Final answer:
When a thermometer is placed in hot water, the mercury or alcohol level first goes down and then rises due to thermal expansion; the glass bulb expands before the liquid, causing a temporary drop before the liquid expands more and the column height increases.
Step-by-step explanation:
When a thermometer is placed in hot water, the column of mercury or alcohol initially descends slightly before it starts to rise. This occurs due to the phenomenon known as thermal expansion. When the thermometer is first placed in the hot water, the glass bulb at the end of the thermometer, which contains the mercury or alcohol, heats up faster than the liquid inside. As glass is a solid, it expands slightly when heated. Since the volume of the bulb increases faster than the liquid inside can expand to fill it, this causes a temporary decrease in the height of the liquid column. Once the liquid inside catches up and starts to heat and expand, its volume increases significantly more than the glass, and the column rises to indicate a higher temperature. This behavior demonstrates how the rapid expansion of liquids like mercury or alcohol, relative to the slower expansion of solid glass, makes them suitable for measuring temperature changes.