How Hot Is Hot? How Cold Is Cold?

By James F. Krieger

Did you ever burn your fingers on a hot potato? Did you ever wonder how hot or cold it is on the moon? A comfortable temperature for a home is about 70º F (21º C). A person’s body temperature is normally 98.6º F (37º C). Water boils at 212º F (100º C). The daytime temperature on the moon is about 273º F (134º C).

The feeling of heat is caused by the movement of atoms. Everything that we can see, smell, taste, or touch is made of atoms, particles so small that they can hardly be seen with the most powerful microscopes. Atoms are always in motion; the hotter a thing feels, the faster its atoms are moving.

The bodies of plants, animals, and people are made of atoms arranged in a special way. When we touch something hot, like boiling water, the water atoms are moving faster than the atoms in our bodies. They smack into us, and the arrangement of atoms in our flesh is damaged, and we are burned.

Metals are made of atoms that hold together very tightly. If aluminum is heated to 1220º F (660º C), its atoms move faster than in its normal state, and they break away from each other. The metal changes to a bright, hot, silvery liquid that pours like water. If this liquid aluminum is heated further to 3272º F (1800º C), it boils, and its atoms move so fast that they fly apart, become a gas, and go off into the air. Other metals require even higher temperatures to change. Iron melts at 2795º F (1535º C) and boils at 5432º F (3000º C).

Even these temperatures are not very high when compared to the heat of the sun. The surface of the sun is 10,832º F (6000º C). The hottest part of the sun is its center, which is about 27 million degrees Fahrenheit (15 million degrees Celsius)! This great heat is caused by nuclear fusion such as occurs in the explosion of a hydrogen bomb.

Astronomers tell us that sometimes a star will explode; for a few days or weeks it will outshine thousands of other stars combined. The temperatures of these exploding stars (suns) must be incredible!

The opposite of hot, of course, is cold. The slower a substance’s atoms are moving, the colder that substance will be. Things can be cooled to a very low temperature by slowing the motion of their atoms. When a person wants to heat something, a fire, a stove, or some other heat source is used. But how do you cool anything?

There is a very unusual liquid called freon, which boils at ordinary room temperature. As part of the liquid boils away, the remaining liquid becomes extremely cold. By using this unique property of freon, other substances, such as food in a refrigerator, can be cooled.

Very low temperatures are described as degrees below zero: they have a minus sign (-) before their numbers. The greater the number of degrees below zero, the colder a thing is.

At 32º F (0º C) water freezes. As water is cooled, the movement of its atoms is slowed down, and the atoms remain next to each other.

When air is cooled to more than -328º F (-200º C), its atoms are practically motionless, and the air becomes a liquid. Hydrogen is a gas that is used to inflate airborne balloons. When hydrogen is cooled past -418º F (-250º C), it changes to a liquid and can be used as part of the fuel for spaceships.

Scientists found that as a gas is cooled, it takes up less and less space. And they discovered that they could calculate how much space any gas will occupy at any temperature. Yet there was a mystery: If a gas reaches -459º F (-273º C), calculations show that it will not take up any space!

After a long time scientists found the answer: Before any gas can be cooled to -459° F (-273º C), it changes to a liquid or a solid, and the motion of atoms stops before reaching that magic, frigid number, which is called Absolute Zero.

Illustrated by Lori Anderson