Overview
This chapter begins with a simple question: if a child feels feverish, can we tell just by touching the forehead? A three-bowl activity shows that our sense of touch is easily fooled, so we need a reliable measure of hotness and coldness. That measure is temperature, and the device that measures it is the thermometer. You will learn how to use a digital clinical thermometer safely, why the normal body temperature of 37.0 °C is only an average, and why digital thermometers have replaced mercury ones. You will meet the three temperature scales, Celsius, Fahrenheit and Kelvin, with their units and symbols, learn that the kelvin is the SI unit, and convert Celsius to kelvin by adding 273.15. Next you study the laboratory thermometer: how to find its range and the value of its smallest division, how to read it, and the rules for using it correctly, including why the temperature of melting ice and boiling water stays steady. Finally you look at air temperature in weather reports, the work of the Indian scientist Anna Mani, and the idea of absolute zero. Throughout, the lesson is that careful, correct measurement matters.
हिंदी में पढ़ें
स्पर्श हमें धोखा क्यों देता है, तापमापी कैसे काम करते हैं, सेल्सियस, फ़ारेनहाइट और केल्विन पैमाने, और ताप को सही ढंग से कैसे मापें।
Learning Objectives
- Explain why the sense of touch is not a reliable way to judge hotness or coldness
- Define temperature and state what a thermometer does
- Use a digital clinical thermometer safely and interpret normal body temperature
- Name the Celsius, Fahrenheit and Kelvin scales with their units, and write temperatures correctly
- Convert temperatures between degree Celsius and kelvin
- Find the range and the smallest division of a laboratory thermometer and read it
- Follow the correct method and precautions for using a laboratory thermometer
- Use maximum and minimum air temperatures and describe the contribution of Anna Mani
Topics in this chapter
7 topics · tap a topic title to jump straight to it.
Hot or cold: can we trust our touch?
1Think first
2The idea, in plain words
When a child at home says “I feel feverish”, the first thing an elder often does is place a hand on the child’s forehead. A warm forehead seems to mean fever. But is a hand a good judge of how hot something is? Let us test it.
The three-bowl test. Take three bowls. Fill the first with warm water (not hot enough to hurt), the second with ordinary tap water and the third with ice-cold water. Keep your right hand in the warm bowl and your left hand in the cold bowl for a minute or two. Now take both hands out and put them together into the middle bowl of tap water.
Something surprising happens. Your right hand, which came from warm water, says the tap water is cool. Your left hand, which came from cold water, says the very same water is warm. One bowl of water cannot be warm and cool at the same moment. So the trouble is not with the water. It is with our sense of touch.
Why does this happen? Our skin does not measure how hot a thing is. It only notices whether the thing feels hotter or colder than the skin itself was a moment ago. A hand that has just been in warm water is warm, so tap water feels cool to it. A hand that has just been in ice water is cold, so the same tap water feels warm.
Touch also fails in other ways:
- A thing that is too hot or too cold can burn or hurt the skin, so we cannot touch it at all.
- Two people may give different answers for the same object.
- Touch can only say “hotter” or “colder”. It cannot give a number that we can write down, compare or share with a doctor.
So, to say how hot or cold something is, we need a reliable measure and a device that gives the same answer every time, whoever uses it. That measure is temperature, and the device is a thermometer. The rest of this chapter is about both.
3Worked example
- Water from a matka feels cooler than water from a tap in summer, but touch cannot say by how much.
- After holding an ice cube, a glass of tap water feels warm.
- A mother feels a child’s forehead, but uses a thermometer to be sure about fever.
4Your turn
Why can touch NOT be used to measure how hot a thing is?
5Remember this
Temperature and the thermometer
1Think first
2The idea, in plain words
We need a word for “how hot or how cold a thing is”. That word is temperature. A hotter body has a higher temperature and a colder body has a lower temperature. A cup of tea just made has a high temperature; an ice cube has a low one.
