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Water

Water

Properties

  • Colourless, odourless liquid
  • 0°C Melting Point
  • 100°C Boiling Point
  • V shaped or bent; 105° bond angle
  • Molecules join via hydrogen bonding
  • Turns anhydrous copper sulphate from white to blue CuSO4 (s) + 5 H2O (l) -> CuSO4.5H2O (s) + heat
  • Turns anhydrous cobalt chloride from blue to pink CoCl2 (s) + 6 H2O (l) -> CoCl2.6H2O (s) + heat
  • Water is used in generating electricity, cleaning, cooling, dissolving, manufacturing products, making concrete, and consumption in the house (g. toilets and showers)

Desiccator – sealed glass basins used to keep substances and papers (e.g. cobalt chloride paper) dry

The Hoffman’s Voltamater

Since water is not ionic, we cannot electrolyse pure water as there are no charged particles to carry the current. If we add some dilute sulphuric acid this dissociates in the water and allows electrolysis to occur. The apparatus is known as Hoffmann’s Voltameter.

Cathode: 4 H2O (l) + 4 e- -> 2 H2 (g) + 4 OH- (aq)

Anode: 2 H2O (l) -> O2 (g) + 4 H+ (aq) + 4 OH- (aq)

Full: 2 H2O (l) -> 2 H2 (g) + O2 (g)

 

Water of Crystallisation

Ionic solids often have molecules of water bonded into their ionic crystal lattice. This water is called water of crystallisation and often has a consistent, simple ratio in the formula. Formulae are often quoted with the water of crystallisation included. E.g.Copper sulphate pentahydrate, Sodium carbonate decahydrate.

 

Agents

Drying Agents – various drying agents are used to absorb water out of air or gas mixtures. Common drying agents are:

  • Concentrated sulphuric acid is used as a drying agent in the preparation of some neutral or acidic gases
  • Calcium oxide which is used to dry alkaline ammonia gas
  • Anhydrous calcium chloride is used to prepare dry hydrogen
  • Silica gel is used as a drying agent in equipment which is sensitive g. cameras

Dehydrating Agents – drying agents that are so powerful that they will remove all atoms required to make up water from certain substances in the solution of solid form. The most common example of a dehydrating agent is concentrated sulphuric acid which will dehydrate sucrose and hydrated copper sulphate. It will also extract the elements of water from cloth or skin.

 

Purification of Water

  • Water is stored in dams and reservoirs. It is never completely pure and may contain bacteria, dissolved substances and solid material which need to be
  • Concerns over levels of pesticides in river water have led to improvements in water purification.
  • Water treatment essentially involves the stages, filtration, sedimentation, (ozone treatment) and chlorination.