Mercury is used in thermometers for the following reasons:

1. High thermal expansion:
Mercury is chosen as the liquid in thermometers due to its high coefficient of thermal expansion. This means that it expands and contracts significantly with changes in temperature. This property allows for accurate measurement of temperature variations.

2. Wide temperature range:
Mercury has a wide temperature range in which it remains in a liquid state. It has a freezing point of -38.83 degrees Celsius (-37.89 degrees Fahrenheit) and a boiling point of 356.73 degrees Celsius (674.11 degrees Fahrenheit). This range covers the majority of temperatures encountered in everyday applications, making it suitable for use in thermometers.

3. Easy visibility:
Mercury is a dense, silvery liquid that is highly visible. Its distinct appearance allows for easy reading of temperature measurements on the thermometer scale. The visibility of mercury enables users to accurately determine the temperature with precision.

4. Good thermal conductivity:
Mercury possesses excellent thermal conductivity, meaning it can transfer heat efficiently. This property ensures that the temperature of the mercury in the thermometer quickly adjusts to the temperature of the surrounding environment, resulting in more accurate readings.

5. Stability and reliability:
Mercury is a stable and reliable substance, resistant to oxidation and evaporation at normal temperatures. It remains liquid and does not react with the materials commonly used in thermometers, ensuring the longevity and accuracy of the device.

6. Traditional use and availability:
Mercury has been used in thermometers for centuries, and its use has become a standard practice. The availability of mercury and the established manufacturing processes for mercury-based thermometers contribute to its continued use.

7. Safety considerations:
While mercury is an effective and commonly used liquid in thermometers, it is important to handle it with caution due to its toxicity. Special care must be taken when handling, storing, and disposing of mercury-containing thermometers to prevent environmental contamination and potential health risks.

In conclusion, mercury is used in thermometers due to its high thermal expansion, wide temperature range, easy visibility, good thermal conductivity, stability, and traditional use. However, safety precautions must be followed when using mercury-containing thermometers.

we know density of water also? y can't we use water