Temperature lets us compare two things with numbers. If one cup of water is at 60 degrees and another at 30 degrees, we know at once which is hotter and also by how much: the first is 30 degrees hotter. The difference in temperature tells us how much hotter one body is than another.
The thermometer. A device that measures temperature is called a thermometer. The word comes from two Greek parts: “thermo” meaning heat and “meter” meaning measure. Unlike our hand, a thermometer gives a number, and it gives the same number to everyone who reads it correctly.
Two kinds you will meet in school.
| Kind | Used for | Typical range |
| Clinical thermometer | Measuring the temperature of the human body | about 35 °C to 42 °C |
| Laboratory thermometer | Measuring the temperature of water, other liquids, air and many other things in experiments | usually −10 °C to 110 °C |
The range of a thermometer is the lowest and the highest temperature it can show. A clinical thermometer has a short range because the human body never goes far outside it. A laboratory thermometer needs a wider range because in experiments we heat and cool things a lot.
Why use the right one? Each thermometer is made for its own job. A clinical thermometer cannot measure boiling water (about 100 °C) or ice (0 °C), because both are far outside its range. A laboratory thermometer is not used for body temperature: it has to be read while it is still in contact with the thing being measured, and it would be unsafe and awkward to hold in the mouth.
Besides these two, you will see room thermometers hanging on walls, and weather instruments that record the temperature of the air. All of them answer the same question: how hot or cold is it, in numbers?
- Range of a thermometer = lowest reading to highest reading
3Worked example
- Hot tea has a higher temperature than cold milk.
- A clinical thermometer measures body temperature.
- A laboratory thermometer measures the temperature of boiling water in an experiment.
4Your turn
Which thermometer would you choose to measure the temperature of ice?
5Remember this
The digital clinical thermometer and body temperature
1Think first
2The idea, in plain words
At home and in clinics we now mostly use a digital clinical thermometer. It runs on a small battery, has a metal tip with a heat sensor, and shows the temperature as numbers on a small screen. It usually reads in degrees Celsius, written °C.
How to use it safely, step by step.
- Read the instruction booklet that comes with it.
- Wash your hands and the tip with soap and water. Keep the screen part out of the water.
- Hold it by the body, never by the tip.
- Press the reset button so that the old reading is cleared.
- Place the tip under the tongue and close the mouth.
- Wait until it beeps or flashes, then take it out and read the display.
- Write the reading down, then wash and dry the tip before putting it away.
What is “normal” temperature? The normal temperature of a healthy person is taken as 37.0 °C. But if you measure ten healthy classmates, you will not get 37.0 °C every time. Some may show 36.7 °C, others 37.2 °C. That is because 37.0 °C is an average of many healthy people. A person’s temperature also changes a little with age, with the time of day, and after running or playing.
Human body temperature normally stays between about 35 °C and 42 °C. That is why a clinical thermometer only needs this short range.
Under the arm. For small children and old people, the thermometer can be kept in the armpit. A reading taken there is about 0.5 °C to 1 °C lower than the true body temperature, so we keep that in mind.
Why digital and not mercury? Older clinical thermometers had mercury in a glass tube. Mercury is very poisonous and hard to clean up if the glass breaks. Digital thermometers avoid that danger and are easier to read.
During the COVID-19 pandemic, infrared (non-contact) thermometers were used. They read the temperature from a short distance without touching the person, which lowers the risk of spreading infection.
- Normal body temperature ≈ 37.0 °C
- Armpit reading ≈ 0.5 °C to 1 °C lower than body temperature
3Worked example
- A reading of 37.2 °C in a healthy child is normal.
- An armpit reading of 36.4 °C may mean a body temperature of about 37 °C.
- Infrared thermometers were used at school gates during COVID-19.
4Your turn
Why are mercury clinical thermometers being replaced?
