Which one of the following metals is kept immersed in Kerosene oil to protect it and to prevent accidental fire?
- (a)Calcium
- (b)Sodium
- (c)Vanadium
- (d)Magnesium
Correct — B, Sodium. Sodium is an alkali metal and among the most reactive of all metals. Left in the open it reacts with the oxygen and moisture of the air, and its reaction with water is violent enough to be dangerous: it produces sodium hydroxide and hydrogen, and the heat released is enough to set the escaping hydrogen alight. That is the accidental fire the question refers to. Kerosene is the standard storage medium because it is a liquid hydrocarbon that does not react with sodium and keeps both air and water vapour off the metal surface, and because sodium is denser than kerosene it stays submerged. Potassium is kept the same way. The paper's own wording, 'to protect it and to prevent accidental fire', names both jobs the kerosene is doing.
- (a)Calcium — Calcium does react with cold water, but slowly and without catching fire — the hydrogen bubbles stick to the metal and float it to the surface. It is stored in an airtight container, not under kerosene.
- (c)Vanadium — A hard transition metal used in alloy steels. It carries a protective oxide film and is stable in air at room temperature, so it needs no liquid cover.
- (d)Magnesium — Magnesium is reactive enough to burn brilliantly once lit, but it does not react with cold water to any useful extent and sits safely in air behind a dull oxide film. Ribbon is simply cleaned with sandpaper before use.
How a metal must be stored follows directly from its place in the reactivity series. The alkali metals at the top react with air and with cold water at once and are kept under an unreactive liquid; metals in the middle react only slowly or only with steam and need no such protection; the noble metals at the bottom sit in the open indefinitely. Kerosene works because it is chemically inert towards sodium, contains no dissolved water and is less dense than the metal.
The list is arranged so that only one option belongs to the top of the reactivity series. A candidate who remembers the order of the series can answer without recalling anything about kerosene at all — the question is only asking which of the four is dangerous in air. The near miss is calcium, which does react with cold water and is often taught in the same breath as sodium; the difference is that its reaction is gentle enough not to ignite. It is also worth being exact about what kerosene prevents. It is not a fire-retardant coating in any general sense — it works by cutting the metal off from air and moisture, which are the reactants.
- Sodium reacts with cold water to give sodium hydroxide and hydrogen, releasing enough heat to ignite the hydrogen.
- Sodium and potassium are both stored under kerosene, which excludes air and moisture.
- Sodium is denser than kerosene, so it stays submerged rather than floating clear.
- Calcium reacts with cold water slowly and floats on the hydrogen bubbles that collect on it; it does not catch fire.
- Magnesium reacts with hot water or steam rather than cold water, and is safe to keep in air.
- Picking calcium because it too reacts with cold water — the reaction is slow and does not ignite.
- Assuming magnesium is stored under liquid because it burns so brightly; it is stable in air.
- Believing the kerosene puts out fire rather than preventing the reaction that starts it.
As a storage item like this one, or turned round into an assertion-and-reason pair about why sodium is kept under kerosene.
Assertion (A): Sodium metal is stored under kerosene. Reason (R): Metallic sodium melts when exposed to air.
- (a) Both A and R are true, and R is the correct explanation of A
- (b) Both A and R are true, but R is NOT a correct explanation of A
- (c) A is true, but R is false
- (d) A is false, but R is true
Answer(c) A is true, but R is false
The same fact set as an assertion-and-reason pair, and a good test of whether the reason has been learnt or only the rule. Storing sodium under kerosene is correct; the stated reason, that sodium melts on exposure to air, is not — its melting point is close to 98 °C.
- practice — not a real PYQ
Which one of the following metals reacts with cold water so vigorously that the hydrogen released catches fire?
- (a)Magnesium
- (b)Zinc
- (c)Sodium
- (d)Iron
Answer(c) Sodium — the reaction with cold water is violently exothermic and the heat ignites the hydrogen being produced.
- practice — not a real PYQ
White phosphorus is stored under water because it
- (a)dissolves in air
- (b)catches fire when exposed to air
- (c)reacts violently with water
- (d)evaporates quickly
Answer(b) catches fire when exposed to air — its ignition temperature is so low that contact with air alone can set it alight, and water keeps the air off.