Which one of the following pairs of elements is liquid at room temperature and at normal pressure?
- (a)Gallium and Bromine
- (b)Mercury and Bromine
- (c)Gallium and Mercury
- (d)Gallium and Caesium
Correct — B, Mercury and Bromine. These are the only two elements that are liquid at ordinary room temperature (about 25 degrees Celsius) and normal pressure. Mercury, a metal, melts at about minus 39 degrees Celsius, and bromine, a non-metal, melts at about minus 7 degrees Celsius, so both are already liquid at room temperature. Gallium (melts near 30 degrees Celsius) and caesium (melts near 28 degrees Celsius) are solids at 25 degrees Celsius, needing a little warming, so any pair containing them is wrong.
- (a)Gallium and Bromine — Bromine is liquid at room temperature, but gallium melts only near 30 degrees Celsius, so it is a solid at ordinary room temperature; the pair fails.
- (c)Gallium and Mercury — Mercury is liquid, but gallium is a solid just above room temperature, melting near 30 degrees Celsius, so this pair is not both liquid.
- (d)Gallium and Caesium — Both gallium (about 30 degrees Celsius) and caesium (about 28 degrees Celsius) melt slightly above room temperature, so both are solids at 25 degrees Celsius.
At about 25 degrees Celsius and normal pressure, only two elements exist as liquids: the metal mercury and the non-metal bromine. A substance is liquid at a given temperature if that temperature lies above its melting point but below its boiling point. Both mercury and bromine have melting points well below room temperature.
The trap is gallium and caesium, which melt just above room temperature and so are famous for melting in the warmth of a hand. Because they are still solid at 25 degrees Celsius, they do not count as room-temperature liquids, so the correct pair excludes them.
- Mercury and bromine are the only two elements liquid at room temperature and normal pressure.
- Mercury (a metal) melts at about minus 39 degrees Celsius.
- Bromine (a non-metal) melts at about minus 7 degrees Celsius.
- Gallium (about 30 degrees Celsius) and caesium (about 28 degrees Celsius) melt just above room temperature, so they are solids at 25 degrees Celsius.
- Counting gallium or caesium as liquids because they melt easily; they are still solid at 25 degrees Celsius.
- Forgetting that bromine, a non-metal, is one of the two room-temperature liquid elements.
Asked as which element or pair is liquid at room temperature; the two answers are mercury and bromine, with gallium and caesium as the near-miss traps.
No directly related past PYQ was found.
- practice — not a real PYQ
Which one of the following elements is the only metal that is liquid at room temperature?
- (a)Gallium
- (b)Mercury
- (c)Sodium
- (d)Caesium
Answer(b) Mercury — it melts at about minus 39 degrees Celsius, so it is liquid at room temperature.
- practice — not a real PYQ
Which one of the following metals melts in the warmth of the human hand?
- (a)Iron
- (b)Gallium
- (c)Aluminium
- (d)Copper
Answer(b) Gallium — its melting point of about 30 degrees Celsius lies near body temperature.