Question 

(a) Water must be treated so that it is safe to drink.

Draw lines to link each stage in the water treatment process to the reason why it is used.

▶️Answer/Explanation

• Filtration – Traps finer particles using sand
• Chlorination – Kills microbes

(b) Water can be tested to identify some of the chemicals in it. 

A scientist tests a sample of water from a river with acidified aqueous silver nitrate and also with acidified aqueous barium chloride. Table 8.1 shows the results.

Test	With acidified aqueous silver nitrate	With acidified aqueous barium chloride
Result	Cream precipitate	White precipitate

The scientist thinks that the water contains both chloride and sulfate ions. State whether the scientist is correct. Explain your answer.

▶️Answer/Explanation

The scientist is not entirely correct. The cream precipitate with silver nitrate indicates the presence of bromide ions (Br⁻) or chloride ions (Cl⁻), but since bromide ions give a cream precipitate and chloride ions give a white precipitate, it is likely that bromide ions are present. The white precipitate with barium chloride confirms the presence of sulfate ions (SO₄²⁻). Therefore, the water contains sulfate ions, but the presence of chloride ions is not confirmed.

(c) Barium chloride, BaCl₂, reacts with sodium sulfate, Na₂SO₄. 

Barium sulfate, BaSO₄, and sodium chloride, NaCl, are made. Construct the balanced symbol equation for this reaction.

▶️Answer/Explanation

The balanced symbol equation for the reaction is:
BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl

(d) Carbon dioxide dissolves in rainwater to make the water weakly acidic. 

Suggest the pH of the rainwater produced.

▶️Answer/Explanation

The pH of rainwater containing dissolved carbon dioxide is typically around 5.6, making it weakly acidic.

(e) The atoms in carbon dioxide, CO₂, are bonded by sharing electrons.

Fig. 8.1 shows some dot-and-cross diagrams.

(i) State which diagram A, B, C or D, shows the arrangement of the outer shell electrons in carbon dioxide.

(ii) State the name of this type of bonding that holds the atoms together in carbon dioxide.

▶️Answer/Explanation

Diagram C shows the correct arrangement of the outer shell electrons in carbon dioxide, where carbon shares electrons with two oxygen atoms.

(f) Complete the following sentences about some of the problems caused by carbon dioxide. 

Choose words from the list. Each word or phrase may be used once, more than once or not at all.

Word List
climate change
greenhouse
oxidation
noble
rusting

Carbon dioxide is a …… gas. Increased concentrations of carbon dioxide in the atmosphere contribute to …… .

▶️Answer/Explanation

Carbon dioxide is a greenhouse gas. Increased concentrations of carbon dioxide in the atmosphere contribute to climate change.