Substances in Water

Natural water is rarely pure. Even when it looks clear, it often contains dissolved and suspended substances. Some are helpful, and some are harmful to living things and to our health.

What might be in natural water?

• Dissolved oxygen (O₂) – tiny oxygen molecules mixed in water; fish and other aquatic life need this to survive.
• Metal compounds – minerals like calcium and magnesium can come from rocks; toxic metals like lead or mercury can come from pollution.
• Plastics and microplastics – small pieces from litter and worn synthetic materials.
• Sewage – waste water that may carry harmful microbes (bacteria and viruses) that cause disease.
• Nitrates (from fertilisers) and phosphates (from fertilisers and some detergents).

Which substances are helpful?

• Dissolved oxygen – supports aquatic life.
• Some metal compounds – minerals like calcium provide essential nutrients for life.

Which substances are harmful?

• Toxic metal compounds – can poison living things even in small amounts.
• Plastics – can be eaten by animals and may carry other pollutants.
• Sewage and harmful microbes – can cause water-borne diseases.
• Nitrates and phosphates – can lower oxygen levels in water, damaging aquatic life.

How do these substances get into water?

• Natural sources: rocks dissolve slowly, adding minerals.
• Human activities: farm fertiliser runoff (nitrates, phosphates), detergents, industry waste, sewage leaks, litter (plastics).

Making water safe to drink (domestic treatment)

• Sedimentation – water stands so larger solids settle at the bottom.
• Filtration – passes through filters (e.g., sand) to remove fine solids.
• Activated carbon – removes tastes and odours.
• Chlorination – chlorine is added to kill harmful microbes.

Distilled water vs tap water

Distilled water is very pure because it is boiled and the steam is condensed, leaving most impurities behind. It is used in practical chemistry because impurities in tap water (like dissolved salts) can affect results.

Common misconceptions

• “Clear” water may still be unsafe – microbes, nitrates, or toxic metals are invisible.
• Boiling kills microbes but does not remove nitrates, phosphates, metals, or plastics.
• Dissolved oxygen is oxygen gas mixed in water, not the oxygen inside the H₂O molecule.

Key points

• Natural water can contain helpful oxygen and minerals but also harmful plastics, microbes, and certain chemicals.
• Drinking water is cleaned by sedimentation, filtration, activated carbon, and chlorination.
• Distilled water is used in experiments because it has fewer impurities than tap water.