NDA (II) 2023 GAT Q65: Consider the following reaction : 2HgO —Δ→ 2Hg + O₂ The

Q: What is the answer to NDA (II) 2023 GAT Q65?

D. Correct — D, Solid, liquid, gas. Written out with state symbols the equation is 2HgO(s) —Δ→ 2Hg(l) + O₂(g). Mercury(II) oxide is a solid powder. Mercury is the one metal that is liquid at ordinary room temperature, its melting point being about −39 °C. Oxygen is a gas. So the three states run solid, liquid, gas in that order, and the Δ over the arrow simply tells you the change is driven by heating.

NDA (II) 2023 General Ability Test · Question 65 of 146

Question text verified against the official NDA question paper.

Consider the following reaction : 2HgO —Δ→ 2Hg + O₂ The respective state of HgO, Hg and O₂ in the above reaction is

(a)Liquid, solid, gas
(b)Solid, solid, gas
(c)Liquid, solid, liquid
(d)Solid, liquid, gas

Answer

Why

Correct — D, Solid, liquid, gas. Written out with state symbols the equation is 2HgO(s) —Δ→ 2Hg(l) + O₂(g). Mercury(II) oxide is a solid powder. Mercury is the one metal that is liquid at ordinary room temperature, its melting point being about −39 °C. Oxygen is a gas. So the three states run solid, liquid, gas in that order, and the Δ over the arrow simply tells you the change is driven by heating.

Why the others are wrong

(a)Liquid, solid, gas — This swaps the first two. Mercuric oxide is not a liquid — it is a solid powder — and the mercury it releases is not a solid.
(b)Solid, solid, gas — It gets the oxide and the oxygen right but calls mercury a solid. Mercury freezes only around −39 °C, so at ordinary temperature the metal collected here is a liquid.
(c)Liquid, solid, liquid — Wrong on all three counts. It makes the oxide a liquid, the mercury a solid, and turns oxygen — a gas at room temperature — into a liquid.

Concept

This is a thermal decomposition — a single compound broken by heat into two simpler substances. Reading such an equation means knowing the ordinary state of each species. Mercury(II) oxide is a red or orange solid; it breaks down on strong heating or on exposure to light. Mercury is the only metallic element that is liquid at room temperature. Oxygen is a colourless gas.

You do not need any chemistry beyond the three physical states — the item is really testing whether you know mercury is a liquid metal. Fix that one fact and options (b) and (c) fall away at once, and (a) fails because a powdered oxide cannot be a liquid. Be honest about one wrinkle: the decomposition itself needs about 500 °C, and at that temperature the mercury comes off as vapour. The state symbols in the printed equation are the conventional room-temperature states, and that convention is what this question is asking you to reproduce.

Key facts

Mercury is the only metal that is liquid at ordinary room temperature, melting at about −39 °C.
Mercury(II) oxide is a solid, and it decomposes on heating above roughly 500 °C or on exposure to light.
The balanced equation with state symbols is 2HgO(s) → 2Hg(l) + O₂(g).
In 1774 Joseph Priestley obtained a gas by heating mercuric oxide, which he called dephlogisticated air and which was later identified as oxygen.

A single bead of liquid mercury, silvery and mirror-bright, resting in the bottom of a clear glass vessel at room temperature. — Mercury is the metal that stays liquid at ordinary room temperature, which is what fixes the middle state in this reaction.Image: А · Wikimedia Commons (Public domain)

Study next

Thermal decomposition reactions and their state symbolsDecomposition by heat, light and electricity (thermolysis, photolysis, electrolysis)Laboratory preparation of oxygen and the history of its discoveryPhysical states of the elements at room temperature — the liquid and gaseous exceptionsBalancing chemical equations and using (s), (l), (g), (aq)

Common traps

Assuming every metal must be a solid, and so marking mercury as solid.
Reading Δ as a catalyst or as electrolysis instead of as simple heating.
Mixing up the reactant and product order — the question asks for HgO first, then Hg, then O₂.

Asked as a state-symbol identification, as a 'name the type of reaction' item on the same equation, or as a general-science item on which element is a liquid metal.

Related PYQs

NDA_GAT_2022_I_Q912022Open & attempt →

Which one of the following reactions is an example of decomposition reaction?

(a) CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)
(b) 2AgCl(s) —Sunlight→ 2Ag(s) + Cl₂(g)
(c) CuO + H₂ —Heat→ Cu + H₂O
(d) Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)

Answer(b) 2AgCl(s) —Sunlight→ 2Ag(s) + Cl₂(g)

The same reaction family with state symbols already printed — one compound splitting into two simpler ones. Silver chloride is broken by light where mercuric oxide is broken by heat.

NDA_GAT_2022_II_Q722022Open & attempt →

Which of the following compounds undergoes/undergo thermal decomposition?

(a) Zinc oxide
(b) Silver oxide and zinc oxide
(c) Silver oxide
(d) Magnesium oxide

Answer(c) Silver oxide

Directly on thermal decomposition of a metal oxide — the same idea as heating mercuric oxide, and it makes the point that only the oxides of the least reactive metals break up this easily.

Practice

practice — not a real PYQ
The reaction 2HgO —Δ→ 2Hg + O₂ is an example of
- (a)a combination reaction
- (b)a thermal decomposition reaction
- (c)a displacement reaction
- (d)a double displacement reaction
Answer(b) a thermal decomposition reaction — one compound is split by heat into two simpler substances.
practice — not a real PYQ
Which one of the following elements is a liquid at ordinary room temperature?
- (a)Sodium
- (b)Iodine
- (c)Mercury
- (d)Phosphorus
Answer(c) Mercury — the only metal that is liquid at room temperature.