Inorganic Salts: Properties, Preparation and Identification of Ions
Senior 2 · Chemistry
Inorganic Salts: Properties, Preparation and Identification of Ions
Lesson 2 of 5
25 min
Sugar disappears faster in hot tea than in cold water. A bottle of soda fizzes when you open it and goes flat if you leave it. And the deep water of Lake Kivu holds an estimated 256 cubic kilometres of carbon dioxide and 65 cubic kilometres of methane in solution, kept there by the enormous pressure of the water above.
All three are about solubility. Whether a salt dissolves decides whether it can be prepared by crystallisation or by precipitation, whether a fertiliser reaches a plant's roots, and how a chemist tests for ions. By the end of this lesson you will be able to use the solubility rules, explain the factors that affect solubility, and calculate with solubility data.
A solution is a uniform mixture of a solute (the substance that dissolves) in a solvent (the substance it dissolves in). Salt water has salt as the solute and water as the solvent. A solution in water is called an aqueous solution. A dilute solution has a little solute in a lot of solvent; a concentrated solution has a lot.
The solubility of a substance is the maximum mass of it that will dissolve in 100 g of solvent at a stated temperature. It is usually given in grams per 100 g of water.
Try it: open Concentration (version 1.8.3). Shake a solid solute into the beaker and watch the concentration meter. When the solution can hold no more, the simulation shows "Saturated!" and solid collects at the bottom. Compare two different solutes.
| Salts | Solubility in water |
|---|---|
| All sodium, potassium and ammonium salts | Soluble |
| All nitrates | Soluble |
| Chlorides | Soluble, except silver chloride and lead(II) chloride |
| Sulfates | Soluble, except barium sulfate and lead(II) sulfate (calcium sulfate is only slightly soluble) |
| Carbonates | Insoluble, except sodium, potassium and ammonium carbonates |
| Hydroxides | Insoluble, except sodium, potassium and ammonium hydroxides (calcium hydroxide is slightly soluble) |
Stirring and grinding the solid into a powder make a solid dissolve faster, but they do not change its solubility: the maximum mass that can dissolve stays the same.
A solubility curve is a graph of solubility against temperature. The table below gives the data from which the curves are drawn.
| Temperature (°C) | 0 | 20 | 40 | 60 | 80 |
|---|---|---|---|---|---|
| Potassium nitrate (g per 100 g water) | 13 | 32 | 64 | 110 | 169 |
| Sodium chloride (g per 100 g water) | 35.7 | 36.0 | 36.6 | 37.3 | 38.4 |
Mutesi dissolves 60 g of potassium nitrate in 100 g of water at 40 °C. Is the solution saturated?
Solution. At 40 °C the solubility is 64 g per 100 g of water. 60 g is less than 64 g, so the solution is unsaturated; 64 − 60 = 4 g more could still dissolve.
A saturated solution of potassium nitrate in 100 g of water at 60 °C is cooled to 20 °C. What mass of crystals forms?
Solution. At 60 °C the water holds 110 g; at 20 °C it can hold only 32 g. The rest crystallises: 110 − 32 = 78 g of crystals.
What is the greatest mass of potassium nitrate that dissolves in 50 g of water at 60 °C?
Solution. 100 g of water dissolves 110 g, so 50 g of water dissolves half as much: 110 × 50 ÷ 100 = 55 g.
A dish containing 26.0 g of a saturated salt solution at 30 °C is heated until all the water has evaporated, leaving 6.0 g of dry salt. Calculate the solubility at 30 °C.
Solution. Mass of water = 26.0 − 6.0 = 20.0 g. Solubility = 6.0 ÷ 20.0 × 100 = 30 g per 100 g of water.
Tip
Per 100 g of WATER, not per 100 g of solution. In evaporation questions, first find the mass of water: mass of solution − mass of dry salt. Then scale to 100 g of water.
Rate is not solubility. Stirring, heating and powdering all speed up dissolving, but only temperature (and, for gases, pressure) changes how much can dissolve.
Use the table in this lesson. Try each item before you read the solution.
Which of these are soluble in water: silver chloride, potassium carbonate, barium sulfate, zinc nitrate, calcium carbonate?
Solution: Potassium carbonate (all potassium salts) and zinc nitrate (all nitrates) are soluble. Silver chloride, barium sulfate and calcium carbonate are insoluble.
A saturated solution of potassium nitrate in 100 g of water is cooled from 80 °C to 40 °C. What mass of crystals forms?
Solution: 169 − 64 = 105 g.
Why is sodium chloride not usually purified by cooling a hot saturated solution, while potassium nitrate is?
Solution: The solubility of sodium chloride hardly changes with temperature (36.0 g at 20 °C, 38.4 g at 80 °C), so very little would crystallise on cooling. Potassium nitrate's solubility falls steeply as it cools, so a large mass of crystals forms. Sodium chloride is obtained by evaporating the water instead.
What mass of sodium chloride can dissolve in 250 g of water at 20 °C?
Solution: 36.0 × 250 ÷ 100 = 90 g.
Explain why fish in a shallow pond may struggle on a very hot afternoon.
Solution: Gases are less soluble in warm water. As the pond heats up, some dissolved oxygen comes out of solution, leaving less for the fish to breathe.
Summary
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
