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2022 O Level Chem P1 Q31 — States of Matter Solution

States of Matter 2022 O Level Pure Chemistry · Paper 1 · Q31

Question

A sample of clean, dry air is cooled to −190 °C. The carbon dioxide forms a solid at −79 °C. The table shows the melting and boiling points of argon, oxygen and nitrogen. What is the state of argon, oxygen and nitrogen at −190 °C? Question diagram

Question diagram

Question diagram (continued)

A
B
C
D

Correct answer: C

Video Solution

Explanation

Answer: C. Apply the rule: at temperature \(T\), a substance is solid if \(T < \text{m.p.}\), liquid if \(\text{m.p.} \leq T \leq \text{b.p.}\), and gas if \(T > \text{b.p.}\). All comparisons are with \(T = -190\) °C.

Argon: m.p. \(-189\), b.p. \(-186\). \(-190 < -189\) → solid.
Oxygen: m.p. \(-218\), b.p. \(-183\). \(-218 < -190 < -183\) → liquid.
Nitrogen: m.p. \(-210\), b.p. \(-196\). \(-190 > -196\) → gas.

States (argon, oxygen, nitrogen) = (solid, liquid, gas) → row C.

The common mistake here is comparing magnitudes of negative numbers the wrong way round. Remember \(-190\) is HIGHER than \(-196\) on the temperature scale, even though 190 is the smaller magnitude.

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