CBSE Class 7 Science Temperature and Thermometers Notes Set 04

Hot, Cold and Temperature

Heat is a form of energy that makes things feel hot or cold. Its SI unit is the joule (J). Temperature tells us how hot or how cold an object is. Our sense of touch can mislead us, because the same warm water can feel hot to a cold hand and cool to a hot hand. A thermometer gives a reliable measure of temperature.

When an object gains heat its temperature usually rises. Heat can also change the state of matter, make substances expand and cause chemical changes.

How a Thermometer Works

A thermometer is an instrument that measures temperature. It has a thin glass tube with a bulb at the bottom filled with a thermometric liquid, usually mercury. When the bulb is heated, the liquid expands and rises along the tube. When it cools, the liquid contracts and falls. The level is read against a scale marked on the tube.

Why Mercury Is Used

1. It is a shiny liquid metal, so the thread is easy to see.

2. It does not stick to the glass.

3. It expands and contracts evenly as temperature changes.

4. It freezes at −39 °C and boils at 357 °C, so it works over a wide range.

Mercury is poisonous. If a thermometer breaks, do not touch the liquid with bare hands and inform an adult.

Temperature Scales

A scale is made using two fixed points. The lower fixed point is the temperature at which pure ice melts. The upper fixed point is the temperature at which pure water boils under normal pressure.

1. Celsius scale: lower point 0 °C, upper point 100 °C, with 100 equal divisions.

2. Fahrenheit scale: lower point 32 °F, upper point 212 °F, with 180 equal divisions.

3. Kelvin scale: lower point 273 K, upper point 373 K, with 100 equal divisions. Kelvin is the SI unit of temperature.

Clinical and Laboratory Thermometers

Clinical thermometer. It measures the temperature of the human body. Its range is 35 °C to 42 °C (or about 94 °F to 108 °F) and it shows both °C and °F. It has a kink near the bulb that stops the mercury from falling back on its own, so the reading stays. After use it is shaken to bring the mercury down. It has no hook. The normal human body temperature is 37 °C (98.6 °F).

Laboratory thermometer. It measures the temperature of liquids and other substances in experiments. Its range is −10 °C to 110 °C and it is marked only in °C. It has no kink, so the mercury falls as soon as the bulb cools. It is not removed from the liquid while reading, and the bulb must not touch the walls of the container.

Worked Examples

Example 1. Why is a clinical thermometer not used to measure the temperature of boiling water? Its range ends at 42 °C, while boiling water is at 100 °C. The mercury would expand beyond the scale and could break the thermometer.

Example 2. The reading of a laboratory thermometer is 36 °C in a beaker. Is the water suitable for a person to drink as warm water? 36 °C is close to body temperature, so it is lukewarm and safe to sip.

Example 3. Why is the clinical thermometer shaken before use? The kink holds the mercury at the previous reading. Shaking brings it below 35 °C so that a fresh reading can be taken.

Quick Check Questions