# A Temperature Guide for Kids

Updated by Jim Deckebach

People experience temperature all the time. Temperature can be defined as the measure of the average energy of the particles in a substance. To physicists, that means the kinetic energy of a substance's particles, but to us, we experience this energy as heat. In the simplest terms, temperature is how hot or cold something is.

## Thermometer Types

A thermometer is the tool used to measure temperature. One type of thermometer has a glass tube with colored alcohol inside the tube. An older type of thermometer has liquid mercury inside the glass tube instead of colored alcohol. Thermometers have different scales and standards to measure temperature. Marks on the outside of a thermometer are measured and placed at precise intervals. To measure the temperature, you check the level of the colored alcohol, noting the corresponding mark on the thermometer.

There are lots of other types of thermometers besides the basic ones you're used to, though. For instance, a refrigerator will often have a thermometer in it so you can monitor the temperature inside. Electronic thermometers work with a thermistor, which changes the resistance to an electric current based on the temperature. A computer is used to measure the resistance of the thermistor, which converts it to a reading on the thermometer. A cryometer is a special type of thermometer used to measure very low temperatures, and a pyrometer is a thermometer that measures very high temperatures. Infrared thermometers measure temperature based on the infrared radiation something gives off, which allows people to measure the temperature of something without touching it.

## How Thermometers Work

As the temperature rises around a thermometer, this causes the liquid inside the tube to rise. The molecules in the liquid move faster and spread out, causing the liquid to move up the glass tube. When the temperature goes down, the molecules in the liquid move slower and get closer together. This causes the liquid level inside the tube to go down.

## Temperature Scales

Temperature scales include Fahrenheit, Celsius, and Kelvin. Gabriel Fahrenheit was the scientist who created the Fahrenheit scale, and this scale uses 32 degrees as the freezing point and 212 degrees as the boiling point. Anders Celsius invented the Celsius scale. The Celsius scale uses 0 as the freezing point and 100 as the boiling point. The Kelvin scale was devised by William Thomson, who was also called Lord Kelvin. Using the Kelvin scale, units are called Kelvins, and there are 100 units between the point where water freezes and the point where it boils. But in Kelvin, 0 is absolute zero, the coldest possible temperature, at which the motion of all molecules stops.

## How to Read a Thermometer

Most glass tube thermometers have two scales printed on them: Fahrenheit and Celsius. To read the thermometer, avoid handling it as much as possible because this might affect the temperature of the colored alcohol. Hold the thermometer by the top and vertically so the level of the colored alcohol is at eye level. Note the point on the thermometer where the liquid reaches. You'll probably see numbers on both sides of the thermometer, one side for the Fahrenheit scale and the other side for the Celsius scale. The mark where the liquid reaches is the temperature, either in Fahrenheit or Celsius. You can also convert between Fahrenheit and Celsius with a mathematical formula: Subtract 32 from the Fahrenheit number, then multiply the result by 5/9 to get the Celsius number. For example, if the liquid level is at 80 degrees Fahrenheit, it would be at 26.67 degrees Celsius.

## Temperature and the State of Matter

The temperature of a substance affects its state of matter. Different substances enter into different phases as the temperature changes. These phases include solid, liquid, and gas. For example, at room temperature, water is a liquid. When the temperature of water is very cold, it becomes solid ice. And if liquid water is heated to a hot enough temperature, it becomes water vapor, a gas.

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