Temperature and its Measurement
Why This Matters
Imagine you feel weak and tired. Your head feels warm. Do you have a fever?
Someone touches your forehead. They say, “Yes, you feel hot.” But is that really true? Their hands might just be cold. Then everything would feel hot to them.
So how do we find out for sure? We use a small tool called a thermometer. It gives us a number. That number tells us exactly how hot or cold something is.
This chapter is about that number. It is called temperature. You will learn why our hands cannot be trusted to judge hot and cold. You will learn how a thermometer works. And you will learn to read one the right way.
This is useful for life, not just for the exam. A correct temperature reading tells a doctor whether you are sick. It tells a cook whether the oil is hot enough. It tells a scientist whether an experiment is going right.
The Big Idea
Our sense of touch is not reliable. The same water can feel warm to one hand and cool to the other, at the very same moment. So we cannot trust our hands to tell us how hot a thing is. Instead, we measure hotness with a tool called a thermometer. It gives us a clear number called the temperature, measured in degrees Celsius (°C). A number does not get confused. That is why a measurement beats a feeling.
Let’s Break It Down
Hot and cold are relative — and why our hands fool us
First, a simple idea. The words “hot” and “cold” are not fixed. They depend on what you compare with.
Think about a glass of tap water. On a hot summer afternoon, it feels cool. On a cold winter morning, the same water feels warm. The water did not change much. You changed. So “hot” and “cold” are relative — they only make sense when you compare two things.
Now here is the bigger problem. Our sense of touch can give wrong answers. Let us see this with a famous activity.
Take three big bowls. Put warm water in bowl A. Put plain tap water in bowl B. Put ice-cold water in bowl C.
Now dip your right hand in the warm water (A) and your left hand in the cold water (C). Keep them there for about one minute.
Then quickly take both hands out. Put both hands together into the same tap water (B).
What happens? Your right hand says, “This feels cool.” Your left hand says, “This feels warm.” But both hands are in the same water at the same time! Figure 7.1 below shows exactly what is going on.
So why does this happen? Our skin does not measure the real hotness of the water. It only feels the change from what it was used to. The right hand got used to warm water, so the tap water feels colder by comparison. The left hand got used to cold water, so the same tap water feels warmer by comparison.
This is the “nothing is a given” lesson. Touch does not tell the truth about hotness. It only tells you what changed. That is why we need a proper tool.
Both your hands are in the same bowl of water, yet one feels it as warm and the other as cool. Why?
Because each hand got used to a different temperature first. The hand that was in warm water now feels the bowl as cool. The hand that was in cold water now feels the same bowl as warm. Our skin only senses the change, not the real hotness — so touch is unreliable.
Temperature — a fair measure
Since our hands cannot be trusted, we need something that does not get confused. That something is temperature.
Temperature is a proper measure of how hot or cold a body is. A hotter body has a higher temperature. A colder body has a lower temperature.
The big advantage is this. Temperature gives a number. A number is the same for everyone. It does not depend on whose hand it is or what that hand felt before. Two people may feel the water differently, but the thermometer gives one fixed number they both agree on.
Temperature is a fair, reliable measure of hotness. It gives a number that everyone agrees on, no matter what their hands felt before.
A thermometer is the tool we use to measure temperature. Let us look at it next.
The thermometer and the Celsius scale
A thermometer is a device that measures temperature.
One common type is a glass thermometer. Look at Figure 7.2 below. It has a few simple parts.
Here is how it works. The bulb holds a coloured liquid. When the bulb gets warmer, the liquid expands (gets bigger) and climbs up the thin tube. When it gets cooler, the liquid shrinks back down. So the height of the liquid tells you the temperature. You just read the mark on the scale next to the top of the liquid.
The marks on the side make up a scale. The scale used for temperature is the Celsius scale. On this scale, temperature is measured in degrees Celsius. We write it as °C.
A few everyday numbers help you picture it:
Water freezes into ice at 0 °C.
