Educational Galaxy

Temperature and Its Measurement — EG's own notes, in simple words

Book: Curiosity (CBSE, Class 6)  |  Chapter 7  |  Medium: English

Diagram (labels are printed on the image): A glass thermometer held in a hand, with its bulb and red liquid column. The scale marks are shown only as a design, not as a reading.

Checking body temperature when a child is unwell. A general picture about using a thermometer.

Diagram (labels are printed on the image): A digital clinical thermometer for body temperature (under the tongue, beep, display) and a laboratory thermometer (−10 °C to 110 °C) for experiments. Never use a laboratory thermometer on the body.

What the chapter is about

Temperature tells us how hot or cold a body is. Our sense of touch can mislead us badly. Try the classic test: keep one hand in hot water and the other in cold water for a minute, then put both hands into a bowl of lukewarm water. The same water feels cold to the hand that was in hot water, and warm to the hand that was in cold water. Touch judges by comparison, not by true temperature, so we need an instrument — the thermometer.

A thermometer works because a liquid inside it expands when heated and contracts when cooled. In a glass thermometer, the liquid sits in a bulb at the bottom and rises up a thin tube as it warms. The tube carries a scale marked in degrees Celsius (°C). On the Celsius scale, water freezes at 0 °C and boils at 100 °C at normal pressure. Another scale, Fahrenheit (°F), is used in some countries: water freezes at 32 °F and boils at 212 °F. In India, temperatures are usually given in degrees Celsius.

Two kinds of thermometers are used in Class 6 work. A clinical thermometer measures body temperature. In this chapter it is a digital clinical thermometer: it runs on a battery, shows the temperature on a display, and is used under the tongue — place it under the tongue, close the mouth, and wait for the beep or flashing light, then read the display. Before and after use, wash its tip with soap and water, press the reset button before measuring, do not hold it by the tip, and keep the display part out of water; always read the instruction manual first. For small children or old people it may be placed in the armpit, but that reading is about 0.5 °C to 1 °C lower than the actual body temperature. Human body temperature does not normally go below 35 °C or above 42 °C, and normal body temperature is about 37 °C — though it varies a little with age, time of day and activity. Earlier, mercury thermometers were used for body temperature, but mercury is extremely toxic and difficult to dispose of if the thermometer breaks, so mercury thermometers are being replaced by digital ones, which read temperature with heat sensors. Older mercury clinical thermometers had a kink — a narrow bend near the bulb that held the liquid in place so the reading could be taken after removing the thermometer — and had to be shaken below 35 °C before use; the chapter's digital thermometer has no kink and needs no shaking.

A laboratory thermometer measures the temperature of things in experiments, such as water being heated. Its range is much wider, about −10 °C to 110 °C. It has a long, narrow, uniform sealed glass tube with a bulb of liquid at one end; the liquid is usually alcohol (coloured red so it is easily seen) or mercury, and the top of the liquid column against the Celsius scale gives the reading. It must never be used to measure body temperature: it is made for experiments, not for the body. In the same way, boiling water and ice are outside a clinical thermometer's range, so a clinical thermometer cannot measure them either.

Reading a thermometer correctly matters. Keep the thermometer upright and vertical, never tilted, let the liquid settle, and bring the eye directly in line with the level of the liquid column — never read from above or below. When a laboratory thermometer is immersed in water, its bulb must be fully immersed but must not touch the bottom or the sides of the beaker, and the temperature must be read while the thermometer is still immersed, because the liquid column begins to fall as soon as it is taken out. Do not hold the thermometer by its bulb, because the warmth of the hand changes the reading, and handle it with care because it can break. And never put a laboratory thermometer in the mouth or under the arm.

What you should learn

• Explain why touch is unreliable for temperature, with the three-bowl test.
• Explain how a thermometer works: liquid expands on heating, contracts on cooling.
• Tell clinical (digital, body) from laboratory (experiments) by use and range — never mix them up; the kink belongs only to older mercury clinical thermometers.
• Read a thermometer correctly: upright, settled, eye level with the column, bulb untouched.
• State normal body temperature (37 °C) and the Celsius fixed points (0 °C, 100 °C).