5Remember this
Temperature scales: Celsius, Fahrenheit and kelvin
1Think first
2The idea, in plain words
A number like “37” means nothing unless we know which scale it is on, just as “5” means nothing unless we say 5 metres or 5 kilograms. Three temperature scales are in common use, and each is named after the scientist who made it.
| Scale | Unit | Symbol | Ice melts | Water boils | Healthy body |
| Celsius | degree Celsius | °C | 0 °C | 100 °C | 37.0 °C |
| Fahrenheit | degree Fahrenheit | °F | 32 °F | 212 °F | 98.6 °F |
| Kelvin | kelvin | K | 273.15 K | 373.15 K | 310.15 K |
The Celsius scale is used on clinical and laboratory thermometers in India and in most of the world. Fahrenheit is still seen on some home thermometers; on it a healthy body shows 98.6 °F. Scientists today rarely use it. The kelvin is the SI unit of temperature, the one used in scientific work.
From Celsius to kelvin. This is easy: just add 273.15.
Rule: Temperature in kelvin = Temperature in °C + 273.15
So 0 °C = 273.15 K and 37 °C = 310.15 K. Going back, subtract 273.15.
For interest: Celsius to Fahrenheit. °F = (9/5 × °C) + 32. Check it: 37 × 9/5 = 66.6, and 66.6 + 32 = 98.6 °F. You do not need to learn this rule now, but it explains why 37 °C and 98.6 °F are the same temperature.
Writing temperatures correctly.
- Scale names start with a capital letter: Celsius scale, Fahrenheit scale, Kelvin scale.
- In the unit, “degree” is small and the name is capital: degree Celsius. The unit kelvin is written all small.
- Symbols are capitals: °C, °F, K. Never write a degree sign with K (write 300 K, not 300 °K).
- Leave a space between the number and the unit: 37 °C, not 37°C. Say “degrees” for more than one.
When someone says “I have a fever of 101 degrees”, they must mean Fahrenheit. A body at 101 °C is impossible; 101 °F is about 38.3 °C, a real fever.
- K = °C + 273.15
- °C = K − 273.15
- °F = (9/5 × °C) + 32 (for interest)
3Worked example
- 0 °C = 32 °F = 273.15 K (ice melts).
- 100 °C = 212 °F = 373.15 K (water boils at sea level).
- A fever of 101 °F is about 38.3 °C.
4Your turn
Which of these is written correctly?
5Remember this
The laboratory thermometer: range and smallest division
1Think first
2The idea, in plain words
A laboratory thermometer is a long, thin, sealed glass tube with a bulb at the bottom. The bulb holds a liquid, usually alcohol coloured red so that it is easy to see, or sometimes mercury. When the bulb gets warmer the liquid expands and the thin column rises up the tube; when it cools the liquid contracts and the column falls. A Celsius scale is marked along the tube. The mark level with the top of the column is the reading.
Two things to find before using any thermometer.
1. Its range. Look at the lowest and highest numbers. A common school thermometer goes from −10 °C to 110 °C. The “−10” means ten degrees below zero, colder than ice.
2. The value of its smallest division. Between two numbered (big) marks there are several small marks. To find what one small division is worth:
Rule: Value of one small division = (difference between two numbered marks) ÷ (number of small divisions between them)
If the numbered marks are 0 °C and 10 °C and there are 10 divisions between them, then one division = 10 ÷ 10 = 1 °C. That is the smallest temperature change this thermometer can show.
| Gap between numbered marks | Divisions in the gap | One division is worth |
| 10 °C | 10 | 1 °C |
| 10 °C | 5 | 2 °C |
| 10 °C | 20 | 0.5 °C |
| 100 °C | 50 | 2 °C |
Reading the column. Suppose the numbered marks are 20 °C and 30 °C with 10 divisions between them, and the red column ends 4 small marks above 20. Each division is 1 °C, so the reading is 20 + 4 = 24 °C. If the column stops exactly halfway between two small marks, we can say half a division more, for example 24.5 °C.
Not every thermometer is the same. Some have finer divisions, some a wider range. So always look carefully before you start.