Water boils at 100 °C.
A healthy human body is about 37 °C.
So if you see “30 °C” on a weather report, that is a warm-ish day. If you see “5 °C”, that is very cold.
Laboratory vs clinical thermometer — how and why they differ
There are two kinds of thermometer you will meet. They look similar but they do different jobs.
A clinical thermometer measures the temperature of the human body. A doctor or your family uses it to check for fever.
A laboratory thermometer measures the temperature of many other things — like water in a beaker, or the air in a room. It is used in science labs.
Why are they different? Because they need different things.
The human body never gets very hot or very cold. A living person’s temperature stays roughly between 35 °C and 42 °C. So a clinical thermometer only needs that small range. Its scale runs from about 35 °C to 42 °C — just enough for the body.
A clinical thermometer also has a clever trick: a tiny narrow bend in the tube called a kink. The kink stops the liquid from sliding back down once you take the thermometer out of your mouth. This way the reading “stays” so you have time to read it. (In a modern digital clinical thermometer, the number simply stays on the screen instead.)
A laboratory thermometer must measure all sorts of things, hot and cold. So it needs a wide range — usually about −10 °C to 110 °C. It has no kink, because you read it while it is still in the water, so the liquid does not need to be held in place.
Figure 7.3 below puts them side by side so the difference is clear.
Here is the same comparison as a table.
| Feature | Clinical thermometer | Laboratory thermometer |
|---|---|---|
| What it measures | The human body | Water, air, many other things |
| Range | About 35 °C to 42 °C | About −10 °C to 110 °C |
| Has a kink? | Yes — it holds the reading | No |
| How you read it | After taking it out | While it is still in the liquid |
| Best for | Checking fever | Lab experiments |
Now you can see why a clinical thermometer cannot measure boiling water. Boiling water is at 100 °C. But the clinical scale stops at 42 °C. The number is simply not on it. The liquid would push past the top — and the thermometer could even break.
Reading a thermometer correctly
A thermometer only gives the right answer if you read it the right way. There is one main rule.
Keep your eye level with the top of the liquid. That means look straight across, not from above and not from below. If you look from an angle, the mark you see does not line up with the real level, and you read the wrong number. Figure 7.4 below shows the correct and wrong eye positions.
A few more simple rules for a laboratory thermometer:
- Hold it upright (vertical). Do not tilt it.
- When it is in water, the bulb should not touch the bottom or sides of the beaker — only the water.
- Read it while the bulb is still in the water. The moment you lift it out, the liquid starts to fall and the reading changes.
Let us read one value together.
On a laboratory thermometer, there is a mark for 20 °C and a mark for 30 °C. Between them are 10 equal small lines (divisions). The top of the liquid is sitting exactly 4 small lines above the 20 °C mark. What is the temperature?
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First, find how much one small line is worth. The gap from 20 °C to 30 °C is 10 °C. This gap is split into 10 equal small lines.
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So each small line stands for 10 °C ÷ 10 = 1 °C.
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The liquid is 4 small lines above 20 °C. That means 4 × 1 °C = 4 °C above 20 °C.
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Add it up: 20 °C + 4 °C = 24 °C. So the temperature is 24 °C.
Normal body temperature
So what is a healthy temperature for a person?
The normal temperature of a healthy human body is about 37 °C. That is the number to remember.
But here is a gentle point. Not everybody is exactly 37 °C. Some healthy people are a little above it, some a little below. The 37 °C figure is really an average taken from very many healthy people. Your own normal might be slightly different and still be perfectly fine.
Your temperature also changes a bit during the day. It depends on your age, the time of day, and how active you are. After running around, you might be a touch warmer. That is normal.
A living person’s body almost never goes below 35 °C or above 42 °C. If the thermometer reads clearly above 37 °C — say 38 °C or 39 °C — that usually means a fever. Figure 7.5 below shows these key numbers on a scale.
