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Temperature and its Measurement

NCERT Class 6 · Science Based on NCERT Class 6 Science textbook · Free CBSE study kit

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Chapter Notes

Overview
This chapter introduces the concept of temperature as a reliable measure of hotness or coldness of a body, moving beyond our unreliable sense of touch. It explains the need for thermometers, different types (clinical and laboratory), their scales (Celsius, Fahrenheit, Kelvin), and correct usage. The one big idea a student must walk away with is that temperature is measured accurately using a thermometer, not by touch, and that different thermometers are designed for different purposes and ranges.
Key Concepts (14)
Temperature
A reliable measure of the hotness (or coldness) of a body. A hotter body has a higher temperature than a colder body.
e.g. During summers, tap water feels hotter than cold water from a matka (earthen pot) because the tap water has a higher temperature.
Thermometer
A device that measures temperature.
e.g. A digital clinical thermometer used to check if a person has fever.
Clinical Thermometer
A thermometer used to measure human body temperature. Modern ones are digital and run on batteries, using heat sensors.
e.g. The thermometer Phiban used to check Lambok's temperature.
Laboratory Thermometer
A thermometer used for measuring temperatures in a laboratory, typically having a range from –10 °C to 110 °C. It contains a liquid (usually coloured alcohol or mercury) in a bulb that rises or falls with temperature.
e.g. The thermometer used in Activity 7.5 to measure the temperature of warm water in a beaker.
Celsius Scale
A temperature scale where the unit is degree Celsius, denoted by °C. The normal human body temperature is 37.0 °C on this scale.
e.g. Water boils at 100 °C and freezes at 0 °C on the Celsius scale.
Fahrenheit Scale
A temperature scale where the unit is degree Fahrenheit, denoted by °F. 37.0 °C is equivalent to 98.6 °F.
e.g. A friend's digital thermometer showing normal body temperature as 98.6 °F.
Kelvin Scale
The SI unit of temperature, denoted by K. Temperature in Kelvin = Temperature in Celsius + 273.15.
e.g. 0 °C is equal to 273.15 K.
Normal Body Temperature
The average body temperature of a large number of healthy people, taken to be 37.0 °C (or 98.6 °F). A perfectly healthy person may have a temperature slightly different from this.
e.g. Lambok's temperature was normal, meaning it was close to 37.0 °C.
Range of a Thermometer
The lowest and highest temperature a thermometer can measure.
e.g. The range of a laboratory thermometer is usually from –10 °C to 110 °C.
Smallest Division (Least Count)
The smallest value that a thermometer can read. For the thermometer in Fig. 7.3b, one small division reads 1 °C.
e.g. If a thermometer has 10 divisions between 0 °C and 10 °C, each division is 1 °C.
Air Temperature
The temperature of the air, an important weather parameter monitored at weather stations. It is reported as maximum and minimum air temperature for the day.
e.g. Weather reports on TV mention the maximum and minimum air temperature of a place.
Infrared Thermometer
A non-contact thermometer that can measure temperature without touching a person's body, reducing the risk of spreading disease.
e.g. Thermometers used during the COVID-19 pandemic to measure temperature from a distance.
Mercury Thermometer
An older type of thermometer containing mercury, which is toxic. These are being replaced by digital thermometers due to safety concerns.
e.g. Mercury thermometers were used earlier but are now avoided because mercury is extremely toxic.
Heat Sensor
A component in a digital thermometer that helps determine temperature.
e.g. Temperature in a digital thermometer is determined with the help of heat sensors.
How It Works
Measuring Body Temperature Using a Digital Clinical Thermometer
  1. Wash your hands and the tip of the digital thermometer with soap and water.
