Chapter 7: Temperature and Its Measurement

Bhupendra Rawat -

 Chapter 7

Temperature and Its Measurement 

1. Hot or Cold?

  • Different objects can have different degrees of hotness or coldness.
  • Our sense of touch is not always reliable for judging temperature.
  • An object that feels hot to one hand may feel cool to the other.
  • Therefore, we need a reliable measurement of hotness or coldness.

2. Temperature

  • Temperature is a reliable measure of how hot or cold a body is.
  • A hotter body has a higher temperature than a colder body.
  • The instrument used to measure temperature is called a thermometer.

3. Types of Thermometers:- There are mainly two types discussed in the chapter:

Thermometer                                 Main Use

  • Clinical thermometer                         Measuring human body temperature
  • Laboratory thermometer                         Measuring temperatures of substances in experiments


4. Clinical Thermometer

  • A digital clinical thermometer is commonly used to measure body temperature.
  • It uses heat sensors and displays the temperature digitally.
  • The temperature is generally measured using the Celsius scale (°C).
  • Older mercury thermometers are being replaced by digital thermometers because mercury is toxic.
  • Non-contact/infrared thermometers can measure temperature without touching the body.

Normal Body Temperature

  • The average normal temperature of a healthy human body is about 37.0 °C or 98.6 °F.
  • The temperature of a healthy person may be slightly higher or lower than 37.0 °C.

Body temperature can be affected by:

  • Age
  • Time of the day
  • Activity level

5. Temperature Scales

Scale                                                 Unit                         Symbol

  • Celsius                                                 degree Celsius             °C
  • Fahrenheit                                         degree Fahrenheit     °F
  • Kelvin                                                 kelvin                       K

The SI unit of temperature is kelvin (K).

No degree symbol is used with kelvin.

37.0 °C = 98.6 °F

Celsius to Kelvin conversion:

  • Temperature in K = Temperature in °C + 273.15
  • Example:- 25 °C = 25 + 273.15 = 298.15 K

6. Precautions While Using a Digital Clinical Thermometer

  1. Read the instruction manual before use.

2. Wash the thermometer tip before and after use.

3. Keep the digital display away from water while washing.

4. Do not hold the thermometer by its tip.

5. Take the reading after the thermometer gives the required signal.

6. Clean and dry the thermometer after use.

7. Laboratory Thermometer

 

- A laboratory thermometer is used for measuring the temperature of substances during experiments.

- It consists of a long, narrow glass tube with a bulb containing liquid.

- The liquid column rises or falls when temperature changes.

- The temperature is read against the Celsius scale.

- The liquid may generally be coloured alcohol or mercury.

- A typical laboratory thermometer shown in the chapter has a range of –10 °C to 110 °C.

Smallest Division

  • The smallest temperature a thermometer can measure depends on its scale.
  • Smallest division = Temperature difference ÷ Number of divisions

Example:

Between 0 °C and 10 °C there are 10 divisions:

10 ÷ 10 = 1 °C

Therefore, the smallest division is 1 °C.     

8. Correct Use of a Laboratory Thermometer

  • The bulb should be fully immersed in the substance.
  • The bulb should not touch the bottom or sides of the container.
  • Keep the thermometer vertical.
  • Do not tilt it.
  • Read the temperature while the thermometer is immersed.
  • Keep your eye level with the top of the liquid column.
  • Handle the thermometer carefully because it can break.

Important

A laboratory thermometer is not suitable for measuring human body temperature because it does not have the same design/use as a clinical thermometer.

9. Temperature of Ice and Boiling Water

  • The temperature of ice remains constant while it is melting.
  • The temperature of water remains constant while it is boiling.
  • Different students may obtain slightly different readings because of errors in measurement or incorrect reading technique.    

10. Air Temperature

  • Air temperature tells us how hot or cold the surrounding air is.
  • Room thermometers give an approximate idea of room temperature.

Weather reports commonly provide:

  • Maximum temperature
  • Minimum temperature
  • Air temperature generally varies from day to day.
  • It usually rises as summer approaches and falls during winter.
  • Air-temperature data are important for weather forecasting.

11. Important Facts to Remember

  • Temperature → measure of hotness or coldness
  • Thermometer → measures temperature
  • Clinical thermometer → body temperature
  • Laboratory thermometer → experimental measurements
  • Normal average human body temperature → 37.0 °C
  • 37.0 °C → 98.6 °F
  • SI unit → kelvin (K)
  • Laboratory thermometer range in the chapter → –10 °C to 110 °C
  • Celsius to Kelvin → °C + 273.15
  • Touch alone is not a reliable method of measuring temperature.
  • The thermometer must be read correctly to obtain an accurate measurement.

