IMAT 2014 Q54 [Galileo Thermometer]

An ornamental thermometer, commonly known as a Galileo thermometer, contains a number of spheres of hollow coloured glass, representing different temperatures, immersed in a column of ethanol. A particular sphere (X) rises from the bottom to the top of the column of liquid when the temperature falls below the value it represents.

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Which statement best explains why the sphere rises when the temperature falls?

A. The mass of the sphere has decreased.
B. The pressure exerted by the liquid has decreased.
C. The volume of gas inside the sphere has increased.
D. The density of the liquid has increased.
E. The temperature of the sphere is different from that of the liquid.

Galileo thermometers rely on the principle of buoyancy, which determines whether an object floats or sinks.

We know that an object immersed in a fluid experiences two main forces, the downwards acting force of gravity and the up thrust of buoyancy.

The main explanation will therefore be:

As the temperature of the air surrounding the thermometer changes, or in this case falls, so does the temperature of the fluid (ethanol) immersing these spheres. As the temperature of the ethanol changes, so does it’s density as it expands or contracts.

The density of the air inside the sphere, however, will not change much relative to the liquids new found density.

So at any given density, spheres will either sink or float.

In this case, the density of this liquid increases as the temperature falls.

The sphere (x) has become less dense than ethanol at this certain temperature, and hence, it floats or rises.

This will therefore make the sphere rise to the top of the column.

The best explanation of this system would be option D.

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