1. Chapter at a Glance
When you have fever, your mother touches your forehead to check if you are “hot”. But is touch a reliable way to judge how hot something is? This chapter shows that our sense of touch can mislead us, and introduces temperature — a reliable measure of hotness or coldness — and the instruments used to measure it: clinical and laboratory thermometers.
- Why touch cannot be trusted to judge hotness.
- Temperature and its scales: Celsius, Fahrenheit and Kelvin.
- Clinical thermometers (digital and non-contact) and normal body temperature.
- Laboratory thermometer — range, least count and correct use.
- Maximum and minimum air temperature in weather reports.
2. Hot or Cold? — Touch Can Mislead
Take three bowls: warm water, tap water and ice-cold water. Dip one hand in warm water and the other in ice-cold water for a minute. Now put both hands in the tap water. The hand from warm water feels the tap water cold; the hand from ice-cold water feels the same water warm! The same water feels different to two hands. So we cannot always rely on our sense of touch to judge hotness.
3. Temperature and its Scales
Temperature is a reliable measure of the hotness or coldness of a body. It is measured with a thermometer. Three temperature scales are used:
| Scale | Unit | Ice melts | Water boils | Used for |
|---|---|---|---|---|
| Celsius | degree Celsius (°C) | 0 °C | 100 °C | Daily life, labs, weather |
| Fahrenheit | degree Fahrenheit (°F) | 32 °F | 212 °F | Body temperature in some places |
| Kelvin | kelvin (K) — SI unit | 273.15 K | 373.15 K | Science |
The lowest possible temperature, called absolute zero, is about −273.15 °C, which is 0 K. Nothing can be cooled below it. Note that we write “K” without the degree sign.
4. Clinical Thermometer
A clinical thermometer measures human body temperature. Today digital thermometers are common; older ones used mercury, which is extremely toxic, so mercury thermometers are being replaced.
- Normal human body temperature is taken as 37.0 °C (98.6 °F).
- Body temperature varies slightly with age, time of day and activity; it normally does not go below 35 °C or above 42 °C.
- Placed in the armpit, a thermometer reads about 0.5–1 °C lower than the actual body temperature.
- Non-contact (infrared) thermometers measure temperature without touching the body — useful in airports and hospitals.
- Wash the tip with soap and water (or wipe with antiseptic).
- Switch it on and place the tip under the tongue (or in the armpit).
- Wait for the beep; then read the display.
- Clean it again and switch off.
5. Laboratory Thermometer
A laboratory thermometer measures the temperature of things other than the human body, like water or ice. A common one has a range of about −10 °C to 110 °C.
Between the 0 °C and 10 °C marks there are 10 small divisions. Each division = (10 − 0) ÷ 10 = 1 °C. If there were 20 divisions, each would be 0.5 °C.
Correct way of using a laboratory thermometer
- Hold the thermometer vertically.
- Immerse the bulb in the liquid so that it is surrounded on all sides, but it should not touch the bottom or sides of the container.
- Wait until the reading becomes steady.
- Read it while the bulb is still in the liquid, with the eye level with the top of the liquid column.
| Feature | Clinical thermometer | Laboratory thermometer |
|---|---|---|
| Purpose | Body temperature | Water, solutions, ice, air |
| Typical range | About 35 °C – 42 °C | About −10 °C – 110 °C |
| Reading | Can be read after removing from the mouth | Must be read while immersed |
| Not to be used for | Boiling water (may break) | Body temperature (range and use not suitable) |
When water is boiling, its temperature stays constant (about 100 °C at sea level) as long as it boils. Melting ice also stays at 0 °C until all of it melts. Different students may get slightly different readings for boiling water because of small differences in thermometers, reading positions or impurities.
6. Air Temperature
Weather reports give the maximum (highest, usually afternoon) and minimum (lowest, usually early morning) air temperatures of a day. Recording them for many days shows how weather changes with seasons.
Chart — example of a weather record (illustrative values for one week in winter):
Illustrative numbers in °C; bar length shows the maximum temperature.
7. Master Table: Key Values
| Quantity | Value |
|---|---|
| SI unit of temperature | kelvin (K) |
| Normal body temperature | 37.0 °C = 98.6 °F |
| Ice melts / water boils (Celsius) | 0 °C / 100 °C |
| Ice melts / water boils (Fahrenheit) | 32 °F / 212 °F |
| Absolute zero | −273.15 °C = 0 K |
| Laboratory thermometer range | about −10 °C to 110 °C |
| Least count | (difference between two marked values) ÷ (number of divisions) |
8. Master Summary Table
| Topic | Key idea | Exam point |
|---|---|---|
| Touch | Not reliable | Three-bowl activity |
| Scales | °C, °F, K | K is SI unit, written without ° |
| Clinical | Body temperature | 37 °C normal; digital replaces mercury |
| Laboratory | Other objects | Vertical, not touching, read while immersed |
| Weather | Max and min | Reading data tables |
9. Mind Map
- Degree of hotness
- Touch unreliable
- Celsius, Fahrenheit
- Kelvin (SI)
- 37 °C normal
- Digital, infrared
- −10 to 110 °C
- Correct handling
10. Tips and Tricks
- Remember “37 – 98.6” as the normal body temperature pair.
