A student sets up a mercury barometer, but accidentally allows a small bubble of air to enter the tube. This air rises to the top of the mercury column, and the height of the mercury column is observed to be lower than the true atmospheric pressure reading.
Which statement best explains why the height of the mercury column is lower?
The trapped air molecules exert an upward force on the glass tube, lifting it slightly.
The trapped air molecules cool the mercury, causing its density to increase and height to decrease.
The trapped air molecules exert a downward pressure on the top surface of the mercury column.
The trapped air molecules react chemically with the mercury, reducing its total volume.
188 exam-style questions on CIE IGCSE Physics Pressure, covering Pressure. Each one has a worked solution and a mark scheme showing where the marks go.