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Two or Three Marks Hidden in One Line
Open any General Science paper and you will find a question that reads like a laboratory instruction: *"Which gas turns lime water milky?"* or *"A gas blackens lead acetate paper. Identify it."* These carry full marks and take four seconds — if you have stored the gas, its formula and its identifying test as one unit rather than three.
That is the whole method of this topic. For every gas that appears in papers we fix three things together:
> name → formula → the one test that proves it
The same discipline applies to water. Water is not one fact but a small cluster: its anomalous expansion, its hardness and the two chemicals that remove each type, and the two look-alike compounds — heavy water (D₂O) and hydrogen peroxide (H₂O₂) — that examiners love to swap for plain H₂O.
Five cards, roughly twenty formulas. Learn them as pairs, never as a list.
1. A colourless gas is passed through lime water and the lime water turns milky. On passing an excess of the same gas, the milkiness disappears. The gas is:
2. Heavy water, used as a moderator in nuclear reactors, has the chemical formula:
3. Which single chemical can remove BOTH temporary and permanent hardness of water?
4. Water has its maximum density at which temperature?
5. A gas with the smell of rotten eggs turns moist lead acetate paper black. The gas is:
6. Carbon monoxide is far more dangerous than carbon dioxide in a closed room mainly because:
7. Which isotope of hydrogen is radioactive?
8. Hydrogen peroxide is stored in dark or opaque bottles because: