Shedding Light on Heat Episode 1: Temperature

The Heat is on! A huge amount of the technology and comforts that we have in our world just wouldn’t exist if it wasn’t for what we’ve learned about the way heat behaves. So keep cool and use this excellent series to teach your students everything that they need to know about heat, including its effect on things and how it transfers from one thing to another.

In Episode 1, Temperature, we get down to the atomic level to explain how hot things are different to cold things, we describe what a temperature scale actually tells us, and demonstrate how we measure the amount of heat energy that water absorbs when it’s heated.

A 3-minute excerpt.


Part A: Introduction. The scope of the series.
Part B: Temperature Scales: Anders Celsius and Gabriel Fahrenheit were two brilliant scientists who came up with the two temperature scales that now bear their names. But which temperature scale is better?
Part C: The Kinetic Theory of Matter: You probably already know that everything is made of atoms, but did you know that atoms are constantly moving? The Kinetic Theory is one of the best theories ever!
Part D: Heat Energy: Heat energy is measured in Joules. But how much energy is a Joule of energy? And how many Joules of energy are needed to boil a pot of water? Well, it depends!

The Transcript
Part A: Introduction.

In this series, we’re going to look at heat. Heat is a huge part of our lives of course.

The heat energy coming from the sun warms our planet (some places more than others), and the heat energy from heaters warms our homes. Heat can be produced by burning things, including natural gas and wood, and by electricity, in say, a toaster.

Heat is a form of energy, and, like all forms of energy, it can make things change in some way. Heat can change the temperature of water, and it can change the proteins and other molecules in for example meat when we cook it.

Heat energy can even change the size of an object, because when most things get hot, they expand. Sometimes explosively so!

A huge amount of the technology and comforts that we have in our world just wouldn’t exist if it wasn’t for what we’ve learned about the way heat behaves.

In this series, we’re going to look at this idea of heat, including its effect on things and how it transfers from one thing to another. There’s a fair bit of heat transferring onto me right now!

We’ll begin by looking at temperature, which is related to heat but which is not the same thing as heat.

Part B: Temperature Scales.

Temperature is a measure of how hot something is. We know that boiling water is much hotter than melting ice, and therefore it has a higher temperature than melting ice. To express the temperature of something, we need a temperature scale.

Most of the world uses the Celsius scale for temperature, named after Anders Celsius, a Swedish scientist who, in 1742, nearly 300 years ago, devised it. Basically, he called the temperature at which ice melts 0° (we now say 0°C) and he called the temperature at which water boils 100°. Anders Celsius could have chosen any numbers that he wanted to for his scale, but he went with 0 and 100, nice round numbers. Something at, say, 37°C, which is approximately the temperature of the human body, is obviously hotter than melting ice, but it’s not as hot as boiling water.

In the United States and a few other countries, the Fahrenheit scale is used. This scale was devised by Polish-born scientist Daniel Gabriel Fahrenheit in 1724. He took a mixture of ice, water, and a chemical called ammonium chloride and was able to consistently reach a temperature that was way lower than the temperature of melting ice. Fahrenheit made this temperature zero degrees on his scale. It corresponds to about -18°C. The melting point of ice was set to 32°F and the boiling point of water was set to 212°F, with 180 equal divisions in between.

The inside of a typical freezer is about -18°C, which is the same as 0°F. Your fridge is about 4°C or about 39°F. Human body temperature is about 37°C or about 99°F.

The Celsius scale was adopted by countries that converted to the metric system throughout the 20th century. The metric system uses round numbers like 10 mm in a cm, 100 cm in a metre and 1000 m in a kilometre or kilometre.

The metric system is generally an easier system to work with, but neither of the two temperature scales is really better than the other; it’s just what you get used to. I live in Australia, so I’ve learned that if the weather forecast says it’s going to be 35°C, then it’s going to be really hot and I might go to the beach.

If I hear that the top temperature will be only 1°C, then I’ll need a coat because it’s going to be cold.

In The Bahamas though, where like the United States they use Fahrenheit, what I would call 35°C would be stated as 95°F.

