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## Thermometers and Absolute Zero

As we begin our study of solids, liquids and gases in Middle School Chemistry, we quickly learn that everything that exists is made up of atoms and molecules (and ions).

Then, we start to explore matter through two basic principles:

1. Atoms and molecules are attracted to each other
2. These atoms and molecules are in constant motion.

We further see that the motion that particles of a substance have are actually a measure of their temperature.  The more movement of atoms and molecules a substance has, the higher its temperature.  Less movement of atoms and molecules means a lower temperature.

As we saw in lesson 1.3, this is how a thermometer works.  In fact, a thermometer simply provides us with a way of precisely judging the  differences in molecular movement that substances at different temperatures have.

Of course, we can use our senses to feel when something is cold or hot, but what if we want to know exactly how cold it is, or exactly how much colder one substance is than another?  Thermometers help us to make precise measurements of temperature, and give us a way to accurately compare the temperature of one substance to the temperature of another.

So, if temperature is a measure of molecular movement, can temperature just increase or decrease forever?  Are there any limits to how hot or cold something can be?

Yes and no.

Although there does not seem to be an upper bound on how hot something can get, scientists have long postulated an absolute lower bound of coldness, mysteriously (and awesomely) called absolute zero.

Theoretically, absolute zero corresponds to the temperature at which all molecular motion stops.  But you may be wondering, why is it called absolute zero?  Why not just … zero?

Some Thermometer History

Although thermometers are incredibly useful instruments of science, it’s important to keep in mind that the scale of a thermometer is relative.  In fact, you may already be aware that there are two common temperature systems currently in use today: Fahrenheit and Celsius.  What makes the temperature scales relative is that while on the Fahrenheit system, the freezing point of water is marked as 32 degrees, in Celsius, freezing is marked as 0 degrees.  So which system is right?  Which temperature is actually freezing?

In fact, both scales are “right”.  The temperature is simply an agreed upon standard that scientists use to define freezing.

To understand this, we have to think back to the time when thermometers were first introduced as cutting edge devices of science.  At that time, there wasn’t any agreement about how to record temperatures.  What system should all scientists use as a common reference so that everyone could compare their data with one another?

One of the most prominent early Thermometer makers was a man named Daniel Fahrenheit.  He designed small thermometers with mercury inside that were easy to use in experiments, but which also had two common reference points: 32 degrees for freezing, and 96 degrees, which corresponds to the internal body temperature of a person.  Because his thermometers were compact and easy to use, his temperature scale quickly gained acceptance and is still used in the United States today.  The other common temperature scale, Celsius (was formerly known as Centigrade) is named after Swedish Astronomer Anders Celsius, who devised a very similar scale.

But we have to keep in mind that 32 degrees is just a number that Fahrenheit chose.  He could just as easily have chosen another number and designed his thermometers in a different way.  So, when the Celsius system designates freezing as 0 degrees, it is also correct.  The scales are just different.

If both systems are correct, then why do they use zero at different points?  Remember that Fahrenheit was introducing his thermometer as a scientific instrument.  When he introduced his thermometer in the 1720s, it was believed that you couldn’t make something much colder than by mixing ice, salt, and water which results in a temperature below the freezing point of water.  Believing this to be the lower bound of “coldness”, Fahrenheit decided to set the value of zero for his scale there.

Today, however, we know that it can get colder than a mixture of salt, ice, and water.

Much colder.

Absolutely Still at Absolute Zero?

But because both common temperature scales already used zero to refer to temperatures, scientists coined the term absolute zero to clarify the true lowest possible temperature, not based on what they could observe (as Fahrenheit did), but instead based on the principle that temperature reflects molecular movement, and that at absolute zero all molecular motion would theoretically cease.

Today, scientists still haven’t been able to reach absolute zero, but they have gotten pretty close.

In fact, to help measure how close, there is a third temperature scale used in science called Kelvin.  In Kelvin, unlike Celsius and Fahrenheit, 0 actually does correspond to absolute zero.  This temperature scale was developed much later than Fahrenheit or Celsius, around 1850, and bears the name of Lord Kelvin, who first suggested it.

You can learn more about the history of science and cold by watching a recently PBS documentary entitled “Absolute Zero”:

Adam M. Boyd is a Sr. Education Associate in the office of K–8 science at the American Chemical Society. In his work at ACS, Adam focuses on chemistry, writing, and the web.