Environmental Chemistry Questions and Answers

Answer: C

It is Henry's law.

Henry's law states that at constant temperature the amount of a gas dissolved in a liquid is proportional to the partial pressure of that gas above the liquid:

\[ C = k_{H}\, p \]

It explains why opening a soda releases dissolved CO₂ and governs oxygen transfer in aeration tanks. Lambert's law and Beer's law relate the absorption of light to path length and concentration, the basis of spectrophotometry. Rutherford's name belongs to atomic structure and scattering, not to gas solubility.

Answer: D

The correct order is NaCl \(\gt\) LiF \(\gt\) Ca(OH)\(_{2}\) \(\gt\) CaF\(_{2}\), comparing solubilities in moles per litre.

Approximate solubilities at room temperature:

• NaCl: about 360 g/l, roughly 6 mol/l.
• LiF: about 1.3 g/l, roughly 0.05 mol/l.
• Ca(OH)₂: about 1.7 g/l, roughly 0.023 mol/l.
• CaF₂: about 0.016 g/l, roughly 0.0002 mol/l.

Common salt is very soluble and calcium fluoride (fluorite) almost insoluble, which fixes the two ends and rules out options A and C. Between the middle pair, LiF dissolves to about twice the molar concentration of Ca(OH)₂, because its formula mass is far smaller, so option B has those two the wrong way round.

Answer: C

Temporary hardness is caused by magnesium carbonate among the options.

Temporary hardness comes from the carbonates and bicarbonates of calcium and magnesium. It is called temporary because boiling removes it: the bicarbonates decompose and the carbonate precipitates, as in

\[ \begin{aligned} &\mathrm{Mg(HCO_{3})_{2}} \\ &\rightarrow \mathrm{MgCO_{3}} + \mathrm{H_{2}O} + \mathrm{CO_{2}} \end{aligned} \]

Strictly the dissolved species is the bicarbonate, but textbooks list the carbonate under temporary hardness, which is the reading the key follows. Chlorides and sulphates of calcium and magnesium, such as magnesium chloride, magnesium sulphate and calcium sulphate, cause permanent hardness, which boiling does not remove.

Answer: B

The causes are (II) and (III).

The event was the Lake Nyos disaster of 21 August 1986, which killed about 1,700 people. Volcanic CO₂ seeping in from below had built up over years in the cold, dense bottom layers of the stratified lake, which held it dissolved under the pressure of the water above (III). A disturbance, possibly a landslide or wind-driven overturn (II), brought deep water up, the pressure fell, and the gas came out of solution in a sudden cloud.

The upper layers were not the saturated ones (I), and gas solubility under pressure is described by Henry's law, not Raoult's law (IV).

Answer: A

Permanent hardness is caused by magnesium sulphate.

Hardness comes from dissolved calcium and magnesium salts. Permanent (non-carbonate) hardness is due to their sulphates and chlorides, such as magnesium sulphate, which do not precipitate on boiling and must be removed by ion exchange or lime-soda treatment.

Bicarbonates of calcium and magnesium cause temporary hardness, which boiling removes by precipitating the carbonates. Magnesium carbonate itself is barely soluble, and sodium chloride does not cause hardness at all, since sodium ions do not form scum with soap.

Answer: B

The oxidation is carried out in the presence of \(\mathrm{H_{2}SO_{4}}\), sulphuric acid.

In the chemical oxygen demand test the sample is refluxed with potassium dichromate in strongly acidic conditions provided by concentrated sulphuric acid, with silver sulphate as catalyst and mercuric sulphate to bind chloride. The dichromate left over is titrated with ferrous ammonium sulphate to find how much was used.

Hydrochloric acid would itself be oxidised by dichromate, nitric acid is an oxidant that would interfere, and hydrofluoric acid is not used in the test.

Answer: D

The correct relationship is low D.O. and high free \(\mathrm{CO_{2}}\).

Dissolved oxygen and free carbon dioxide usually vary inversely. Where organic pollution is decomposed by bacteria, respiration uses up oxygen and releases carbon dioxide, so polluted water has low D.O. and high free \(\mathrm{CO_{2}}\). Where algae photosynthesise in daylight, they use carbon dioxide and release oxygen, giving the reverse.

Both high, both low, or both equal would ignore the fact that the same processes produce one gas while consuming the other.

Answer: C

Water is densest at 4°C.

More exactly, pure water reaches its maximum density of about 1000 kg per cubic metre at 3.98°C. Above that, it expands on warming as any liquid does. Below it, hydrogen bonding starts to build an open, ice-like structure, so the water expands as it cools to 0°C, and ice is less dense still.

This anomaly matters ecologically: in winter, water at 4°C sinks to the bottom of a lake while colder water and ice stay on top, so lakes freeze from the surface down and aquatic life survives below. 0°C, 5°C and 10°C are all less dense than water at 4°C.