- One division = (difference between two numbered marks) ÷ (number of divisions between them)
3Worked example
- A school thermometer reading −10 °C to 110 °C has a range of 120 degrees.
- 50 divisions between 0 °C and 100 °C means each division is 2 °C.
- Red alcohol is used so the column is easy to see.
4Your turn
Between the marks 30 °C and 40 °C there are 5 small divisions. What is one division worth?
5Remember this
Measuring temperature correctly with a laboratory thermometer
1Think first
2The idea, in plain words
A thermometer gives the right answer only if it is used the right way. Here are the rules for measuring the temperature of warm water in a beaker:
- Hold it upright. Keep the thermometer straight up and down, not tilted.
- Bulb inside, but free. The whole bulb should be in the water, but it must not touch the bottom or the sides of the beaker. The beaker itself may be hotter or cooler than the water.
- Wait, then read. Watch the column rise. Read it when it stops moving, but do not wait too long or the water itself will start cooling.
- Read while it is still in the water. As soon as you lift it out, the column starts to fall.
- Eye level with the column. Look straight at the top of the liquid column, with your eye at the same height. Looking from above or below gives a wrong reading.
- Handle with care. Do not hold it by the bulb (your hand would warm it) and do not knock it, as the glass can break.
Ice and boiling water. In a teacher’s demonstration, a thermometer in melting crushed ice shows about 0 °C, and one in boiling water shows a reading close to 100 °C. If you read them again after some time, you find something important: the reading stays the same while the ice is melting, and it also stays the same while the water is boiling, even though the burner is still on.
Why do different students get different readings? Five students in Shillong measured the same boiling water and wrote 97.8, 98.0, 97.9, 98.0 and 98.1 °C. Small differences like these usually come from how the reading was taken: a tilted thermometer, the eye too high or low, or the bulb touching the pot. This is why scientists repeat a measurement and follow the same careful method every time. (Water also boils at a slightly lower temperature in hilly places, which is why none of the readings is exactly 100 °C.)
Safety. Boiling water and burners must only be handled by the teacher. Watch from a safe distance.
3Worked example
- The column falls as soon as the thermometer is taken out of warm water.
- Melting ice stays near 0 °C until all of it has melted.
- Readings of 97.8 °C and 98.1 °C for the same water show small reading errors.
4Your turn
Where should your eye be while reading a laboratory thermometer?
5Remember this
Air temperature, weather and Anna Mani
1Think first
2The idea, in plain words
Thermometers hang on the walls of clinics, hospitals and school laboratories. These room thermometers give a rough idea of how warm the air in a room is. Outside, the temperature of the air is one of the most important facts about the weather.
Maximum and minimum temperature. Weather reports in newspapers, on television and online give two numbers for each place every day: the maximum (highest) and the minimum (lowest) air temperature. If you write these numbers down for ten days in a row, you will see that they change from day to day, because weather depends on many things such as clouds, wind and rain. Over months, they rise as summer comes and fall as winter comes.
Weather stations. Air temperature is measured at weather stations all over the world, together with rainfall, wind, humidity and other facts. All this information is used to make weather forecasts, which help farmers, fishermen, pilots and all of us to plan.
Anna Mani, the “Weather Woman of India”. Anna Mani (1918–2001) was an Indian scientist who designed and built many instruments for measuring the weather. Because of her work, India no longer had to depend on other countries for these instruments. She also studied how India could use energy from the wind and the Sun. The chapter opens with a thought of hers: a wrong measurement can do more harm than having no measurement at all. That is the reason we learnt careful reading in the last topic.
How hot and how cold can things be? The centre of the Sun is about 15 million °C. There is no known upper limit to temperature. But there is a lowest possible temperature, about −273.15 °C, which is 0 K. It is called absolute zero. That is why the kelvin scale starts there and has no negative numbers.