But what exactly is temperature a measure of? In what way is this boiling hot water different to the icy-cold water next to it?

To answer these questions we need to get down to the atomic level, so let’s go there right now.

Part C: The Kinetic Theory of Matter

You probably already know that everything is made of atoms. Atoms are the tiny tiny building blocks that make up everything.

There are over 100 different types of atoms on earth and they’re listed on what we call the Periodic Table.

Pure copper for example is made entirely of copper atoms, pure sulfur is made entirely of sulfur atoms and pure zinc is made entirely of zinc atoms.

Water is not made of single atoms but groups of atoms that have chemically joined together. Each group, which we call a molecule, is made up of 2 hydrogen atoms and one oxygen atom which have all chemically bonded together. Water’s chemical formula is H2O.

Now back to temperature and what it means. It turns out that all atoms are constantly moving. You can’t tell just by looking at something that its atoms are moving, but they are.

So for example, an iron bar is made entirely of iron atoms. When it’s cold, the atoms move, or vibrate slowly in their fixed positions. When it’s hot, its atoms on average vibrate much faster.

Temperature is a measure of the average kinetic energy of the atoms that make up something. The higher the temperature of an object, the faster on average its atoms are moving. It’s true for everything.

The water molecules that make up cold water bounce around and vibrate more slowly than the water molecules that make up hot water.

This concept that everything is made of atoms that are constantly moving is called the kinetic theory of matter. The word kinetic comes from the Greek word kinesis which means movement.

One way of showing that atoms are moving faster on average in a hot substance compared to a cold substance is by dropping some food dye into hot water and into cold water. The glass of water on the left was in the fridge and cooled down to a temperature of about 4°C. The glass of water on the right was filled with boiling hot 100°C water from a kettle. We waited 5 minutes to allow the water to be absolutely still before adding the food dye. It’s pretty obvious that the food dye in the hot water spreads out much more quickly than the food dye in the cold water. This is because the water molecules that make up the hot water are vibrating and moving and bouncing around much more quickly than the water molecules that make up the cold water. They therefore crash into the molecules that make up the food dye more often and with more force and this makes the food dye spread much more quickly within the hot water.

This process of one substance randomly spreading out into another substance is called diffusion. The dye has diffused through the water.

I can demonstrate diffusion using this simulation. The liquid on the left is cold and so the particles are moving, on average, slowly, while the liquid on the right is hot and the particles are moving, on average, much more quickly. If I inject a red dye into both, then it’s pretty obvious that the red dye diffuses very slowly in the cold liquid and it diffuses much more quickly in the hot liquid. The random collisions in the hot liquid are occurring far more often and with more speed and with more force, and so the red dye in the hot liquid spreads much more quickly. The collisions occurring in the cold liquid occur far less often and with less force and speed, so diffusion occurs more slowly. Let’s check back with the real thing.

Ten minutes later, the dye in the cold water had still not diffused very much.

In fact in water at 0°C, the water molecules are bouncing into each other with an average speed of about 600 m/s (614 m/s), which is incredibly fast. In 90°C water though, they’re moving at an average speed of about 700 m/s which of course is much faster. As I said, these are only averages because there is a huge variation in speeds, just depending on how exactly they crash into each other. At any given moment, a water molecule might be moving much faster than the average but then if it crashes into another water molecule it might slow down to way below the average, just like a cue ball slows down when it hits a coloured ball.

Diffusion also occurs in the air. In this room, the air is still, but the atoms and molecules that make up the air are actually moving around.

If I spray some deodorant into the air, the atoms and molecules that make up the air randomly smash into the molecules that make up the fragrance of the deodorant and make them diffuse throughout the room.

In this experiment, these volunteers were asked to raise their hands when they could smell the deodorant, in other words, when the chemicals that make up the deodorant entered their noses. As you can see, they don’t all smell the deodorant straight away, because it takes time for the deodorant to diffuse throughout the room, in a similar kind of way to what we saw earlier with the food dye. Remember, there’s no wind carrying the fragrance, that would be different. The molecules of fragrance spread thanks to the trillions of atoms and molecules that make up the air smashing into them. It took about four minutes for the volunteer furthest away to finally smell the deodorant.

