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School Chemistry Notes: Solubility of gases or salts in water - using solubility curves

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4. Gas and salt solubility

Gas and salt solubility and solubility curves are considered

(Suitable for AQA, Edexcel and OCR GCSE chemistry students)

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5a. How well do different gases and solids dissolve in water?

  • First, some definitions of words you may encounter in talking about solubility and other water related situations:

    • solute: the material which is to be dissolved in a solvent.

    • solvent: the liquid which dissolves the material (the solute). You will come across water more than any other liquid solvent BUT lots of important organic solvents like hexane (petrol like), ethanol (alcohol) and propanone (acetone) are in common laboratory use.

    • solution: the result of dissolving something in a liquid (solute + solvent => solution).

    • solubility: to what extent a solute material will dissolve.

    • soluble: the material will dissolve in a particular liquid solvent.

    • saturated: means that no more of a substance (the solute) will dissolve in its solution i.e. maximum solubility achieved at a particular temperature.

    • Therefore a saturated solution is one in which no more solute will dissolve at that temperature giving the maximum solubility at that particular temperature.

    • insoluble: not soluble, will not dissolve in a particular liquid (don't assume it means will not dissolve in anything).

    • hydration: means the addition of water to a material.

    • dehydration: means to remove water from a substance.

  • Factors affecting rates of dissolving.

    • heat: heating the mixture to raise the temperature will increase the rate of a substance dissolving - the energy of all the particles involved is increased - increased rate of more energetic collisions between solute and solvent particles speeding up the dissolving process.

    • surface area: if a solid is broken up and crushed into smaller pieces or a powder it will dissolve faster. This breaking down of a solid increases the surface are for the solvent to 'attack' and dissolve the solid.

    • stirring: this increases the rate of dissolving because it prevents 'local' saturation of the solution which will inhibit dissolving.

    • volume of solvent: adding more solvent increases the speed of dissolving, the less concentrated

    • These factors are similar with those affecting the rates of chemical reactions except there is no catalyst that speed up dissolving as far as I know? Also, increasing the volume of the solvent will decrease the rate of reaction because concentrations are reduced.

  • Some gases and solid substances are more soluble in water than others and some are hardly soluble at all.

  • The solubility of gases and solids in water also depends on the temperature of the water:

  • Many gases are soluble in water and the solubility increases as the temperature decreases and as the pressure increases.

  • Carbonated water is produced by dissolving carbon dioxide under high pressure. When the pressure is released the gas bubbles out of the solution. Carbonated water is used to give fizzy drinks a 'tang' to the taste.

    • It is a weakly acid solution, explaining why rainwater containing dissolved carbon dioxide from the air, can very slowly dissolve limestone.

      • The solution of CO2(aq) is sometimes described as 'carbonic acid', H2CO3, but this does not really exist!

        • However, the solution is acidic due to the formation of hydrogen ions.

        • CO2(aq) + H2O(l) (c) doc b H+(aq) + HCO3-(aq) 

        • Note: the equilibrium is almost completely on the left.

  • Thermal Pollution: Dissolved oxygen is essential for aquatic life and the colder the water, the more of it dissolves. Hot water from power stations may be discharged into rivers or lakes. This discharge reduces the amount of oxygen dissolved in the water and this can damage aquatic life and disrupt the natural eco-systems.

  • Chlorine water is made by dissolving Chlorine gas in water and can be a useful chemical reagent, both in the laboratory and industry (e.g. displaces iodine from sea water).

  • Chlorine water is used to bleach materials and kill bacteria.

  • Many ionic compounds are soluble in water and many covalent compounds are insoluble in water (but don't make assumptions!).

  • The solubility of a solute in water, or any other solvent, is usually given in grams of solute per 100 grams of solvent (e.g. water) at that temperature.

  • The solubility of most solid solutes increases as the temperature increases (opposite of gases, but the ambient air pressure has no effect).

  • A saturated solution is one in which no more solute will dissolve at that temperature giving the maximum solubility at that particular temperature.

  • When a hot saturated solution cools some of the solute will separate from the solution (crystallisation). The crystals form because the solubility is lower at the lower temperature.

  • From solubility graphs-data you can calculate how much will dissolve at a given temperature and how much will crystallise out on cooling.