Chart / Table: Two kinds of thermometers

Table: Two kinds of thermometers

Table: Celsius fixed points

Practice set — try it yourself

1. Why can we not trust touch alone to judge temperature?
   Hint: Think about one hand warm and one cold.
2. What is normal body temperature?
   Hint: Recall the standard value in Celsius.
3. How do you use a digital clinical thermometer?
   Hint: Think about under the tongue and the beep.
4. Which thermometer would you use to measure the temperature of hot tea? Why?
   Hint: Think about range.
5. A thermometer reads 39 °C for a sick child. What does this suggest?
   Hint: Compare with normal body temperature.

Questions with Answers

1. What is temperature? Why is touch not reliable for measuring it?
   Temperature is a measure of how hot or cold a body is. Touch is not reliable because it depends on what the hand felt before — the same water feels warm to a cold hand and cold to a warm hand — so two people can judge the same object differently.
2. What is the difference between a clinical thermometer and a laboratory thermometer?
   A clinical thermometer measures body temperature. In this chapter it is digital: it is used under the tongue, beeps, and shows the reading on a display, and body temperature normally stays between 35 °C and 42 °C. A laboratory thermometer measures temperatures in experiments, has a wider range of about −10 °C to 110 °C, and its reading comes from the top of a liquid column against a scale.
3. Why did older clinical thermometers have a kink, and why does the chapter's digital thermometer not need one?
   The kink was a narrow bend near the bulb in older mercury clinical thermometers. It stopped the liquid falling back quickly when the thermometer was removed from the mouth or armpit, so the temperature could be read after taking it out, and the thermometer had to be shaken below 35 °C before use. The chapter's digital thermometer shows the reading on a display and holds it, so it needs neither a kink nor shaking.
4. What is normal body temperature? What does a reading of 39 °C mean?
   Normal body temperature is about 37 °C. A reading of 39 °C is above normal and suggests the person has a fever and may need rest and medical advice.
5. Why should a laboratory thermometer not be used to measure body temperature?
   Because it is made for experiments, not for the body: its bulb and glass are not made for safe use in the mouth, and its reading must be taken while it is still immersed — the liquid column begins to fall as soon as it is taken out, so no stable body reading can be taken. The digital clinical thermometer, used under the tongue, is the right instrument.