Two healthy friends measure their temperature. One reads 36.8 °C and the other reads 37.2 °C. Does this mean one of them is sick?
No. Normal body temperature is about 37 °C, but it is an average. Healthy people can be a little above or below it. Both 36.8 °C and 37.2 °C are very close to 37 °C, so both friends are perfectly normal.
A quick word on safe use. Always wash the tip of a clinical thermometer with soap and water before and after use, so germs do not spread. Handle a glass thermometer gently — if it hits something hard, it can break.
Common Mistakes
Let us clear up a few ideas that trip students up.
If something feels hot when you touch it, your hand is telling you its true temperature.
Touch feels so direct and instant that it seems like a reliable test — after all, you can clearly feel the difference between hot and cold things every day.
Touch only senses the change from what your skin was used to, not the real hotness. The two-bowls activity proves it: the same water can feel warm to one hand and cool to the other at the same time. Only a thermometer gives the true temperature.
A clinical thermometer can be used to measure the temperature of boiling water.
A thermometer is a thermometer, so it seems like one type should work for anything hot or cold.
A clinical thermometer's scale only goes up to about 42 °C, because the body never gets hotter than that. Boiling water is 100 °C — far off the scale. To measure boiling water you need a laboratory thermometer, which goes up to about 110 °C.
Every healthy person must have a temperature of exactly 37 °C.
The number 37 °C is repeated so often as 'normal' that it sounds like a fixed rule that everyone must match exactly.
37 °C is just an average of many healthy people. A healthy person can be slightly above or below it. Your temperature also shifts a little with age, time of day, and activity. Small differences are normal.
It does not matter what angle you look from when reading a thermometer.
The mark and the liquid both look clear, so it feels like you would read the same number from any position.
If you look from above or below, the liquid level appears to line up with the wrong mark, and you read a wrong number. You must keep your eye level with the top of the liquid and look straight across.
Quick Check
Test yourself with these.
Why can't we trust our sense of touch to measure how hot something is?
What is the normal temperature of a healthy human body?
Which thermometer would you use to measure the temperature of warm water in a beaker?
When reading a thermometer, where should your eye be?
Practice Problems
Easy
What is the unit of temperature on the Celsius scale, and how do we write it?
The unit is the degree Celsius. We write it as °C. For example, normal body temperature is about 37 °C.
Name the two main types of thermometer and say what each is used for.
The two types are the clinical thermometer and the laboratory thermometer. The clinical thermometer is used to measure the human body temperature (to check for fever). The laboratory thermometer is used to measure the temperature of other things, like water and air, in experiments.
Medium
A clinical thermometer's scale goes from 35 °C to 42 °C. Can it measure the temperature of ice (0 °C) or boiling water (100 °C)? Explain.
No, it cannot measure either one. Its scale only covers 35 °C to 42 °C, the range of the human body. Ice is at 0 °C, which is far below the scale. Boiling water is at 100 °C, which is far above the scale. Both numbers are simply not on a clinical thermometer. You would need a laboratory thermometer (range about −10 °C to 110 °C) for these.
On a laboratory thermometer, the gap between the 0 °C mark and the 10 °C mark is divided into 10 equal small lines. How much does one small line stand for?
The gap from 0 °C to 10 °C is 10 °C. It is split into 10 equal small lines. So each small line = 10 °C ÷ 10 = 1 °C. Each small line stands for 1 °C.
Challenge
Riya dips one hand in hot water and the other in cold water for a minute. Then she puts both hands in a bucket of tap water. Her hot-water hand feels the bucket as cool, but her cold-water hand feels it as warm. Riya says, 'The bucket has two temperatures at once!' Is she right? Explain what is really happening.
Riya is not right. The bucket of tap water has only one temperature. The two hands give different feelings, but that is about the hands, not the water.
