Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):
• Topic 12.5 — Identification of ions and gases (Part (a)(iii))
• Topic 3.2 — Relative masses of atoms and molecules (Part (a)(iv))
• Topic 10.1 — Water (Parts (a)(v), (b), (c)(i), (c)(ii), (d))
(a)(i)
Calcium
From Table 3.1, the mass of calcium ions (\( \mathrm{Ca}^{2+} \)) is 7.3 mg, which is the highest mass among the positive ions listed for the 600 cm³ sample.
(a)(ii)
Phosphate
The \( \mathrm{PO}_4^{3-} \) ion is called the phosphate ion.
(a)(iii)
Test: Add aqueous sodium hydroxide (NaOH) and aluminium (Al) foil, then warm carefully.
Test result: Ammonia gas is produced (detected by damp red litmus paper turning blue).
This specific reduction reaction is the standard test for nitrate ions in qualitative analysis.
(a)(iv)
mass = \( 0.9 \) mg
The concentration of \( \mathrm{Na}^+ \) is 2.7 mg in 600 cm³. To find the mass in 200 cm³, we use the ratio: \( \text{Mass} = 2.7 \times \frac{200}{600} = 2.7 \times \frac{1}{3} = 0.9 \text{ mg} \).
(a)(v)
Fertilisers
Nitrate ions are a major component of agricultural fertilisers. Runoff from farmland carries these ions into rivers, causing deoxygenation through eutrophication.
(b)
Sedimentation / filtration
Sedimentation allows solids to settle at the bottom, while filtration physically removes solid particles from the liquid.
(c)(i)
Sewage
Untreated or partially treated sewage entering a river is the main source of harmful microbes.
(c)(ii)
Chlorination
Adding chlorine (or chlorine compounds) to water kills harmful microbes (disinfection), making the water safe to drink.
(d)
Carbon / graphite
Activated carbon (charcoal) is used in water treatment to adsorb organic compounds that cause bad tastes and odours.
