Name two substances naturally found in river water that are beneficial to living organisms.

A student collects water from a clean river. State two differences between this river water and pure water.

Classify the following as either a beneficial or harmful substance when dissolved in river water: dissolved oxygen.

Explain why dissolved substances are present in natural river water even though it has rained recently.

The table below lists three sources of water. For each source, suggest whether dissolved oxygen levels would be HIGH or LOW and give one reason. (i) Fast-flowing mountain stream (ii) Stagnant pond covered in algae (iii) Deep underground reservoir

State the role of dissolved oxygen in river water for fish and other aquatic animals.

A river becomes polluted with sewage. Explain why the fish population in the river decreases.

Aquatic plants in a river produce dissolved oxygen during the day. State the biological process responsible and write a word equation for it.

Dissolved oxygen levels in a river are measured at 5 mg/dm³. Suggest whether this value is adequate for most fish species and explain your reasoning.

Explain why dissolved oxygen levels in a river are generally higher during the day than at night.

State the beneficial purpose of dissolved metal compounds (mineral salts) found naturally in water.

Name one metal compound found in natural water and state its specific benefit to the human body.

Hard water contains dissolved calcium and magnesium compounds. Suggest one benefit and one disadvantage of hard water for human health and infrastructure.

A scientist tests two water samples: • Sample A: pure distilled water • Sample B: natural river water Describe two chemical tests that would distinguish Sample B from Sample A, including expected results for each.

Describe how water is treated to make it safe for drinking. Include reference to at least TWO stages of treatment and explain the purpose of each stage. [Extended response]