Try it. Hang a room thermometer in your class and keep a laboratory thermometer in a beaker of water near it. Read both in the first period, at lunch and in the last period for two weeks. Is the water always at the same temperature as the air?
- Absolute zero = −273.15 °C = 0 K
- Daily range = maximum − minimum
3Worked example
- A weather report: maximum 34 °C, minimum 24 °C.
- Air temperatures rise as summer approaches.
- Absolute zero is −273.15 °C, which is 0 K.
4Your turn
Who is known as the “Weather Woman of India”?
5Remember this
Key Concepts
- Temperature
- A measure of how hot or cold a body is; a hotter body has a higher temperature.
- Thermometer
- A device that measures temperature.
- Clinical thermometer
- A thermometer for measuring human body temperature, with a range of about 35 °C to 42 °C.
- Digital thermometer
- A battery-run thermometer that uses a heat sensor and shows the temperature as numbers on a screen.
- Infrared thermometer
- A non-contact thermometer that measures temperature from a short distance without touching the person.
- Laboratory thermometer
- A sealed glass thermometer with a bulb of liquid, usually ranging from −10 °C to 110 °C, used in experiments.
- Range
- The lowest to the highest temperature that a thermometer can measure.
- Smallest division
- The temperature change shown by one small mark on a thermometer scale.
- Normal body temperature
- The average temperature of many healthy people, taken as 37.0 °C or 98.6 °F.
- Celsius scale
- The scale on which ice melts at 0 °C and water boils at 100 °C; its unit is degree Celsius (°C).
- Fahrenheit scale
- A scale on which a healthy body is 98.6 °F; its unit is degree Fahrenheit (°F).
- Kelvin scale
- The scientific scale whose unit, the kelvin (K), is the SI unit of temperature; K = °C + 273.15.
- Absolute zero
- The lowest possible temperature, about −273.15 °C or 0 K.
- Maximum and minimum temperature
- The highest and lowest air temperatures recorded at a place in a day.
- Anna Mani
- Indian scientist (1918–2001), the ‘Weather Woman of India’, who built many weather instruments.
End-of-Chapter Trial Paper & Test Questions
Topic-wise questions to test your understanding of every concept in this chapter.
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Why can we not always rely on our sense of touch to decide whether something is hot or cold? / हम किसी वस्तु के गर्म या ठंडे होने का निर्णय करने के लिए हमेशा अपने स्पर्श पर भरोसा क्यों नहीं कर सकते?
Show answer
Touch only compares an object with how warm the skin already is, so the same water can feel warm to a cold hand and cool to a warm hand; it also gives no number. / स्पर्श केवल वस्तु की तुलना त्वचा की पहले की गर्माहट से करता है, इसलिए वही पानी ठंडे हाथ को गर्म और गर्म हाथ को ठंडा लग सकता है; साथ ही इससे कोई संख्या नहीं मिलती।
-
What is the normal temperature of a healthy human body on the Celsius and Fahrenheit scales? / सेल्सियस और फ़ारेनहाइट पैमानों पर स्वस्थ मानव शरीर का सामान्य ताप कितना है?
Show answer
The normal body temperature is taken as 37.0 °C, which is the same as 98.6 °F; it is an average, so a healthy person may be slightly above or below it. / सामान्य शरीर ताप 37.0 °C माना जाता है, जो 98.6 °F के बराबर है; यह एक औसत है, इसलिए स्वस्थ व्यक्ति का ताप इससे थोड़ा अधिक या कम हो सकता है।
-
Why is a laboratory thermometer not used to measure body temperature? / शरीर का ताप मापने के लिए प्रयोगशाला तापमापी का उपयोग क्यों नहीं किया जाता?
