To avoid the condition of bends, as a precautionary step, most professional divers today use a helium-oxygen mixture instead of a nitrogen-oxygen mixture in the compressed gas tanks. This is mainly because of lower solubility of helium in the blood than nitrogen. Moreover, because of small size of He atoms as compared to nitrogen molecules these pass through cell walls without damaging them. The excess 02 dissolved in the blood is used in metabolism and does not cause the condition of bends.

(i) Manoj has studied that certain metals like iron react with air or water in the environment to form undesirable compounds (usually oxides) and the process is called corrosion. In case of iron, corrosion is called rusting. Therefore, the normal iron hinges will rust after certain time. Manoj has also learnt that iron is protected from rusting by galvanisation. The galvanised iron articles have coating of zinc on their surfaces, which protects them from rusting. If some scratches occur on the protective film on coated iron, even then iron will not be rusted. This is due to the fact that because of scratches, both zinc and iron get exposed to oxidation but zinc undergoes oxidation in preference to iron. Thus, galvanised iron hinges will have longer life and lustre.
(ii) Rusting is a phenomenon which damages the iron articles. It destroys iron bridges, buildings, automobiles etc. and we must prevent rusting of iron at any cost. So, Manoj gave a good suggestion to his father to buy galvanised iron hinges though these are costlier. In longer term, these will prove to be cheaper.

The reaction is exothermic and therefore, according to Le Chatelier's principle, it should be favoured by low temperature. However, the reaction should be carried out at an optimum temperature of about 750 K and not at 350 K. This is because the collision frequency of the reactant molecules decreases with decrease in temperature. As a result, the rate of reaction would decrease at a lower temperature. Consequently, at low temperature, the yield of the product will be less. Hence, the reaction should be carried out at about 750 K.

Nitrogen fertilisers are very useful for the growth of plants because these provide essential plant nutrients to the soil. However, these should not be used in excessive quantities. The excessive use of fertilisers has many disadvantages also. Some of these are: 
(i) The excessive use of nitrogenous fertilisers concentrates nitrates in the soil and water. Nitrate rich water is unfit for drinking. When this water is carried off into surface water bodies such as rivers, lakes and ponds, it proliferates the growth of algae. These algae consume dissolved oxygen from water and therefore, decrease the oxygen content in water leading to the death of useful aquatic life such as fish. Such an increase in growth of algae in rivers, lakes, ponds etc. resulting in the reduction of oxygen content in water is called eutrophication. Eutrophication destroys the life-supporting environment in rivers, lakes and ponds.
(ii) Excessive use of fertilisers over a long period may affect the alkalinity or acidity of the soil and may adversely affect the crop production. Thus, as a student of chemistry, we should suggest the farmer use only required quantities of nitrogen fertilisers.

(i) In the presence of air and light, chloroform is oxidized to poisonous gas phosgene.
\(CHC{ l }_{ 3 }+\frac { 1 }{ 2 } { O }_{ 2 }\overset { Air\quad or\quad light }{ \longrightarrow } \quad COC{ l }_{ 2 }+HCl\\ Phosgene\)
(ii) Continuous chloroform exposure may damage liver and kidneys because chloroform is metabolised to poisonous phosgene. Some people develop sores when the skin comes in contact with chloroform for a long time. Therefore, use of chloroform as an anesthetic has been replaced by less toxic and safer anesthetic such as ether.