Chapter 7 — Temperature and its Measurement
Detailed solutions to every in-text “curiosity” question and every “Let us enhance our learning” exercise, with worked calculations, drawn thermometer diagrams, and the original textbook figures.
💭In-text Questions
No. Our sense of touch is not reliable for judging hotness or coldness accurately.
In Activity 7.1, the same water in container B feels cool to the hand that was in warm water and warm to the hand that was in cold water. So touch can only give a rough idea — to confirm a fever we must measure the body temperature with a thermometer.
By measuring its temperature using a thermometer. Temperature is a reliable measure of the hotness or coldness of a body — a hotter body has a higher temperature than a colder one.
They are non-contact thermometers, also called infrared thermometers. They measure a person’s temperature without touching the body, which helps reduce the risk of spreading disease.
Body temperature is influenced by age (as well as time of day and activity level). The value 37.0 °C is only an average of many healthy people, so a perfectly healthy person can have a normal temperature a little different from 37.0 °C.
In general, small children may have a normal body temperature slightly higher than that of adults — the exact “normal” varies from person to person.
Yes, generally. Since normal body temperature varies with age, healthy older people often have a normal temperature a little lower than that of young adults. Again, 37.0 °C is just an average — a healthy person’s normal value can be slightly above or below it.
That thermometer uses the Fahrenheit scale instead of the Celsius scale. It is the same temperature shown in a different unit:
So both readings describe the same normal body temperature — only the scale differs.
No. The temperatures of boiling water (about 100 °C) and of ice (0 °C) lie outside the range of a clinical thermometer, which is designed only for the narrow band of human body temperature (about 35 °C to 42 °C). For those, a laboratory thermometer is needed.
By using a laboratory thermometer. It has a much wider range — typically from −10 °C to 110 °C — so it can measure things like ice, warm water and boiling water that a clinical thermometer cannot.
By feeling the pulse rate. Fever raises a person’s pulse rate, and this was known even in olden days in India.
However, since several other situations also change the pulse rate, pulse rate alone is not a fully reliable indicator of fever.
No. A laboratory thermometer is not suitable for body temperature. Its reading starts falling as soon as it is taken out of contact, so it must be read while in contact — which is impractical and unhygienic for the body. It is also long and fragile and not designed to be placed in the mouth or armpit. For body temperature we use a clinical thermometer.
📘Let us enhance our learning
Answer: (ii) 37.0 °C — this is the accepted average normal body temperature (equal to 98.6 °F).
Answer: (iv) 98.6 °F. On the Fahrenheit scale, 37 °C equals 98.6 °F.
- (i) The hotness or coldness of a system is determined by its temperature.
- (ii) The temperature of ice-cold water cannot be measured by a clinical thermometer.
- (iii) The unit of temperature is degree Celsius.
Answer: (ii) −10 °C to 110 °C. (Option (iv), 35–42 °C, is the range of a clinical thermometer.)
Answer: Student 3. The correct way is to hold the thermometer vertically with the bulb fully immersed in the water but not touching the bottom or the sides of the beaker, and to read it while it is still immersed. Only Student 3 does all of this.
The others go wrong: the thermometer is tilted, or the bulb touches the bottom of the beaker, or the bulb is not properly immersed.
The red liquid should rise up to each marked temperature, as shown below:
For 7.5 °C the column stops halfway between the 5 °C and 10 °C marks (one small division = 1 °C, so half a division above 7 °C).
- (i) Type: a laboratory thermometer — its scale reads below 0 °C (from about −10 °C up to 40 °C), a range a clinical thermometer does not have.
- (ii) Reading: read the mark where the top of the liquid column lies — it coincides with the 10 °C mark, so the reading is 10 °C.
- (iii) Smallest value: between two big marks (10 °C apart) there are 10 small divisions, so one division = 10 ÷ 10 = 1 °C.
A laboratory thermometer’s liquid level starts falling the moment it is removed from the body (it has no arrangement to “hold” the reading), so it would have to be read while kept in the mouth — which is impractical and unhygienic.
It is also long and fragile and its range is much wider than needed, making it unsuitable and less precise for body temperature. A clinical thermometer, designed for this purpose, is used instead.
| Day | 7 am | 10 am | 1 pm | 4 pm | 7 pm | 10 pm |
|---|---|---|---|---|---|---|
| One | 38.0 | 37.8 | 38.0 | 38.0 | 40.0 | 39.0 |
| Two | 38.6 | 38.8 | 39.0 | 39.0 | 39.0 | 38.0 |
| Three | 37.6 | 37.4 | 37.2 | 37.0 | 36.8 | 36.6 |
- (i) Highest recorded temperature: 40.0 °C.
- (ii) When: on Day One at 7 pm.
- (iii) Returned to normal: on Day Three — the temperature reaches 37.0 °C (normal) by 4 pm and stays at/below normal after that.
Answer: thermometer (a). To read a value like 22.5 °C (which falls halfway between two whole degrees), we need a thermometer whose smallest division is 0.5 °C. Thermometer (a) has the finest divisions (0.5 °C), so it can show 22.5 °C accurately.
The other two have coarser divisions (1 °C or 2 °C), so they cannot read the exact half-degree value.
Answer: (ii) 27.5 °C. The top of the column lies halfway between the 25 °C and 30 °C marks — one small division is 0.5 °C, so the reading is 27.5 °C.
So each division measures 2 °C.
Between 10 °C and 20 °C there must be 10 ÷ 0.5 = 20 equal divisions. The scale looks like this:
She means the Fahrenheit scale (101 °F).
- 101 °C is close to the boiling point of water — a human body can never reach it (body temperature never goes much above about 42 °C).
- On the Fahrenheit scale, normal body temperature is 98.6 °F, so 101 °F is a typical fever reading. Hence Komal means 101 °F.