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Hot and cold

Measurement · level 1, lesson 6 of 6

A question to start

Try this with a grown-up. Fill three bowls with water: one cold (from the tap with an ice cube in it), one warm (nice bath warm, never hot), and one in between.

coldin betweenwarm

Put one hand in the cold bowl and the other hand in the warm bowl. Count slowly to 30. Now put both hands into the in-between bowl at the same time.

Something strange happens. The hand that was in the cold water says, "This is warm!" The hand that was in the warm water says, "This is cold!" But it is the same water. Your two hands disagree!

So hands are not good judges of hot and cold. In this lesson you will learn the words for hot and cold, how to put things in order from coldest to hottest, and the tool that does not get fooled: the thermometer.

The idea

Hot, warm, cool, cold

How hot or cold something is, is called its temperature. We have many words for it, from coldest to hottest:

  • freezing: so cold that water turns to ice, like a snowy winter day.
  • cold: like a glass of water with ice in it, or the inside of a fridge.
  • cool: a bit cold, like a fall morning or a shady spot.
  • warm: like a sunny spring day, or a bath.
  • hot: like a summer afternoon at the beach, or soup on the stove.

When we compare two things, we say one is warmer (or hotter) and the other is colder (or cooler). A cup of cocoa is warmer than a glass of milk from the fridge. An ice cube is colder than both.

Be safe: never touch something to find out if it is hot. Stoves, kettles, irons and soup can burn you. Ask a grown-up. That is another good reason to have a tool that measures temperature for us.

Why your hands get fooled

Your skin does not really feel "how hot the water is". It feels whether heat is coming into your hand or going out of it. The cold hand had become cold. When it went into the in-between water, heat flowed into it, so it felt warm. The warm hand had become warm. In the same water, heat flowed out of it, so it felt cold. Your hands told you about themselves, not about the water. A fair judge must not change with what it touched before.

The thermometer

A thermometer is a tool that measures temperature. One kind is a thin glass tube with a little bulb at the bottom. Inside is a coloured liquid.

summer day winter day

Here is the most important thing to know:

When it gets warmer, the line goes up. When it gets colder, the line goes down.

On the summer day, the line is high. On the winter day, the line is low. So the higher the line, the warmer it is.

Why does the line move? Almost everything gets a tiny bit bigger when it warms up, and a tiny bit smaller when it cools down. The liquid in the bulb does this too. When it warms up, it grows, and the only place it can go is up the thin tube, so the line climbs. When it cools down, it shrinks back into the bulb, so the line drops. The tube is very thin, so even a tiny bit of growing makes the line move a long way, and we can see it easily.

The little marks on the side are like the marks on a ruler. They help us say how much the line went up or down. You will learn to read the numbers next to the marks in level 3. For now, just remember: higher line, warmer; lower line, colder.

Many thermometers today are digital: they show a number on a little screen instead of a line. A bigger number means warmer. Doctors and parents use a thermometer to check if you have a fever, because when you are sick your body can get a little warmer than usual.

Putting temperatures in order

To put things in order from coldest to hottest, compare them two at a time, just like lengths and weights. If the soup is hotter than the bath, and the bath is warmer than the tap water, then the soup is hotter than the tap water.

Seasons

In much of Canada, the weather changes a lot through the year.

  • Winter is the coldest season. Snow, ice, mittens.
  • Spring gets warmer. Snow melts, plants grow.
  • Summer is the warmest season.
  • Fall (also called autumn) gets cooler. Leaves change colour and fall.

Then winter comes again. The seasons go round and round, like the days of the week.

Cold, cold Mars

Mars is farther from the Sun than Earth is, so it gets less of the Sun's warmth. Mars is very cold. On a usual day there, it is colder than almost any winter day anywhere in Canada. A thermometer on Mars would have its line very, very low most of the time. The robot rovers that drive around on Mars carry heaters to keep their parts from getting too cold.

Worked examples

Example 1. Coldest to hottest. Put these in order from coldest to hottest: a bath, an ice cube, a cup of hot cocoa, a glass of tap water.

  1. The coldest is the ice cube: it is frozen.
  2. Tap water is cool, warmer than ice. So tap water is next.
  3. A bath is warm, warmer than tap water. So bath is next.
  4. Hot cocoa is hottest of all. So the order is ice cube, tap water, bath, hot cocoa.

Example 2. Read the thermometers. Which thermometer shows the warmest temperature? Which shows the coldest?

A B C
  1. The warmer it is, the higher the line.
  2. B's line is the highest, so B is the warmest.
  3. C's line is the lowest, so C is the coldest.
  4. From coldest to warmest: C, A, B.

Example 3. Morning and afternoon. A thermometer hangs outside a window. Look at it in the morning and in the afternoon.

morning afternoon
  1. In the afternoon the line is higher than in the morning.
  2. A higher line means warmer.
  3. So it got warmer from morning to afternoon. The line went up by 3 little marks.
  4. Why? The Sun had been shining on the ground all day, warming it up. On many days the afternoon is warmer than the early morning.

Try it

  1. Which is colder: a snowball or a cup of tea?
  2. Put in order from coldest to hottest: a sunny summer day, a fridge, a fire.
  3. A thermometer's line goes down. Is it getting warmer or colder?
  4. Two thermometers: in one, the line is at the 6th mark; in the other, at the 2nd mark. Which shows the warmer place?
  5. Why is it not a good idea to touch a pot to find out if it is hot?
  6. Jo held her hand in snow, then touched cool water. She said, "The water is warm!" Is the water really warm? What happened?
  7. Which season is usually the warmest in Canada? Which is the coldest?
  8. In the morning a thermometer outside is at the 3rd mark. In the afternoon it is at the 8th mark. Did it get warmer or colder? By how many marks?
Answers
  1. The snowball. It is frozen; tea is hot.
  2. A fridge (cold), then a sunny summer day (warm or hot), then a fire (very hot).
  3. Colder. The liquid shrinks when it cools, so the line goes down.
  4. The one at the 6th mark. A higher line means warmer.
  5. You might burn yourself. A hot pot can look exactly like a cold one. Ask a grown-up, or use a thermometer.
  6. The water is cool, not warm. Jo's hand was very cold from the snow, so heat flowed from the water into her hand, and that felt warm. Hands tell you about themselves, not about the water. This is why we use a thermometer.
  7. Summer is usually the warmest. Winter is usually the coldest.
  8. Warmer, because the line went up. From the 3rd mark to the 8th mark is 4, 5, 6, 7, 8: 5 marks.

Common mistake

Trusting your hands. Your hands are fooled by what they touched before. A thermometer is not.

Reading the thermometer upside down. The line goes up when it is warmer, not colder. Think of the liquid waking up and climbing when it gets warm, and curling up small in the bulb when it gets cold.

Where it leads