10 Chemistry of the environment Pretest

Which colour change is seen when water is added to anhydrous cobalt(II) chloride?

• White → blue

• Blue → pink

• Pink → blue

• White → pink

Answer explanation

When water is added to anhydrous cobalt(II) chloride, it changes from blue (the anhydrous form) to pink (the hydrated form), indicating the presence of water. Thus, the correct answer is Blue → pink.

What is the colour change when water is added to anhydrous copper(II) sulfate?

• Blue → white

• White → blue

• Pink → blue

• Blue → pink

Answer explanation

Anhydrous copper(II) sulfate is white. When water is added, it hydrates to form copper(II) sulfate pentahydrate, which is blue. Therefore, the correct color change is white → blue.

Which property is used to test the purity of water?

• Colour

• Melting and boiling points

• Density

• Taste

Answer explanation

The melting and boiling points of water are specific and well-defined. Pure water has a melting point of 0°C and a boiling point of 100°C at standard pressure, making these properties reliable indicators of its purity.

Pure water has a boiling point of:

• 0 °C

• 50 °C

• 100 °C

• 120 °C

Answer explanation

Pure water boils at 100 °C under standard atmospheric pressure. This is a fundamental property of water, making 100 °C the correct answer.

Why is distilled water used in chemistry experiments instead of tap water?

• It is cheaper

• It contains fewer impurities that could affect results

• It contains no chemicals at all

• It is easier to make

Answer explanation

Distilled water is preferred in chemistry experiments because it contains fewer impurities than tap water, which can interfere with experimental results and lead to inaccurate conclusions.

Which two substances in natural water are beneficial?

• Dissolved oxygen and some metal compounds

• Toxic metals and plastics

• Sewage and nitrates

• Phosphates and plastics

Answer explanation

Dissolved oxygen is essential for aquatic life, while some metal compounds can be beneficial in trace amounts, aiding in biological processes. The other options contain harmful substances that negatively impact water quality.

Which harmful substance in water can cause disease?

• Dissolved oxygen

• Toxic metals

• Sewage

• Phosphates

Answer explanation

Sewage contains harmful pathogens and bacteria that can lead to diseases when present in water. Unlike dissolved oxygen, toxic metals, or phosphates, sewage directly poses a health risk.

Which process removes tastes and odours in water treatment?

• Sedimentation

• Filtration

• Carbon treatment

• Chlorination

Answer explanation

Carbon treatment effectively removes tastes and odours from water by adsorbing impurities onto activated carbon, making it the best choice for this process compared to sedimentation, filtration, or chlorination.

Which sequence shows the correct order of main steps in domestic water treatment?

• Sedimentation → chlorination → carbon treatment

• Sedimentation and filtration → carbon → chlorination

• Carbon treatment → filtration → chlorination

• Filtration → sedimentation → chlorination

Answer explanation

The correct order for domestic water treatment is sedimentation and filtration to remove larger particles, followed by carbon treatment to eliminate contaminants, and finally chlorination to disinfect the water.

Which of the following are used as fertilisers?

• Ammonium salts and nitrates

• Sulfates and chlorides

• Oxides and hydroxides

• All of the above

Answer explanation

Ammonium salts and nitrates are commonly used as fertilizers due to their high nitrogen content, which is essential for plant growth. The other options, sulfates, chlorides, oxides, and hydroxides, are not primarily used as fertilizers.