When water molecules freeze at 0°C, the molecules still have some energy compared to ice at -50°C. In both cases, the molecules are not moving, so there is no heat energy.

So what is the temperature at which all the molecules have absolutely zero energy? A temperature scale can be defined theoretically for which zero degree corresponds to zero average kinetic energy. Such a point is called **absolute zero**, and such a scale is known as an **absolute temperature scale**. At absolute zero, the molecules do not have any energy.

The Kelvin temperature scale is an absolute scale having degrees the same size as those of the Celsius temperature scale. Therefore, all the temperature measurements related to energy measurements must be made on Kelvin scale.

You can convert a temperature in Celsius (c) to Kelvin (k) with this formula:

**K = c + 273.15**

You can also change a temperature in Kelvin to Celsius:

**c = k – 273.15**

To make calculations for this class easier, you may round off and use just 273 in your conversions.

### Try This!

**Instructions**: Click the "Play" button below and notice what happens to the ice cube. Answer the questions that follow based on your observations. To determine if you answered correctly, click the "check" button after each question.