Water Hardness LO3

Pure Water

Water that is free from contaminants and other substances, possessing physical properties such as being odourless, tasteless, and colourless, boiling at 100°C and freezing at 0°C, and having a maximum density of 1 g/cm³ at 4°C.

Hydrated Salts

Salts that contain water molecules in their structure, known as water of crystallisation, which can be removed by heating to form an anhydrous salt.

Water of Crystallisation

Water molecules present in the structure of hydrated salts, influencing the shape of crystals and can be removed by heating the hydrated salt.

Anhydrous Salt

A salt that has had the water of crystallisation removed, which can regain water molecules to become hydrated.

Homogenous Mixture

A mixture where all parts are in the same state of matter, such as rock salt or air.

Heterogeneous Mixture

A mixture where the parts are in different states of matter, like sand and water.

Soluble Substances

Substances that can dissolve in a liquid to form a solution, with the dissolved solid called the solute and the liquid doing the dissolving called the solvent.

Insoluble Substances

Substances that do not dissolve in a liquid, forming a suspension if present as a fine powder in the liquid.

Factors that affect solubility are:

1. Size of particles

2. Stirring

3. Amount of solute already dissolved

4. Temperature

Substances which are soluble:

• All nitrates

• All hydrogen carbonates

• All group 1 metal salts

• All ammonium salts

• Halides except silver and lead halides

• Sulfates except barium, calcium and lead sulfates

Substances which are insoluble:

• Carbonates except group 1 metal and ammonium carbonates

• Metal oxides except group 1 and 2 metal oxides that react with water

• Hydroxides except group 1 metal and ammonium hydroxides

Filtration

A separation technique used to separate an insoluble solid from a liquid or aqueous solution based on particle size and density.

Evaporation to dryness

A method used to obtain a soluble solid from a liquid solvent by evaporating the solvent until dry, affecting crystal size.

Crystallisation

A process to obtain a soluble solid from a liquid solvent by cooling a saturated solution to form crystals, often used for obtaining hydrated salts.

Ionic bond

A strong bond between metal and non-metal ions in salts, characterized by electrostatic forces of attraction in a crystal lattice structure.

Solubility

The maximum quantity of solute that can dissolve in a certain quantity of solvent at a specified temperature.

Solvent

The substance in which the solute dissolves to form a solution.

Saturated Solution

A solution that cannot dissolve any more solute at a given temperature.

Dilute Solution

A solution with a relatively small amount of solute dissolved in the solvent.

Concentrated Solution

A solution with a relatively large amount of solute dissolved in the solvent.

Solubility Curve

A graph showing how the solubility of a substance varies with temperature in a specific solvent.

Solubility of a Substance

The ability of a substance to dissolve in a solvent, often influenced by factors like temperature and pressure.

Hard water

Hard water is water that will not readily form a lather with soap, instead it forms a scum.

Temporary hardness

Hardness in water caused by the presence of dissolved calcium hydrogen carbonate or magnesium hydrogen carbonate, which can be removed by boiling.

Word equation for the reaction when water passes over hydrogen carbonate

calcium carbonate + hydrogen carbonate → calcium hydrogen carbonate

magnesium carbonate + hydrogen carbonate → magnesium hydrogen carbonate

Word equation for when calcium hydrogen carbonate undergoes thermal decomposition

Calcium hydrogen carbonate → calcium carbonate + water + carbon dioxide

Permanent hardness

Hardness in water caused by the presence of dissolved calcium or magnesium ions like calcium sulfate or magnesium sulfate, which cannot be removed by boiling.

Boiling

A method of softening water by removing temporary hardness through the thermal decomposition of calcium and magnesium hydrogen carbonates.

Distillation

A technique used to separate the solvent (water) from dissolved solutes, capable of removing both temporary and permanent hardness from water.

Washing soda

Sodium carbonate decahydrate (Na₂CO₃.10H₂O) used to remove both temporary and permanent hardness from water by forming precipitates with calcium or magnesium ions.

Reaction that occurs with washing soda and temporary hard water

sodium carbonate + calcium hydrogen carbonate → sodium hydrogen carbonate + calcium carbonate

Na₂CO₃(aq) + Ca(HCO₃)₂(aq) → 2NaHCO₃(aq) + CaCO₃(s)

Reaction that occurs with washing soda and permanent hard water

sodium carbonate + calcium sulfate → sodium sulfate + calcium carbonate

Na₂CO₃(aq) + CaSO₄(aq) → 2NaSO₄(aq) + CaCO₃(s)

Ion-exchange

Method of removing both types of water hardness by passing water through a resin containing sodium ions, which exchange with calcium or magnesium ions present in hard water.

Reaction that happens in ion exchange

calcium ions + sodium resin → sodium ions + calcium resin

Precipitation reaction

Chemical reaction where two soluble solutions react to form an insoluble precipitate, as seen in the removal of hardness from water using washing soda or ion-exchange resin.

Scum

Solid material formed on the surface of water when soap reacts with dissolved calcium or magnesium ions in hard water, leading to the formation of insoluble calcium salts.

Lather

Foam formed when soap dissolves in water, indicating the ability of water to form a lather with soap and distinguishing between hard and soft water.

Chemical tests for water (Word equations)

anhydrous copper (II) sulfate + water → hydrated copper (II) sulfate

anhydrous cobalt (II) chloride + water → hydrated cobalt (II) chloride

Chemical test for water (Balanced equations)

CuSO₄(s) + 5H₂O(l) → CuSO₄.5H₂O(s)

CoCl₂(s) + 6H₂O(l) → CoCl₂.6H₂O(l)

Osmosis

Osmosis is the movement of water molecules across a selectively permeable membrane from an area of higher water concentration to an area of lower water concentration. It is used to purify water (remove the salts present in it)