JEE Main 2021 · 22 July · evening · Single correct · +4 / −1
The water having more dissolved O₂ is :
- (a) boiling water
- (b) water at 80°C
- (c) polluted water
- (d) water at 4°C
Answer and solution
Answer (d) water at 4°C
Solution
Chemistry ma’am · 3 steps
A gas dissolves best in cold, clean water, so pick the coldest and cleanest sample.
- Temperature effect. Dissolving O₂ in water releases heat, so heating drives the gas back out. The colder the water, the more oxygen it holds.
- Pollution effect. Polluted water carries organic waste. Microbes use up dissolved oxygen as they break that waste down, so the O₂ level falls and the BOD goes up.
Sample What happens to dissolved O₂ Boiling water Almost all of it is driven out Water at 80°C Very little left, since solubility is low when hot Polluted water Used up by microbes breaking down waste Water at 4°C Highest, because cold water holds the most gas - So. Water at 4°C is the coldest, unpolluted sample, so it has the most dissolved oxygen. The answer is (d).
The idea to keep. The solubility of a gas in a liquid falls as temperature rises. Dissolved oxygen is also lowered by organic pollution, because microbes consume it.
Answer from the NTA final answer key. Solution by PrepWiser.