Because it will bust.
That depends on how hot the water is in the cup. Use the thermometer to measure it and see.
When a thermometer bulb containing mercury is placed in hot water, the mercury inside expands, causing the level of mercury to rise. This expansion of mercury is used to measure the increase in temperature accurately.
The bulb of a clinical thermometer breaks when placed in very hot water because the sudden increase in temperature causes the air inside the bulb to expand rapidly. This rapid expansion of air creates pressure within the bulb, leading to its breakage.
A thermometer is used to measure the temperature of hot water.
When a thermometer is quickly dipped in hot water, it won't do anything. If you leave the thermometer in the hot water, the temperature shown will read higher. The temperature shown will not exceed the temperature of the water.
The mercury level in a thermometer placed in a hot tub of water will rise as the temperature of the water increases. This is because the volume of liquid mercury expands with higher temperatures, causing it to climb up the measuring scale in the thermometer.
If you ever want to measure a temperature above 212 degrees, you're better off hanging the thermometer into the air in the oven. Since it's a hot-air oven, you want to measure the air. If it were a hot-water oven, you might prefer to measure water. But even then, what are you gonna do above 212, when the water goes away and all you have in there is steam ??
When a thermometer is placed into something cold, the liquid inside contracts and decreases in volume. This causes the liquid level to decrease, indicating a lower temperature on the scale of the thermometer.
a thermometer
When a thermometer is placed in hot water, the heat causes the liquid inside (mercury in this case) to expand. This expansion initially pushes the mercury down slightly due to increased pressure. However, as the mercury absorbs more heat, it expands further and eventually rises to reflect the higher temperature of the hot water.
Because the hot water heat transfers into the rod.
The idea is based avoiding use of the thermometer when it is reading "higher" than what it is that is being measured. Let's look at what's happening. If we wash a clinical thermometer in hot water, it will respond to that hot water. That will cause the reading to shoot up well above a hundred degrees. If we then try to use it, it will have to "cool down" under the tongue of the person we're using it on. It is actually being cooled by that person. The thermometer will take a relatively long time to reach the temperature of the individual whose temperature is being measured. And this time will vary, but will be longer than it would be for the thermometer to "come up" to temperature. One of the directions for using these thermometers is to first shake it down until it reads less than 95 degrees or so. If you're starting with a thermometer than has just been washed in hot water, you're not going to be able to shake it down per the directions. You'll have to run it under cool water for a bit to cool it so you can use it.