When a thermometer is placed in a beaker of hot water, WHY does the mercury level DROP, then rise?
See my answer below to the same question.
Mercury expands when heated and that forces it from the bulb at the thermometer base but due to the hot water the thermometer bulb expands hence heat gets transferred to the mercury making it drop
When a thermometer is placed in a beaker of hot water, the mercury level initially drops due to thermal expansion.
To understand this phenomenon, let's first look at the structure of a thermometer. A typical mercury thermometer consists of a long, narrow glass tube with a bulb at one end containing mercury. The glass tube is sealed at both ends, and the empty space above the mercury in the tube is usually filled with nitrogen or argon gas.
When the thermometer is placed in hot water, the heat from the water is transferred to the bulb part of the thermometer. As the temperature increases, the mercury inside the bulb and the lower part of the glass tube expands. As a result, the volume of mercury increases, but since the glass tube is narrow, the mercury level drops.
However, as the heat continues to transfer up the glass tube, the temperature of the mercury in the upper part of the tube also increases. As a consequence, the mercury in the upper part of the thermometer expands, causing the mercury level to rise and eventually reach a new equilibrium position.
So, the initial drop in the mercury level occurs due to the expansion of the mercury in the bulb and the lower part of the glass tube, while the subsequent rise occurs due to the expansion of the mercury in the upper part of the glass tube as it equilibrates with the temperature of the surrounding hot water.