Here is why. Our skin does not measure real hotness. It only senses the change from what it was used to. Riya’s hot-water hand had got used to hot water, so the tap water feels cooler by comparison. Her cold-water hand had got used to cold water, so the same tap water feels warmer by comparison.
So the different feelings come from the two hands, not from the water. The water has one true temperature — and a thermometer would show that single number. This is exactly why we trust a thermometer and not our touch.
On a laboratory thermometer, there is a mark for 30 °C and a mark for 40 °C, with 10 equal small lines between them. The top of the liquid sits 7 small lines above the 30 °C mark. What temperature is it?
First, find the value of one small line. The gap from 30 °C to 40 °C is 10 °C, divided into 10 equal lines. So one small line = 10 °C ÷ 10 = 1 °C.
The liquid is 7 small lines above 30 °C. That is 7 × 1 °C = 7 °C above 30 °C.
So the temperature = 30 °C + 7 °C = 37 °C.
Summary
- “Hot” and “cold” are relative — they depend on what you compare with. The same tap water can feel warm in winter and cool in summer.
- Our sense of touch is not reliable. The two-bowls activity shows the same water can feel warm to one hand and cool to the other, because skin only senses the change, not the real hotness.
- Temperature is a fair measure of how hot or cold a body is. It gives a number everyone agrees on.
- A thermometer measures temperature. Its bulb holds a liquid that rises when warmed and falls when cooled.
- Temperature is measured on the Celsius scale, in degrees Celsius (°C). Water freezes at 0 °C and boils at 100 °C.
- A clinical thermometer (range about 35 °C to 42 °C, has a kink) measures the body. A laboratory thermometer (range about −10 °C to 110 °C, no kink) measures other things.
- Read a thermometer with your eye level with the top of the liquid, looking straight across.
- The normal temperature of a healthy human body is about 37 °C — an average, so small differences are normal.
What’s Next
You now know how to measure how hot or cold a thing is. Next, we follow water as it gets hotter and colder, and watch it change form.
In Chapter 8 — A Journey through States of Water, you will see how water turns into ice when it cools, and into steam when it heats up — and how it changes back again. The temperature numbers you learned here (0 °C and 100 °C) will be the key to that whole story.
Frequently Asked Questions
Why can't we use our hand to judge if something is hot or cold?
Our hands are not reliable because they compare the new object to whatever temperature they already are. If your hands are very cold, even a lukewarm object will feel hot. If your hands are already warm, a slightly cool object will feel cold. This makes our hands a bad measuring tool because two people can feel the same object differently. A thermometer always gives the same number.
What is normal body temperature in Celsius?
Normal human body temperature is about 37 °C (degrees Celsius), though healthy people vary a little either side. Doctors say you have a fever when your temperature is clearly above 37 °C, around 38 °C or more. A clinical thermometer is designed to measure only the range around body temperature, usually from about 35 °C to 42 °C, because that is the only range doctors need for checking health.
What is the difference between a clinical thermometer and a laboratory thermometer?
A clinical thermometer measures body temperature and has a narrow range of about 35 °C to 42 °C. It has a kink (a bend) in the glass tube just above the bulb to stop the coloured liquid from falling back on its own, so you can read it after removing it from the body. A laboratory thermometer has a much wider range (usually -10 °C to 110 °C) and has no kink because it must be read while still inside the liquid being measured.
How do you read a thermometer correctly class 6?
Hold the thermometer at eye level with the bulb end away from you. Look straight at it — not from above or below. Find where the top of the coloured liquid column stops and read the number on the scale at that exact point. Reading from an angle gives a wrong value because the line of sight crosses the wrong marking. Always read before the coloured liquid falls back.
Why does the coloured liquid rise in a thermometer when heated?
Liquids expand (take up more space) when they get hot and contract (take up less space) when they cool down. In a thermometer the liquid is inside a very thin tube, so even a tiny expansion pushes the liquid a long way up the tube where you can easily read it. The thin tube makes the expansion visible — in a wide tube the same expansion would barely be seen.