Show answer
A laboratory thermometer must be read while it is still in contact with the body, and its long glass tube is unsafe and awkward to keep in the mouth; a clinical thermometer is made for this job. / प्रयोगशाला तापमापी को वस्तु के संपर्क में रहते हुए ही पढ़ना पड़ता है और इसकी लंबी काँच की नली को मुँह में रखना असुरक्षित और असुविधाजनक है; यह काम नैदानिक तापमापी के लिए बना है।
-
A laboratory thermometer has 50 divisions between 0 °C and 100 °C. What does each division measure? / एक प्रयोगशाला तापमापी में 0 °C और 100 °C के बीच 50 भाग हैं। प्रत्येक भाग कितना ताप दर्शाता है?
Show answer
The gap of 100 °C is divided into 50 equal parts, so each division measures 100 ÷ 50 = 2 °C. / 100 °C के अंतर को 50 बराबर भागों में बाँटा गया है, इसलिए प्रत्येक भाग 100 ÷ 50 = 2 °C दर्शाता है।
-
Convert 27 °C into kelvin and 300 K into degree Celsius. / 27 °C को केल्विन में और 300 K को डिग्री सेल्सियस में बदलिए।
Show answer
Using K = °C + 273.15, 27 °C = 300.15 K; and 300 K = 300 − 273.15 = 26.85 °C. / K = °C + 273.15 से 27 °C = 300.15 K होता है; और 300 K = 300 − 273.15 = 26.85 °C होता है।
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Komal says she has a fever of 101 degrees. Which scale does she mean, and why? / कोमल कहती है कि उसे 101 डिग्री बुखार है। वह किस पैमाने की बात कर रही है और क्यों?
Show answer
She means the Fahrenheit scale, because a human body can never be at 101 °C; 101 °F is about 38.3 °C, which is a fever. / वह फ़ारेनहाइट पैमाने की बात कर रही है, क्योंकि मानव शरीर कभी 101 °C पर नहीं हो सकता; 101 °F लगभग 38.3 °C है, जो बुखार है।
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List four precautions to be taken while using a laboratory thermometer. / प्रयोगशाला तापमापी का उपयोग करते समय बरती जाने वाली चार सावधानियाँ लिखिए।
Show answer
Hold it upright, keep the bulb in the liquid without touching the container, read it while it is still immersed, and keep the eye level with the top of the column; also do not hold it by the bulb. / इसे सीधा पकड़ें, बल्ब को द्रव में रखें पर बर्तन को छूने न दें, इसे द्रव में डूबे रहते हुए पढ़ें, और आँख को स्तंभ के शीर्ष के समतल रखें; इसे बल्ब से भी न पकड़ें।
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Why are mercury thermometers being replaced by digital thermometers? / पारे वाले तापमापियों के स्थान पर डिजिटल तापमापी क्यों लाए जा रहे हैं?
Show answer
Mercury is a very poisonous substance that is hard to clean up if the glass breaks, while digital thermometers carry no such risk and are easier to read. / पारा बहुत विषैला पदार्थ है जिसे काँच टूटने पर साफ़ करना कठिन होता है, जबकि डिजिटल तापमापियों में ऐसा कोई ख़तरा नहीं होता और उन्हें पढ़ना आसान होता है।
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Why can a clinical thermometer not measure the temperature of boiling water? / नैदानिक तापमापी उबलते पानी का ताप क्यों नहीं माप सकता?
Show answer
A clinical thermometer has a range of only about 35 °C to 42 °C, and boiling water at about 100 °C is far outside this range. / नैदानिक तापमापी की परास केवल लगभग 35 °C से 42 °C होती है, और लगभग 100 °C वाला उबलता पानी इस परास से बहुत बाहर है।
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What happens to the temperature of ice while it is melting, and of water while it is boiling? / पिघलती हुई बर्फ़ और उबलते हुए पानी के ताप का क्या होता है?
Show answer
The temperature stays the same: melting ice stays at about 0 °C until all of it melts, and boiling water stays at a steady temperature even while it is being heated. / ताप स्थिर रहता है: पिघलती बर्फ़ पूरी पिघलने तक लगभग 0 °C पर रहती है, और उबलता पानी गर्म किए जाने पर भी एक स्थिर ताप पर रहता है।
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