More NCERT-exercise questions — extra practice

1. Why can we not trust our sense of touch to measure temperature correctly?
   Because touch judges by comparison with what the hand felt before. After hot water, lukewarm water feels cold; after cold water, the same water feels warm. Only a thermometer gives the true temperature.
2. Fill in the blanks: A clinical thermometer has a range of about ______ °C to ______ °C, and normal body temperature is about ______ °C.
   35; 42; 37.
3. True or False: A laboratory thermometer can be used to measure body temperature because its range is wider. Give a reason.
   False. A laboratory thermometer is made for experiments, not for the body: it must be read while immersed, and it is not designed for safe use in the mouth. The digital clinical thermometer is the right instrument for body temperature.
4. Older mercury clinical thermometers had a kink. What was it, and why does the chapter's digital thermometer not need one?
   The kink was a narrow bend near the bulb that stopped the liquid falling back when the thermometer was removed from the body, so the temperature could be read calmly afterwards. A digital thermometer shows the reading on a display and holds it, so no kink and no shaking are needed.
5. What precautions do you take with a digital clinical thermometer?
   Read the instruction manual first. Wash the tip with soap and water before and after use, press the reset button, place it under the tongue and close the mouth, and wait for the beep or flashing light before reading the display. Do not hold it by the tip, and keep the display part out of water.
6. How does a liquid thermometer show a rise in temperature?
   The liquid in the bulb expands when heated, so it rises up the thin tube. The top of the liquid column against the scale shows the temperature.
7. List two precautions while reading a thermometer.
   Keep the thermometer upright and let the liquid settle; bring the eye to the level of the liquid column; and do not hold the thermometer by its bulb, because hand warmth changes the reading.
8. A thermometer breaks and mercury spills. What should you do?
   Do not touch the mercury with bare hands. Tell an adult immediately, leave the room ventilated, and let the adult clean it up safely, because mercury is toxic.
9. Convert 37 °C into Fahrenheit (formula: °F = °C × 9/5 + 32). Why is this only an extra skill, not a replacement for Celsius?
   37 × 9/5 + 32 = 66.6 + 32 = 98.6 °F. It is an extra skill because temperatures in India are normally reported in Celsius; the Celsius scale is the one to master first.
10. How do you find the smallest value a laboratory thermometer can read? A thermometer has 50 divisions between 0 °C and 100 °C. What does each division measure?
    Find the temperature difference between two big marks and divide by the number of small divisions between them. In the chapter's example, 10 °C between big marks divided by 10 divisions gives 1 °C per division. For 50 divisions between 0 °C and 100 °C, each division measures 100 ÷ 50 = 2 °C. Always look carefully at the thermometer you are about to use, because range and smallest value differ between thermometers.
11. List the correct way of measuring temperature with a laboratory thermometer.
    Handle it with care (it can break) and do not hold it by the bulb. Immerse the bulb fully in the water so that it touches neither the bottom nor the sides of the beaker. Hold the thermometer vertically, never tilted. Read the temperature while it is still immersed, with the eye directly in line with the level of the liquid column.
12. What is a room thermometer, and what do weather reports tell us about air temperature?
    A room thermometer is hung on a wall (in a laboratory, clinic or hospital) and gives an approximate idea of the room temperature. Weather reports give the maximum and minimum air temperature of the day, and these vary from day to day — generally rising towards summer and falling towards winter.
13. Komal says she has a fever of 101 degrees. Does she mean Celsius or Fahrenheit?
    Fahrenheit. 101 °C is far above any body temperature (the body normally stays between 35 °C and 42 °C), while 101 °F is a fever reading — about 38.3 °C, above the normal 37 °C. The chapter gives the equivalence 37.0 °C = 98.6 °F; the Fahrenheit scale is not used in most scientific studies anymore.
14. Vaishnavi's mother recorded her body temperature for three days. Day One reached 40.0 °C at 7 pm; Day Two stayed around 38–39 °C; Day Three fell from 37.6 °C to 36.6 °C. Answer: (i) highest temperature, (ii) when, (iii) on which day did it return to normal?
    (i) 40.0 °C. (ii) Day One at 7 pm. (iii) Day Three, when readings came back to about 37 °C and below — the fever had broken.
15. You must measure a temperature of 22.5 °C. Which thermometer do you use — a clinical thermometer or a laboratory thermometer? Why?
    The laboratory thermometer. Its range (about −10 °C to 110 °C) includes 22.5 °C, while a clinical thermometer only covers body temperatures (normally 35 °C to 42 °C) and cannot measure 22.5 °C at all.
16. What is the SI unit of temperature? Convert normal body temperature (37 °C) into kelvin.
    The SI unit is the kelvin (K), and temperature in kelvin = temperature in Celsius + 273.15. So 37 °C = 37 + 273.15 = 310.15 K. Note that no degree sign (°) is written with K.
17. For small children or old people, a digital thermometer is sometimes placed in the armpit. How does that reading compare with the actual body temperature?
    It is about 0.5 °C to 1 °C lower than the actual body temperature, so the real temperature is a little higher than the armpit reading.
18. Students in Shillong measured boiling water at about 97.8–98.1 °C, not 100 °C. Why might boiling water not read exactly 100 °C, and can a clinical thermometer measure boiling water?
    The boiling point depends on the place and pressure — at a hill station like Shillong it is a little below 100 °C, and different students may also read slightly differently if they do not all follow the correct way of reading. A clinical thermometer cannot measure boiling water (or ice) at all, because those temperatures are outside its body-temperature range.

How to use this page: Read the summary, say the Key Terms in your own words, then try the practice set without looking. Use the Questions with Answers to check yourself afterwards. Ask your teacher or parent to check your answers.