p-Block Elements: JEE Main Chemistry Question with Solution
Which one of the following elements will remain as liquid inside pure boiling water ?
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Hint 1 of 3
What two physical and chemical conditions must an element satisfy to remain as a liquid inside pure boiling water (at 1 atm)?
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Step-by-step solutionView
Correct answer
Gallium melts at 29.76 °C, boils at over 2200 °C, and does not react vigorously with neutral boiling water, allowing it to remain as a liquid at 100 °C.
Option analysis
Why each option works or fails
A · Cs
Believing that because cesium has a low melting point (28.5 °C), it will simply exist as a liquid metal under boiling water. Consider chemical reactivity with water; cesium reacts explosively with water to form cesium hydroxide and hydrogen gas, so it does not persist as elemental liquid.
B · Ga
None. Gallium melts at ~29.8 °C and boils at ~2229 °C, staying unreactive enough with pure water to remain a stable liquid at 100 °C. Gallium's melting point is well below 100 °C, its boiling point is far above 100 °C, and it forms a protective passivating oxide film that prevents rapid reaction with boiling water.
C · Li
Confusing the low density and low melting point of alkali metals with thermal stability in water. Check both melting point and reactivity: lithium melts at 180.5 °C (so it would remain solid even if unreactive) and reacts vigorously with boiling water.
D · Br
Remembering that bromine is a liquid at room temperature and assuming it remains a liquid at 100 °C. Check the boiling point of the element; bromine boils at 58.8 °C and therefore vaporizes completely into a gas inside boiling water (100 °C).
Reviewed route
Solution
StepWorking
01concept
To remain as a liquid inside pure boiling water (at 100∘C), the element must not chemically react vigorously with water, its melting point must be below 100∘C (so it is molten), and its boiling point must be above 100∘C (so it does not vaporise).
02option_verdict
Caesium (Cs) is an alkali metal that reacts violently and explosively with water to form CsOH and H2 gas, so it cannot remain as elemental liquid.
03option_verdict
Gallium (Ga) has a melting point of ∼29.8∘C and an exceptionally high boiling point of ∼2400∘C. It is protected by a thin oxide layer and does not react with pure water at 100∘C, remaining as a stable liquid pool at the bottom.
04option_verdict
Lithium (Li) is an alkali metal that reacts vigorously with boiling water to form LiOH and H2 gas, plus its melting point is 180.5∘C (it would be a solid reactant below its MP anyway).
05option_verdict
Bromine (Br2) is a liquid at room temperature, but its boiling point is only 58.8∘C. In boiling water (100∘C), it rapidly boils away as bromine vapour.
✓discriminator
Alkali metals (Li, Cs) react chemically with water; bromine boils below 100∘C (59∘C); gallium melts at 30∘C, boils above 2000∘C, and is unreactive with water at 100∘C.
Hints that build this answer step by step
What two physical and chemical conditions must an element satisfy to remain as a liquid inside pure boiling water (at 1 atm)?
Its melting point must be ≤ 100 °C, its boiling point > 100 °C, and it must not rapidly react with water.
Why cannot bromine (Br₂) or the alkali metals (Cs, Li) remain as elemental liquids in boiling water?
Br₂ boils at ~59 °C (turns to gas), while Cs and Li react violently/vigorously with water.
How does gallium (Ga) behave at 100 °C in contact with pure water?
It melts at ~30 °C, boils well above 2000 °C, and is protected from rapid reaction by a passivating oxide layer.
Gallium forms a self-passivating protective oxide film (Ga2O3) that prevents reaction with water at 100°C; it only reacts with steam at much higher temperatures.