is the maximum mass of salt that dissolves in 250 g of water?
3
At 30 °C S = 30. 25 g of the salt are dissolved in 100 g of water. Is the solution saturated, unsaturated or supersaturated?
4
Why do small bubbles appear on the wall of a glass when cold water is left in a warm room?