Revision:Aqueous Chemistry

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Water is essential for all life — most abundant substance on earth.

Water cycle

Water in rivers, lakes and oceans is evaporated by the heat of the sun. Water vapour rises into atmosphere to form clouds. The clouds cool as they rise and produce rain.

Testing for Water

1. Anhydrous copper sulphate — turns from white to blue if water is present.
2. Anhydrous cobalt chloride — turns from blue to pink if water is present.

Test for Purity

Pure water freezes at 0°C and boils at 100°C exactly — impurities will raise the boiling point and lower the freezing point.

Solubility

Water is a good solvent. Many substances dissolve well in water.

Solids

Most ionic solids are soluble in water, whereas most covalent compounds are insoluble in water.

A saturated solution is one in which no more solid will dissolve in the solvent. If a saturated solution is cooled some solid will separate from the solution.

Solubility of an ionic compound increases as the temperature of the solvent increases. Solubility is usually given in grams of solute in 100g of water.

Gases

Many gases are soluble in water. Solubility increases as pressure increases and decreases as temperature increases.

Carbonated water (in fizzy drinks) is produced by dissolving CO2 in water under high pressure. When the pressure is released the gas bubbles out of solution.

Dissolved oxygen is essential for aquatic life.

Chlorine gas is dissolved in water to produce chlorine water. This is used as a bleach and to kill bacteria.

Drinking Water

Water is passed through filter beds to remove any insoluble matter and then chlorine is added to kill bacteria. Sometimes fluoride ions are added to prevent tooth decay.

Water Pollution

Water pollution is caused by fertilisers, chemical waste from factories, animal waste. Dissolved nitrate ions from fertilisers can have harmful effects on babies. Pollution results in excessive water plant growth followed by decay. This results in reduction of oxygen levels, resulting in death of aquatic organisms (Eutrophication).

Hard and Soft Water

Hard water does not easily form a lather with soap, and contains calcium and magnesium ions.

Soft water easily forms a lather with soap.

Water flows over rocks containing soluble calcium or magnesium compounds in order to become hard.

Limestone reacts with slightly acidic rain water to form calcium hydrogencarbonate solution. This causes problems on heating when calcium carbonate is formed as fur on kettles etc.

Advantages and Disadvantages of Hard Water

Water softening

• Distillation — water is distilled to remove soluble impurities
• Excess Soap — eventually all the Ca²⁺ and Mg²⁺ ions are removed by soap as scum and a lather is formed.
• Washing Soda (Na₂CO₃) — The calcium and magnesium ions react to form calcium and magnesium carbonates which are insoluble and are precipitated out of solution.
• Ion Exchange Resins — These resins exchange Na⁺ ions for the Ca²⁺ and Mg²⁺ ions. Water passing out of the resins contains sodium ions which do not cause hardness.

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