Final answer:
The initially descending mercury column in a thermometer when exposed to heat is due to the faster expansion of the glass compared to the mercury, followed by the greater expansion of mercury which then causes the column to rise.
Step-by-step explanation:
When a mercury thermometer is exposed to heat, such as a flame, the column of mercury initially descends before rising because of the expansion properties of both the mercury and the glass. Glass, being solid, tends to heat up and expand faster than the liquid mercury inside. Once the glass expands, it momentarily makes more space available for the mercury, leading to a slight drop in the mercury level. However, as the mercury gets heated, it expands at a much greater rate than the glass, causing the column to rise substantially, indicating an increase in temperature.
Surface tension also plays a role. Mercury has a high surface tension and forms a convex meniscus with glass. With heat, the forces that create the meniscus, as well as the overall volume of the mercury, change, resulting in the liquid first decreasing then increasing in height as the temperature rises.