  2. Reset the thermometer by pressing the reset button.
  3. Place the thermometer under the tongue and close your mouth.
  4. Wait till the thermometer makes a beeping sound or flashes a light.
  5. Take it out from the mouth and read the temperature on the digital display.
  6. Record the temperature.
  7. Clean the tip of the thermometer with soap and water, and dry it.
💡 Do not hold the thermometer by the tip, and keep the digital portion (display) out of water while washing.
Measuring Temperature Using a Laboratory Thermometer
  1. Take some warm water in a beaker.
  2. Dip the thermometer in water so that the bulb is immersed in water.
  3. Ensure the bulb does not touch the bottom or sides of the beaker.
  4. Hold the thermometer vertically; do not tilt it.
  5. Observe the rise of the liquid column in the thermometer.
  6. Wait till the column stops rising.
  7. Read the temperature while the thermometer is still immersed in water, with your eye directly in line with the level of the liquid column.
💡 The temperature must be read while the thermometer is immersed in water because as soon as you take it out, the liquid column begins to fall.
Finding the Range of a Laboratory Thermometer
  1. Take a laboratory thermometer and observe it carefully.
  2. Note the lowest temperature it can measure (e.g., –10 °C).
  3. Note the highest temperature it can measure (e.g., 110 °C).
  4. The range is from the lowest to the highest temperature.
💡 Different thermometers may have different ranges; always check the thermometer you are using.
Finding the Smallest Division (Least Count) of a Laboratory Thermometer
  1. Observe the thermometer carefully.
  2. Note the temperature difference between two bigger marks (e.g., between 0 °C and 10 °C is 10 °C).
  3. Count the number of small divisions between these two bigger marks (e.g., 10 divisions).
  4. Divide the temperature difference by the number of divisions (e.g., 10 °C ÷ 10 = 1 °C).
  5. This value is the smallest temperature the thermometer can read.
💡 The smallest division is also called the least count of the thermometer.
Converting Temperature from Celsius to Kelvin
  1. Take the temperature in Celsius scale.
  2. Add 273.15 to it.
  3. The result is the temperature in Kelvin scale.
💡 The Kelvin scale does not use the degree symbol (°); it is written as K.
Exam Traps — Don't Make These Mistakes
⚠ Confusing the Range of Clinical and Laboratory Thermometers
✗ A clinical thermometer can measure temperatures from –10 °C to 110 °C.
→ The range –10 °C to 110 °C is for a laboratory thermometer, not a clinical thermometer. A clinical thermometer is designed only for body temperature (roughly 35 °C to 42 °C).
✓ A clinical thermometer is used to measure human body temperature and has a range suitable for that (e.g., 35 °C to 42 °C), while a laboratory thermometer typically has a range from –10 °C to 110 °C.
⚠ Thinking Normal Body Temperature is Exactly 37.0 °C for Everyone
✗ The normal body temperature of every healthy person is exactly 37.0 °C.
→ 37.0 °C is the average body temperature of a large number of healthy people. A perfectly healthy person may have a temperature slightly different from 37.0 °C due to factors like age, time of day, and activity level.
✓ The normal body temperature of a healthy human is taken to be 37.0 °C, but it can vary slightly from person to person and is influenced by factors such as age, time of day, and activity level.
⚠ Reading the Thermometer After Taking It Out of the Liquid
✗ Take the laboratory thermometer out of the water and then read the temperature.
→ As soon as you take the thermometer out of the water, the level of the liquid column begins to fall, so the reading will not be accurate.
✓ The temperature must be read while the thermometer is still immersed in the water.
⚠ Writing the Unit for Kelvin Incorrectly
✗ The temperature is 300 °K.
→ The Kelvin scale does not use the degree symbol (°). The correct symbol is just 'K' (capital letter, no degree sign). Also, 'kelvin' is written with a lower-case 'k' when spelled out.
✓ The temperature is 300 K.
⚠ Assuming Pulse Rate Alone Indicates Fever
✗ In olden days, fever was detected by checking the pulse rate, which is a reliable indicator.
→ While fever affects pulse rate, other situations also affect pulse rate. Therefore, pulse rate alone is not a reliable indicator of fever.
✓ In olden days, fever was detected by pulse rate, but pulse rate alone is not a reliable indicator of fever because other factors can also affect it.
⚠ Using a Laboratory Thermometer to Measure Body Temperature
✗ A laboratory thermometer can be used to measure body temperature.
→ A laboratory thermometer has a much wider range (–10 °C to 110 °C) and is not designed for the narrow range of body temperature. Also, it is not safe or practical for oral use.
✓ A laboratory thermometer is not used to measure body temperature because its range is too wide and it is not designed for that purpose. A clinical thermometer should be used instead.
Compare & Contrast
Purpose and usage
Clinical Thermometer
Laboratory Thermometer
A clinical thermometer is used to measure human body temperature, while a laboratory thermometer is used for measuring temperatures in a laboratory for various purposes (e.g., water, air).
Unit and normal body temperature reading
Celsius Scale
Fahrenheit Scale
On the Celsius scale, the unit is degree Celsius (°C) and normal body temperature is 37.0 °C. On the Fahrenheit scale, the unit is degree Fahrenheit (°F) and normal body temperature is 98.6 °F.
Safety and ease of use
Mercury Thermometer
Digital Thermometer
Mercury thermometers contain toxic mercury and are difficult to dispose of if broken, while digital thermometers pose no such risk and have an easy-to-read digital display.
Exam Focus
Marks pattern: This chapter typically appears with 2-3 MCQs (1 mark each), 1 Very Short Answer (1 mark), 1 Short Answer (2-3 marks), and occasionally a Long Answer (5 marks) on the correct method of using a thermometer or comparing thermometers.
Most asked: Definition of temperature and thermometer · Normal human body temperature (37.0 °C / 98.6 °F) · Range and least count of a laboratory thermometer · Correct way to use a laboratory thermometer (bulb not touching sides, read while immersed, eye level) · Difference between clinical and laboratory thermometers · Conversion between Celsius and Kelvin
Don't skip: Describe the correct procedure to measure the temperature of water using a laboratory thermometer (5 marks).; Differentiate between a clinical thermometer and a laboratory thermometer (3 marks).; What is the normal body temperature? Why might it vary slightly from person to person? (2-3 marks)
One-Line Revision (read aloud before the exam)
  1. Temperature is a reliable measure of how hot or cold a body is.
  2. A thermometer is a device used to measure temperature.
  3. A clinical thermometer is used to measure human body temperature.
  4. The normal body temperature of a healthy human is taken as 37.0 °C or 98.6 °F.
  5. A laboratory thermometer typically has a range from –10 °C to 110 °C.
  6. While using a laboratory thermometer, the bulb should not touch the bottom or sides of the container.
  7. The temperature must be read while the thermometer is still immersed in the liquid.
  8. The eye should be directly in line with the liquid level when reading a thermometer.
  9. The SI unit of temperature is kelvin (K).
  10. Temperature in Kelvin = Temperature in Celsius + 273.15.
  11. The Celsius scale uses degree Celsius (°C) and the Fahrenheit scale uses degree Fahrenheit (°F).
  12. Digital thermometers are safer than mercury thermometers because mercury is toxic.
  13. Pulse rate alone is not a reliable indicator of fever.
  14. The smallest division of a thermometer is called its least count.
  15. Air temperature is an important weather parameter and varies daily.

