Water

The One Thing

Water is the compound H₂O, containing hydrogen and oxygen in a fixed 2:1 ratio by volume. Its distinctive physical and chemical properties make it essential for life, but safe and sustainable use requires appropriate purification, pollution control, and conservation.

Reactions, Processes and Experiments

Key Terms

• Water: A chemical compound containing hydrogen and oxygen in a fixed ratio of two hydrogen atoms to one oxygen atom, represented by H₂O.
• Composition of water: By volume, pure water contains two parts hydrogen and one part oxygen; by mass, it contains approximately 11.1% hydrogen and 88.9% oxygen.
• Electrolysis: The decomposition of water into hydrogen and oxygen using electric current: 2H₂O → 2H₂ + O₂.
• Physical properties: Characteristics of pure water such as being colourless, odourless, and tasteless; freezing at 0°C, boiling at 100°C at normal atmospheric pressure, and having maximum density at 4°C.
• Chemical properties: The ways in which water reacts with some metals, metallic oxides, non-metallic oxides, and certain compounds, while also acting as a solvent and participant in chemical reactions.
• Universal solvent: Water, which dissolves many ionic and polar substances, although it does not dissolve every substance.
• Solution: A homogeneous mixture formed when a solute dissolves in a solvent; in salt solution, salt is the solute and water is the solvent.
• Solubility: The maximum amount of a substance that can dissolve in a given amount of solvent at a particular temperature.
• Hard water: Water containing dissolved calcium or magnesium salts that does not form lather easily with soap.
• Temporary hardness: Hardness caused mainly by dissolved calcium and magnesium hydrogencarbonates; it can be removed by boiling.
• Permanent hardness: Hardness caused mainly by dissolved calcium and magnesium sulphates or chlorides; boiling does not remove it.
• Soft water: Water that forms lather readily with soap because it contains very little dissolved calcium or magnesium salt.
• Suspended impurities: Visible, insoluble particles such as mud, sand, and clay that remain floating in water.
• Dissolved impurities: Soluble substances such as salts, gases, and some minerals that cannot be removed by simple filtration.
• Sedimentation: The process in which heavier insoluble particles settle at the bottom of water.
• Loading: Adding a substance such as alum to muddy water to help suspended particles gather and settle more quickly.
• Filtration: Separating insoluble solid particles from a liquid by passing the mixture through a porous filter.
• Distillation: Purifying water by converting it into vapour through boiling and then condensing the vapour back into liquid.
• Chlorination: Adding a controlled amount of chlorine or a chlorine compound to kill harmful microorganisms in water.
• Ozonation: Using ozone to destroy microorganisms and oxidise certain impurities in water.
• Potable water: Water that is sufficiently safe and suitable for drinking.
• Water pollution: The contamination of water by harmful physical, chemical, or biological substances.
• Biochemical oxygen demand: The amount of dissolved oxygen needed by microorganisms to decompose biodegradable organic matter in water; high demand usually indicates greater pollution.
• Eutrophication: Excessive growth of algae in a water body due to nutrient pollution, followed by oxygen depletion and harm to aquatic life.
• Water conservation: The careful and efficient use, protection, recycling, and storage of water resources.

Easily Confused

• Temporary hardness and permanent hardness: Temporary hardness is mainly caused by calcium and magnesium hydrogencarbonates and can be reduced by boiling; permanent hardness is mainly caused by calcium and magnesium sulphates or chlorides and is not removed by boiling.
• Suspended impurities and dissolved impurities: Suspended impurities are visible, insoluble particles removable by methods such as sedimentation and filtration; dissolved impurities are soluble substances that cannot be removed by simple filtration.
• Filtration and distillation: Filtration removes insoluble solids, whereas distillation separates water from dissolved substances by boiling and condensation.
• Boiling and distillation: Boiling kills many microorganisms but does not remove most dissolved salts; distillation leaves dissolved solids behind and can produce very pure water.
• Chlorination and ozonation: Chlorination uses chlorine or a chlorine compound, whereas ozonation uses ozone to destroy microorganisms and oxidise certain impurities.
• Hard water and soft water: Hard water forms little lather and produces soap scum because of calcium or magnesium salts; soft water forms lather readily.
• Water purification and water conservation: Purification treats water to remove impurities or microorganisms; conservation reduces waste and protects available water resources.
• Water pollution and eutrophication: Water pollution is contamination by harmful physical, chemical, or biological substances; eutrophication is a specific pollution-related process involving nutrient enrichment, algal growth, oxygen depletion, and harm to aquatic life.

What Gets Asked

• State the formula and composition of water: H₂O, a 2:1 hydrogen-to-oxygen ratio by volume, and approximately 11.1% hydrogen and 88.9% oxygen by mass. A common mark-losing error is confusing the volume ratio with the mass percentages.
• Write the electrolysis equation, 2H₂O(l) → 2H₂(g) + O₂(g), and identify the 2:1 gas-volume ratio. The equation must show hydrogen and oxygen in the correct proportions.
• Recall physical properties of pure water, including freezing at 0°C, boiling at 100°C at 1 atmosphere, maximum density of about 1 g/cm³ at 4°C, and a pH close to 7. The unusual expansion on freezing and its importance for aquatic organisms may also be required.
• Select a purification method for a stated impurity or water source. Sedimentation and filtration remove suspended particles, whereas distillation removes dissolved solids; boiling kills many microorganisms but does not remove most dissolved salts.
• Distinguish temporary from permanent hardness and identify suitable treatments. Boiling reduces temporary hardness, while permanent hardness may require washing soda treatment, ion exchange, or distillation.
• Explain pollution and conservation issues, including sewage, industrial waste, fertilisers, pesticides, oil, plastics, hot-water discharge, biochemical oxygen demand, eutrophication, and methods such as repairing leaks, rainwater harvesting, wastewater reuse, wetland protection, and sewage treatment. The key distinction is that excess nitrates and phosphates cause algal growth, oxygen depletion, and harm to aquatic life.