- Kelvin = Celsius + 273 (approximately) — handy for quick checks (e.g. 27 °C ≈ 300 K).
- Laboratory thermometer rules: V-N-R — Vertical, No touching, Read while immersed.
- Writing “°K” — the correct symbol is just K.
- Taking the lab thermometer out of water before reading — the reading changes.
- Using a clinical thermometer for boiling water — its range is too small and it may break.
11. Important One-liners
- Temperature is a reliable measure of hotness or coldness.
- Our sense of touch is not a reliable judge of temperature.
- A thermometer measures temperature.
- The SI unit of temperature is kelvin.
- Normal human body temperature is 37 °C (98.6 °F).
- Mercury is toxic, so digital thermometers are preferred.
- Infrared thermometers measure temperature without contact.
- Water boils at 100 °C and ice melts at 0 °C (at normal pressure).
- Absolute zero is about −273.15 °C or 0 K.
12. Exam-pattern Questions with Detailed Answers
The SI unit of temperature is:
- degree Celsius
- degree Fahrenheit
- kelvin
- joule
Show answer
Kelvin (K) is the SI unit. Celsius and Fahrenheit are other scales; joule is the unit of energy.
Answer: (C) kelvin
Fill in the blank: Normal human body temperature is ______ °C.
Show answer
It is taken as 37.0 °C, which equals 98.6 °F.
Answer: 37
Which thermometer measures temperature without touching the body?
Show answer
Infrared (non-contact) thermometers detect heat radiation from the body.
Answer: Infrared / non-contact thermometer
Why can’t we use a clinical thermometer to measure the temperature of boiling water?
Show answer
Its range is only about 35–42 °C, while boiling water is about 100 °C. The thermometer cannot show such a reading and may get damaged.
Answer: Range too small; may break
A laboratory thermometer has 5 divisions between 20 °C and 30 °C. What is its least count?
Show answer
(30 − 20) ÷ 5 = 2 °C per division.
Answer: 2 °C
List three precautions while using a laboratory thermometer.
Show answer
Keep it vertical; the bulb should not touch the bottom or sides of the vessel; read it while the bulb is still in the liquid, with the eye level with the column.
Answer: Vertical, bulb not touching, read while immersed
Why is the digital thermometer preferred over the mercury thermometer?
Show answer
Mercury is extremely toxic; if a mercury thermometer breaks, it harms health and the environment. Digital thermometers are safe, quick and easy to read.
Answer: Mercury is toxic; digital is safer and easier
Explain the three-bowl activity and what it shows.
Show answer
One hand in warm water and the other in ice-cold water, then both in tap water: the tap water feels cold to the first hand and warm to the second. It shows that touch is unreliable for judging temperature.
Answer: Same water feels different — touch unreliable
Four students recorded the temperature of the same boiling water as 99.5 °C, 100 °C, 100.5 °C and 99 °C. Give two possible reasons.
Show answer
Small differences between thermometers; reading errors (eye not level with the column); bulb touching the hot bottom of the vessel; impurities in water or local air pressure. These cause slight variations.
Answer: Instrument differences and reading errors
A doctor reads 36.5 °C with a thermometer in a child’s armpit. Is the child likely to have fever? Explain.
Show answer
Armpit readings are about 0.5–1 °C lower than the actual body temperature, so the actual temperature is about 37–37.5 °C — normal or very slightly high. The child is not likely to have a significant fever.
Answer: Probably no fever — armpit reads lower
Convert 27 °C into kelvin and explain why there is no negative temperature on the Kelvin scale.
Show answer
K = °C + 273.15 → 27 + 273.15 = 300.15 K (≈ 300 K). The Kelvin scale starts at absolute zero (0 K = −273.15 °C), the lowest possible temperature, so nothing can be below 0 K.
Answer: ≈ 300 K; scale starts from absolute zero
13. Quick Revision Summary
- Touch is unreliable; temperature measures hotness with a thermometer.
- Scales: Celsius (°C), Fahrenheit (°F), Kelvin (K, SI unit); absolute zero ≈ −273.15 °C.
- Clinical thermometer: body temperature, normal 37 °C (98.6 °F); digital and infrared types.
- Laboratory thermometer: about −10 to 110 °C; keep vertical, do not touch the vessel, read while immersed.
- Boiling water and melting ice stay at constant temperature while changing state.