  • Solubility curves:

    • A solubility curve is the graph of how much of a substance dissolves at a given temperature versus the temperature.

      • See section 4b. for Solubility data and solubility curves

  • General rules which describe the solubility of common types of compounds in water:

    • All common sodium, potassium and ammonium salts are soluble e.g. NaCl, K2SO4, NH4NO3

    • All nitrate salts are soluble e.g. NaNO3, Mg(NO3)2, Al(NO3)3, NH4NO3

    • Some ethanoate salts are soluble e.g. CH3COONa

    • Common chloride salts are soluble except those of silver and lead e.g.

      • soluble: KCl, CaCl2, AlCl3 or insoluble AgCl, PbCl2

    • Common sulfates are soluble except those of lead, barium and calcium: soluble e.g.

      • soluble: Na2SO4, MgSO4, Al2(SO4)3

      • insoluble: PbSO4, BaSO4, CaSO4 is slightly soluble.

    • Common oxides, hydroxides and carbonates are usually insoluble (e.g. Group 2 and Transition Metals) except those of the Group 1 Alkali Metals sodium, potassium etc. and ammonium:

      • soluble: K2O, KOH, NaOH, NH4OH actually NH3(aq), Na2CO3, (NH4)2CO3  

      • insoluble: MgO, CuO, ZnO, Mg(OH)2, Fe(OH)2, Cu(OH)2, CuCO3, ZnCO3, CaCO3

  • Knowledge of salt solubility is important in deciding which method of salt preparation is employed.

  • See methods of making salts and details of various methods.

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4b. Solubility curves for selected salts

solubility curves for potassium nitrate, potassium sulphate/sulfate, sodium chloride, copper(II) sulphate

  • Interpretation of graph e.g.

    • Reading graph: at 38oC the solubility of copper sulphate, CuSO4, is 28g of anhydrous salt per 100g of water.

    • Reading graph: at 84oC the solubility of potassium sulphate, K2SO4, is 22g per 100g of water.

    • Ex Q1: How much potassium nitrate will dissolve in 20g of water at 34oC?

      • At 34oC the solubility is 52g per 100g of water,

      • so scaling down, 52 x 20 / 100 = 10.4g will dissolve in 20g of water

    • Ex Q2: At 25oC 6.9g of copper sulphate dissolved in 30g of water, what is its solubility in g/100cm3 of water?

      • Scaling up, 6.9 x 100 /30 = 23g/100g of water (check on graph, just less than 23g/100g water).

    • Ex Q3: 200 cm3 of saturated copper solution was prepared at a temperature of 90oC. What mass of copper sulphate crystals form if the solution was cooled to 20oC?

      • Solubility of copper sulphate at 90oC is 67g/100g water, and 21g/100g water at 20oC.

      • Therefore for mass of crystals formed = 67 - 21 = 46g (for 100 cm3 of solution).

      • However, 200 cm3 of solution was prepared,

      • so total mass of copper sulphate crystallised = 2 x 46 = 92g

  • Note: The density of water is close to 1.0g/cm3 or ml, so for approximate purposes. the volume in cm3 or ml of just the water is numerically close to the value in g, i.e. 100 cm3 of water or solution is about 100g of water.

 

Examples of

SALT SOLUBILITY DATA

SOLUBILITY

g salt / 100g water

Salt name

potassium nitrate

potassium sulphate

sodium chloride

hydrated copper(II) sulphate

and formula

Temp. oC

KNO3

K2SO4

NaCl

CuSO4 (anhydrous *)

0

13.9

7.4

35.7

14.3

10

21.2

9.3

35.8

17.4

20

31.6

11.1

36.0

20.7

30

45.3

13.0

36.2

24.2

40

61.4

14.8

36.5

28.7

50

83.5

16.5

36.8

33.8

60

106.0

18.2

37.3

40.0

70

  

19.8

37.6

47.0

80

  

21.4

38.1

56.0

90

  

22.9

38.6

67.5

100

  

24.1

39.2

80.0
       

* multiply by 1.562 for hydrated crystals CuSO4.5H2O

Extra Aqueous Chemistry Index:

1. Water cycle, treatment, pollution

2. Colloids - sols, foam and emulsions

3. Hard and soft water - causes and treatment

4. Gas and salt solubility in water and solubility curves (this page)

5. Calculation of water of crystallisation

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