Answers with Questions

1. The normal temperature of a healthy human being is close to ______.
(i) 98.6 °C
(ii) 37.0 °C
(iii) 32.0 °C
(iv) 27.0 °C  
Answer: (ii) 37.0 °C

2. 37 °C is the same temperature as ______.
(i) 97.4 °F
(ii) 97.6 °F
(iii) 98.4 °F
(iv) 98.6 °F
Answer: (iv) 98.6 °F

3. Fill in the blanks:
(i) The hotness or coldness of a system is determined by its ______.
Answer: Temperature
(ii) The temperature of ice-cold water cannot be measured by a ______.
Answer: Clinical thermometer
(iii) The unit of temperature is degree ______.
Answer: Celsius

4. The range of a laboratory thermometer is usually ______.
(i) 10 °C to 100 °C
(ii) –10 °C to 110 °C
(iii) 32 °C to 45 °C
(iv) 35 °C to 42 °C  
Answer: (ii) –10 °C to 110 °C
The chapter gives –10 °C to 110 °C as the range of the laboratory thermometer shown.    

5. Four students used a laboratory thermometer to measure the temperature of water as shown in Fig. 7.6. Who followed the correct way for measuring temperature?
(i) Student 1
(ii) Student 2
(iii) Student 3
(iv) Student 4  
Answer: Cannot be determined without Fig. 7.6.
Correct method: The bulb should be immersed in water, should not touch the bottom or sides of the beaker, and the thermometer should be held vertically.     

6. Colour to show the red column on the drawings of thermometers (Fig. 7.7) as per the temperatures written below:
- 14 °C
- 17 °C
- 7.5 °C
Answer: The liquid column should be coloured up to the corresponding temperature mark on each thermometer.
This is a drawing-based question; the exact colouring cannot be reproduced without Fig. 7.7.

7. Observe the part of thermometer shown in Fig. 7.8 and answer:
(i) What type of thermometer is it?
Answer: Laboratory thermometer.
(ii) What is the reading of the thermometer?
Answer: Cannot be determined without Fig. 7.8.
(iii) What is the smallest value that this thermometer can measure?
Answer: Cannot be determined without Fig. 7.8.
Method:
Smallest division = Temperature difference between two major marks ÷ Number of small divisions.     

8. A laboratory thermometer is not used to measure our body temperature. Give a reason.
Answer: A laboratory thermometer is designed for measuring the temperature of substances during experiments and is not designed for measuring human body temperature. A clinical thermometer is used for measuring body temperature.

9. Vaishnavi has not gone to school as she is ill. Her mother has kept a record of her body temperature for three days.
            Day   7 am   10 am   1 pm   4 pm   7 pm   10 pm
  • One         38.0 °C 37.8 °C 38.0 °C 38.0 °C 40.0 °C 39.0 °C
  • Two         38.6 °C 38.8 °C 39.0 °C 39.0 °C 39.0 °C 38.0 °C
  • Three 37.6 °C 37.4 °C 37.2 °C 37.0 °C 36.8 °C 36.6 °C

(i) What was Vaishnavi’s highest recorded temperature?
Answer: 40.0 °C
(ii) On which day and at what time was her highest temperature recorded?
Answer: Day One at 7 pm
(iii) On which day did Vaishnavi’s temperature return to normal?
Answer: Day Three
(37.0 °C was recorded at 4 pm.)

10. If you have to measure the temperature 22.5 °C, which of the following three thermometers will you use? Explain.
Answer: Use the thermometer whose range includes 22.5 °C and whose scale has a smallest division suitable for measuring 22.5 °C.
The exact option (a), (b), or (c) cannot be identified without Fig. 7.9.

11. The temperature shown by the thermometer in Fig. 7.10 is:
(i) 28.0 °C
(ii) 27.5 °C
(iii) 26.5 °C
(iv) 25.3 °C  
Answer: Cannot be determined from the text alone because Fig. 7.10 is required to read the thermometer.

12. A laboratory thermometer has 50 divisions between 0 °C and 100 °C. What does each division of this thermometer measure?
Solution:
Temperature difference = 100 °C − 0 °C
= 100 °C
Number of divisions = 50
Smallest division:
100 ÷ 50 = 2 °C
Answer: Each division measures 2 °C.

13. Draw the scale of a thermometer in which the smallest division reads 0.5 °C. You may draw only the portion between 10 °C and 20 °C.
Answer: The scale should have divisions of 0.5 °C:
10.0 → 10.5 → 11.0 → 11.5 → 12.0 → … → 19.5 → 20.0 °C
Thus, the difference between every two consecutive small divisions is 0.5 °C.

14. Komal tells you that she has a fever of 101 degrees. Does she mean it on the Celsius scale or Fahrenheit scale?
Answer: She most likely means 101 °F (Fahrenheit).
Reason: Normal body temperature is approximately 98.6 °F, and 101 °F is a commonly used body-temperature reading indicating fever. The chapter identifies 98.6 °F as equivalent to 37.0 °C.    

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