MCQs — 10 Questions with Answers

Q1. What is the normal body temperature of a healthy human being?

  • A. 37.0°C ✓
  • B. 40.0°C
  • C. 35.0°C
  • D. 42.0°C

Answer: A — The normal body temperature of a healthy human is 37.0°C, which is the average temperature measured in many healthy people.

Q2. Which of the following is a device used to measure temperature?

  • A. Barometer
  • B. Thermometer ✓
  • C. Compass
  • D. Telescope

Answer: B — A thermometer is the tool designed specifically to measure the temperature of a body or substance.

Q3. Why should we not rely only on our sense of touch to decide if something is hot or cold?

  • A. Because our skin is not sensitive
  • B. Because our senses can be tricked by previous sensations ✓
  • C. Because hot things always feel cold
  • D. Because we cannot feel anything with our hands

Answer: B — The activity showed that the same water can feel warm to one hand (that was in cold water) and cold to another hand (that was in warm water) at the same time, proving our senses can trick us.

Q4. What is the Celsius scale used for in a clinical thermometer?

  • A. To measure distance
  • B. To measure weight
  • C. To measure temperature ✓
  • D. To measure time

Answer: C — The Celsius scale (°C) is the temperature measurement scale used in clinical thermometers to show body temperature.

Q5. Why are digital thermometers preferred over mercury thermometers?

  • A. Because they are cheaper
  • B. Because mercury is poisonous and dangerous if the thermometer breaks ✓
  • C. Because they are slower
  • D. Because they can only measure high temperatures

Answer: B — Digital thermometers are safer because mercury is a toxic substance that can harm you if accidentally released from a broken thermometer.

Q6. What should you do before using a digital clinical thermometer?

  • A. Boil it in water
  • B. Wash the tip with soap and water ✓
  • C. Heat it under the sun
  • D. Keep it in the refrigerator

Answer: B — According to the precautions, you must wash the tip of the thermometer with soap and water before and after each use to keep it clean.

Q7. In Activity 7.1, when you dip your hands in containers A and C first, then put both in container B, what happens?

  • A. Both hands feel the same temperature in B
  • B. Right hand feels B is cool, left hand feels B is warm ✓
  • C. Container B becomes colder
  • D. You cannot feel anything in B

Answer: B — The hand from warm water (A) feels B as cool, while the hand from cold water (C) feels B as warm—showing our senses can give different feelings for the same water.

Q8. What is the lowest temperature that a healthy human body normally reaches?

  • A. 30°C
  • B. 32°C
  • C. 35°C ✓
  • D. 40°C

Answer: C — The chapter states that healthy human body temperature does not normally go below 35°C.

Q9. __________ is a reliable measure of how hot or cold a body is.

  • A. Touch
  • B. Temperature ✓
  • C. Color
  • D. Weight

Answer: B — Temperature, measured with a thermometer and shown as a number, is the reliable way to know exactly how hot or cold something is.

Q10. When measuring body temperature using a digital thermometer under the tongue, what should you do while waiting?

  • A. Keep your mouth open
  • B. Talk to your friend
  • C. Close your mouth and wait for the beep ✓
  • D. Move the thermometer around

Answer: C — According to Activity 7.2, you must close your mouth after placing the thermometer under your tongue and wait until it makes a beeping sound.

Flashcards

What is temperature?

Temperature is a reliable measure of how hot or cold something is, shown as a number in degrees.

What is a thermometer?

A thermometer is a tool or device that measures the temperature of something.

What is a clinical thermometer used for?

A clinical thermometer is used to measure the body temperature of a human being.

What is the normal body temperature of a healthy person?

The normal body temperature of a healthy human is 37.0°C.

Why can we not always trust our sense of touch to know if something is hot or cold?

Our sense of touch can be tricked because the same water can feel warm to one hand and cold to another hand at the same time.

What is the Celsius scale?

The Celsius scale is a way to measure temperature where the unit is degree Celsius, written as °C.

Why are digital thermometers better than mercury thermometers?

Digital thermometers are safer because mercury is poisonous and can hurt you if the thermometer breaks.

Where should you place a digital thermometer to measure body temperature?

Place the thermometer under your tongue and close your mouth until it beeps.

What should you do with a thermometer before and after using it?

Wash the tip of the thermometer with soap and water before and after using it.

What is the range of normal body temperature in healthy humans?

