**Introduction**

Water is not used instead of mercury in thermometers due to several reasons. While water has a number of advantages, it also has significant limitations that make it unsuitable for use in thermometers. This can be explained by examining the properties and behaviors of both water and mercury.

**Properties of Water**

Water is a common substance that exists in a liquid state at room temperature. It has a relatively high specific heat capacity, meaning it can absorb or release a large amount of heat energy before its temperature changes. Water also has a relatively low freezing and boiling point, which makes it suitable for a wide range of applications.

**Advantages of Water**

There are some advantages to using water in thermometers:

1. **Availability**: Water is abundant and readily available in most environments, making it a cost-effective option for thermometers.

2. **Safe**: Compared to mercury, water is non-toxic and does not pose a significant health risk if accidentally ingested or exposed.

3. **Environmental Friendliness**: Water is environmentally friendly and does not contribute to pollution or contamination.

**Limitations of Water**

However, water has several limitations that prevent its use in thermometers:

1. **Freezing and Boiling Points**: Water freezes at 0°C (32°F) and boils at 100°C (212°F) at sea level. These relatively low temperature ranges restrict the operational range of water thermometers compared to mercury thermometers, which can measure a wider range of temperatures.

2. **Vaporization**: Water readily evaporates, especially at higher temperatures. This can lead to a loss of water from the thermometer, affecting its accuracy and reliability.

3. **Expansion and Contraction**: Water expands when heated and contracts when cooled. However, the expansion and contraction of water are not as predictable and consistent as mercury. This can introduce errors in temperature measurements.

4. **Pressure Sensitivity**: Water is more sensitive to changes in atmospheric pressure compared to mercury. This can affect the accuracy of water thermometers, particularly if used in different elevations or locations.

**Conclusion**

While water has some advantages such as availability, safety, and environmental friendliness, its limitations including freezing and boiling points, vaporization, expansion and contraction, and pressure sensitivity make it unsuitable for use in thermometers. Mercury, on the other hand, possesses properties that make it an ideal choice for thermometers, including a wide temperature range, low vaporization rate, consistent expansion and contraction, and minimal pressure sensitivity.

In thermometer mercury expands due to the heat. Water does not expand due to heat. This is why mercury is used in the thermometer.