A mercury barometer is faulty because a small amount of air is trapped in the space above the mercury column. When the temperature of the device increases, the height of the mercury column in the tube decreases, even though the external atmospheric pressure remains constant.
Why does the height of the mercury column decrease?
The air molecules above the mercury cool down and exert less pressure.
The density of the mercury increases, making the column heavier.
The glass tube expands significantly, causing the mercury level to drop.
The air molecules above the mercury move faster and collide more frequently and with greater force, increasing the pressure above the mercury.
151 exam-style questions on CIE IGCSE Physics 2.1.2a Particle structure and motion in solids, liquids and gases. Each one has a worked solution and a mark scheme showing where the marks go.