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CAIE IGCSE Chemistry 1.1.4 Effects of Temperature and Pressure on Questions

Explain equilibrium observations using particle concentration, pressure and temperature effects.

Syllabus
2026–2028
Course
Chemistry 0620

Exam points

  • explain how pressure or volume changes can alter the observed colour of an equilibrium mixture without changing equilibrium position

CAIE IGCSE Chemistry 1.1.4 Effects of Temperature and Pressure on Questions question 1

[Maximum number: 1]

Hydrogen iodide, HI, decomposes into iodine and hydrogen. The reaction is reversible.

2HI( g) colourless gas ⇌ purple gas I2( g)+H2( g) colourless gas \underset{\text { colourless gas }}{2 \mathrm{HI}(\mathrm{~g})} \rightleftharpoons \underset{\substack{\text { purple gas } \\ \mathrm{I}_{2}(\mathrm{~g})}}{ }+\underset{\text { colourless gas }}{\mathrm{H}_{2}(\mathrm{~g})}

A gas syringe containing a mixture of hydrogen iodide, iodine and hydrogen gases was sealed. After reaching equilibrium the mixture was a pale purple colour.

Figure for Question CAIE IGCSE Chemistry 1.1.4 Effects of Temperature and Pressure on Questions question 1 — CAIE IGCSE Chemistry

The plunger of the gas syringe is pushed in. The position of equilibrium does not change. The colour of the gaseous mixture turns darker purple.

The temperature remains constant.

2HI( g) colourless gas ⇌⇌ purple gas I2( g)+H2( g) colourless gas \underset{\text { colourless gas }}{2 \mathrm{HI}(\mathrm{~g})} \rightleftharpoons \underset{\substack{\text { purple gas } \\ \mathrm{I}_{2}(\mathrm{~g})}}{\rightleftharpoons}+\underset{\text { colourless gas }}{\mathrm{H}_{2}(\mathrm{~g})}

Suggest why the colour of the gaseous mixture turns darker purple even though the position of equilibrium does not change.

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