What this chapter is about
This chapter introduces you to the idea of temperature. Temperature tells us how hot or cold something is. You already know that a glass of lassi from the fridge feels cold, while a cup of tea feels hot. But how do we measure this hotness or coldness? That is what this chapter teaches.
You will learn about a device called a thermometer, which helps us measure temperature in numbers. You will also learn about different scales used to read temperature, like Celsius. By the end, you should be able to read a thermometer correctly and understand why our sense of touch alone cannot always tell us the true temperature of things.
Understanding temperature is useful in daily life. It helps us know if we have fever, if water is safe to touch, or if the weather is cold enough for a sweater.
Key ideas
- Temperature is a measure of how hot or cold an object is.
- Our sense of touch can give us a rough idea about temperature, but it is not reliable for exact measurement.
- A thermometer is a device used to measure temperature accurately.
- The unit of temperature we commonly use is degree Celsius, written as °C.
- A clinical thermometer is used to measure body temperature. It usually shows readings from 35 °C to 42 °C.
- A laboratory thermometer is used in science experiments and can measure a wider range of temperatures.
- Normal human body temperature is around 37 °C.
- Heat flows from a hotter object to a colder object until both reach the same temperature.
Formulas and facts to remember
- Temperature: A measure of how hot or cold something is.
- Thermometer: A device that measures temperature.
- Degree Celsius (°C): The common unit for measuring temperature.
- Normal body temperature: About 37 °C.
- Clinical thermometer range: Usually 35 °C to 42 °C.
- Freezing point of water: 0 °C (when water turns to ice).
- Boiling point of water: 100 °C (when water turns to steam).
- Heat always moves from a hotter body to a colder body.
Worked examples
Example 1: Reading a thermometer
Riya looks at a clinical thermometer after placing it under her tongue for a minute. The silver liquid line has risen to a mark between 36 and 38. The small lines show that it is exactly at 37 °C.
What does this reading mean?
Solution: The reading of 37 °C means Riya's body temperature is normal. She does not have fever.
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Example 2: Comparing temperatures
Arjun's mother boils water for tea. She says the water temperature is 100 °C. His little sister is drinking milk at 40 °C. Which is hotter?
Solution: The boiling water at 100 °C is hotter than the milk at 40 °C. A higher number in °C means higher temperature.
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Example 3: Why touch is not reliable
Priya dips her right hand in warm water and her left hand in cold water for one minute. Then she puts both hands in a bucket of room-temperature water. Her right hand feels the water is cool, but her left hand feels the same water is warm.
Solution: This happens because our hands got used to different temperatures. The same water feels different to each hand. This shows that our sense of touch cannot give exact temperature. A thermometer would show the same reading for that water no matter which hand we use.
Common mistakes
- Thinking the liquid inside a thermometer is always mercury → Many thermometers now use alcohol or digital sensors.
- Holding the thermometer by the bulb while reading → This warms the bulb and gives a wrong reading.
- Believing our hands can tell exact temperature → Our sense of touch is unreliable; always use a thermometer.
- Forgetting to shake down a clinical thermometer before use → The old reading stays and mixes with the new one.
- Confusing temperature with heat → Temperature tells how hot something is; heat is the energy that flows between objects.
Quick revision
- Temperature measures how hot or cold an object is.
- A thermometer gives us the exact temperature in numbers.
- We measure temperature in degree Celsius (°C).
- Normal human body temperature is about 37 °C.
- Water freezes at 0 °C and boils at 100 °C.
- Our touch can fool us; a thermometer cannot.