A healthy human body temperature does not normally go below 35°C or above 42°C.

Important Board Questions

What is temperature? [1 mark]

Temperature is a reliable measure of how hot or cold something is; it is shown as a number using degrees (°C for Celsius).

Why can we not always trust our sense of touch to know if something is hot or cold? Explain with an example from Activity 7.1. [2 marks]

In Activity 7.1, the same water (container B) felt warm to the left hand (which was in cold water) and cool to the right hand (which was in warm water). This shows our senses can be tricked by what they felt before.

Describe the steps you would follow to measure your own body temperature using a digital clinical thermometer. Why should you wash the thermometer before and after use? [3 marks]

Steps: wash tip, reset, place under tongue, close mouth, wait for beep, read display, wash tip again. Washing prevents spreading germs and keeps the thermometer clean for accurate readings.

Draw and label a digital clinical thermometer. Explain: (a) What is the normal body temperature of a healthy human? (b) Why are digital thermometers better than mercury thermometers? (c) Why does body temperature vary slightly in different healthy people? [5 marks]

Label: tip, display screen, reset button. Normal temp = 37.0°C. Digital thermometers are safer because mercury is poisonous. Temperature varies due to age, time of day, and activity level.

True or False: A person who has a body temperature of 36.5°C must have a fever. Give a reason for your answer. [2 marks]

False—because normal body temperature is 37.0°C, but healthy people can have normal temperatures slightly lower or higher (like 36.5°C) depending on age, time of day, and activity level. A perfectly healthy person is not automatically sick with just a 36.5°C reading.

Assertion-Reason Questions for Temperature and its Measurement

Q1. Assertion (A):

A clinical thermometer should always be washed with soap and water before and after use.

Reason (R):

This prevents the spread of germs and maintains hygiene when measuring body temperature.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true, and washing prevents disease spread, which is exactly why we should do it — the reason correctly explains the assertion.

Q2. Assertion (A):

Our sense of touch cannot always correctly tell us if something is hot or cold.

Reason (R):

Because our sense of touch depends on our previous experience and what we touched before.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both statements are true. Activity 7.1 proved that the same water felt different to each hand depending on whether it was previously in hot or cold water — this is why we cannot rely on touch alone.

Q3. Assertion (A):

Digital thermometers have replaced mercury thermometers in measuring body temperature.

Reason (R):

Because digital thermometers work with electricity while mercury thermometers work with heat.

Answer: C — A is true but R is false — The assertion is true, but the reason is false. Mercury thermometers were replaced because mercury is toxic and dangerous, not because of how they work. Digital thermometers use heat sensors and are safer.

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Case-Based Questions (CBQ) for Temperature and its Measurement

Case Study:

Priya's mother was cooking dal and rice for lunch in their kitchen in Delhi. Suddenly, Priya complained that her head was aching and she felt very warm. Her mother touched her forehead with her hand and thought Priya might have a fever. To confirm this properly, her mother took out a digital clinical thermometer from the medical box, cleaned its tip with soap and water, and measured Priya's temperature. The thermometer showed 38.5°C. Her mother gave her some rest and medicine.

Q1. Why did Priya's mother use a thermometer instead of just relying on her touch to check for fever?

  • A. Because touch can be deceived and a thermometer gives accurate readings ✓
  • B. Because thermometers are more expensive
  • C. Because touch cannot feel heat at all
  • D. Because her hand was wet from cooking

Answer: A

Q2. What was Priya's body temperature?

  • A. 37.0°C which is normal
  • B. 38.5°C which is higher than normal ✓
  • C. 35.0°C which is very low
  • D. 40.0°C which is very high

Answer: B

Q3. What precaution did Priya's mother take before measuring temperature?

  • A. She boiled the thermometer in water
  • B. She cleaned the tip with soap and water ✓
  • C. She submerged the digital display in water
  • D. She reset the thermometer after measuring

Answer: B

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