The fragrance eventually spreads out so much, that we can no longer smell it. Our noses can’t detect it if the concentration isn’t high enough.

The Kinetic Theory is also often called the Particle Theory. Dry air is made up mostly of pairs of nitrogen atoms, nitrogen molecules in other words, oxygen molecules, individual argon atoms, carbon dioxide molecules, and tiny amounts of a few other gases. (Air also contains water molecules. The amount varies but 1% is fairly normal.) Now rather than saying atoms or molecules, we often simply refer to all of these things as particles. So the particle theory is another name for the kinetic theory.

The word theory can sometimes be a little confusing. It can mean just a guess, but in science though, the word “theory” means an idea or a way of thinking that explains lots of different things.

Now we can’t see individual atoms, but the kinetic theory (or the particle theory), the idea that things are made of atoms that are constantly moving, is one of the best theories ever. It helps explain so many things including:

  • As we’ve just seen, diffusion;
  • What solids, liquids, and, gases are;
  • How and why things change state, like for example why solids melt into liquids;
  • Why things expand when they get hot;
  • How heat energy can pass through a substance like, say, from the fire underneath a pot to the water inside it;
  • What air pressure is;
  • Why chemical reactions occur faster when the chemicals that are chemically reacting are hot;
  • How rocket engines work;

and a whole lot of other things. We’ll be looking at some, but not all, of these things in this series. As you learn more and more about Science, the kinetic theory will come up again and again. Right now though, we’re talking heat and temperature.

Part D: Heat Energy

So giving an object’s temperature is a way of expressing, on a scale, the average kinetic energy of the atoms that make it up.

Heat is related to temperature but what exactly is heat? Well, heat is a form of energy and it’s the actual amount of energy that is transferred from a hotter substance to a colder substance. The amount of heat energy that is transferred is measured in Joules. The Joule, named after English scientist James Prescott Joule, is the unit for Energy, including Heat Energy.

If I place a container of hot water into a container of cold water, heat energy will transfer from the hot water to the cold water. You can tell because the hot water’s temperature decreases, whereas the cold water’s temperature increases. After five minutes the hot water’s temperature had decreased from 39°C to 29°C while the colder water’s temperature had increased from 20°C to 25°C. Not all of the heat energy that the hot water was losing was going into the cold water of course, a lot of it was escaping into the air, but a lot of it was flowing into the colder water. And when does the heat energy stop flowing? Well, when the two are at the same temperature. Heat energy will always transfer from a hotter object to a colder object and it will not transfer if the two objects are at the same temperature.

As I said heat energy is measured in Joules. We covered Joules in our Shedding Light on Energy unit but we should go over it again. Basically, if 1 kg of water is heated and its temperature increases by 1°C, let’s say 29°C to 30°C, then the water has absorbed 4200 Joules of heat energy. 1 Joule of energy is a very small amount of energy.

If you want to raise the temperature of 1 kg of water by 2°C, then you need a total of 8400 Joules of energy. Obviously the higher the temperature change, the more energy you need. To raise the temperature of 1 kg of water by 3°C requires 12,600 Joules. (3 times what you need for a 1°C change)

Now if you have 2kg of water and its temperature increases by 1°C, then it has to absorb double the amount of energy that 1 kg needs to absorb: 8,400 Joules.

The energy absorbed by water as its temperature increases is 4200 x the mass of the water x the temperature change of the water.

So how much energy is required to raise the temperature of 4 kg of water from 20°C to 90°C? Quite simply the amount of energy is 4200 x 4 x 70 (70 is the temperature change from 20°C to 90°C) which equals 1,176,000 Joules of energy.

Now heat energy transferring into a substance doesn’t necessarily cause a change in temperature. It can also cause the substance to change state, for example, to turn from a solid to a liquid, that is, to melt, or to change from a liquid to a gas, that is, to boil.

The Kinetic Theory of Matter can help us to explain how and why things change state and that’s what we’ll be looking at in our next episode. See you then.


The simulations of the solids, liquids, and gases were created by

PhET Interactive Simulations
University of Colorado Boulder