This question was asked on a cold Winter’s day. The water in the bird bath had frozen and I used hot water from the kettle to thaw it so the birds could drink. When I explained to my 6-year-old that water freezes at zero degrees Celsius, she wondered if that was the coldest that things can get. But it isn’t – in fact some things can get much, much, much colder!
So what happens when things get hot or cold and how do we measure temperature?
Everything on Earth is made up of chemical elements. The smallest units or building blocks of these elements are called atoms, and atoms can join together to make larger building blocks called molecules.
Atoms have energy and will jiggle about. The amount of jiggling allows us to describe three states of matter: solids, liquids and gases. The atoms inside a solid (like wood) don’t jiggle much and line up close together in neat lines. The atoms in a liquid (like water) are more jiggly and can move around each other. The atoms in a gas (like air) are very jiggly and can move apart from each other in all directions.
But why am I telling you about elements, atoms and states of matter? Because you can change something’s state of matter by making it hotter or colder:
- Heating a substance will give its atoms and molecules more energy, so they get more jiggly. A solid will warm to a liquid; a liquid will warm to a gas.
- Cooling a substance will remove energy, so its atoms and molecules will get less jiggly. A gas will cool to a liquid; a liquid will cool to a solid.
So, temperature is a way of describing how much jiggly energy atoms and molecules have.
We use temperature in our daily lives: for example, in weather forecasts, for cooking and in medicine. Temperature is measured using a thermometer and given a numerical value:
- The [Celsius](http://www.livescience.com/39959-celsius.html) scale is based on the physical properties of water. Zero degrees is the temperature at which water freezes (turns to ice) and 100 degrees is the temperature at which water boils (turns to a gas). Celsius is the most commonly used temperature scale.
- The [Fahrenheit](http://www.livescience.com/39916-fahrenheit.html) scale uses a different set of reference points. On this scale, water freezes at 32 degrees and boils at 212 degrees. The Fahrenheit scale is often used by doctors to measure human body temperature (which is normally about 98.6 degrees).
- The [Kelvin](http://www.livescience.com/39994-kelvin.html) scale is based on physics. It sets the coldest possible temperature as the point at which atoms and molecules stop moving. This is called ‘absolute zero’ and is equivalent to minus 273.15 degrees Celsius – super-cold! On this scale, water freezes at 273 Kelvin and boils at 373 Kelvin. This temperature scale is often used in science.
Check out this video about measuring temperature!