4. Gas and salt solubility in water

Gas and salt solubility and solubility curves are considered - variation of solubility with temperature

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 CO_2(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.

      • CO_2(aq) + H₂O_(l) 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 NH_3(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.

TOP OF PAGE and sub-index

4b. Solubility curves for selected salts

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