Temperature And Its Measurement

Hello students, welcome to today's science lesson. I am so happy to see you all here, ready to learn something new and interesting. Today, we are going to study Chapter 7 from your science book, and the chapter is called "Temperature and its Measurement." This is a very important chapter because temperature is something we all experience every single day, and understanding how to measure it correctly is a very useful skill. So, let's begin our journey into the world of temperature.

Before we start, let me tell you a small story. There is a boy named Lambok and his elder sister Phiban, and they live in a beautiful place called Shillong. One day, Lambok came home from school feeling unwell. He told his sister that he was feeling feverish. Now, what did Phiban do? She touched his forehead with her hand and felt that he might be having a fever. But then, to confirm this properly, she took out a thermometer from the almirah, washed its tip with soap and water, measured Lambok's temperature, and found that his temperature was actually normal. She then washed the thermometer again, dried it, and put it back. She asked Lambok to change his uniform, eat his lunch, and rest for some time.

Now, here is a very important question for you to think about: Can it always be correctly judged that a person has fever, only by touching the person? Think about this for a moment. When you touch someone's forehead, you can get an idea if they feel hot, but is your sense of touch always reliable? Let's find out.

In this chapter, we are going to learn why our sense of touch is not always reliable, what temperature actually means, how we measure temperature using different types of thermometers, and much more. So, pay attention and let's learn together.

Now, let's start with the first section of our chapter, which is called "Hot or Cold?"

We all know from our daily experience that some bodies are hotter than others. For example, during summers in India, the water that comes out of the tap can be quite warm, while the water from a matka or earthen pot, or water from a refrigerator, is quite cold. We can feel this difference just by touching the two samples of water. But the question is: can we always rely upon our sense of touch to tell us which is hotter and which is colder? Let us find out through an activity.

Let's do Activity 7.1 together. Take three large containers and label them A, B, and C. Pour warm water in container A, tap water in container B, and ice-cold water in container C. Now, we are going to do this activity in two parts — first, we will make predictions, and then we will make observations.

Before you actually do this, I want you to predict what will happen. First, dip your right hand in container A (warm water) and your left hand in container C (ice-cold water), and keep them there for about one to two minutes. After that, take out your hands from containers A and C, and place both hands simultaneously in container B, which has tap water.

Now, write down your predictions. What will your right hand feel when you dip it in container B? What will your left hand feel when you dip it in the same container B? Write your predictions in your notebook.

Now, actually perform this activity and write your observations. Compare whether your observations match with your predictions. Did your right hand feel that the water in container B is cool, while your left hand felt that the same water is warm? This is really interesting, isn't it?

What do we infer from these observations? We infer that our sense of touch cannot always correctly tell us how hot or cold a body is. The reason is that our sense of touch only tells us about our own sensation at that moment, and it gets affected by what we touched just before. So, if we touch something cold, our hand becomes cold, and then when we touch something at room temperature, it feels warm to us. Similarly, if we touch something hot, our hand becomes hot, and then when we touch something at room temperature, it feels cool to us. This is why our sense of touch is not reliable for measuring temperature accurately.

So, the question is: then how do we find out how hot or cold a body is? The answer is that we use a device called a thermometer to measure temperature accurately. And that is exactly what we are going to learn in this chapter.

Now, let's move on to the next section, which is called "Temperature."

A reliable measure of the hotness or coldness of a body is called its temperature. A hotter body has a higher temperature than a colder body. The difference in temperature between two bodies tells us how hot one body is in comparison to another body. A device that measures temperature is called a thermometer.

There are two kinds of thermometers that you are likely to come across in your daily life — clinical thermometers and laboratory thermometers. Clinical thermometers are used to measure human body temperatures, while laboratory thermometers are used for many other purposes, like measuring the temperature of water, air, or other substances in a laboratory. Let us now learn more about thermometers and how to use them to measure temperature.

Now, let's learn about "Measuring Temperature" in detail. First, we will learn about the clinical thermometer.

You might be familiar with a thermometer that is used for measuring our body temperature. It is called a clinical thermometer. Such thermometers show temperatures digitally. These are also known as digital clinical thermometers and they run on batteries. These measure temperature when the thermometer is placed in contact with a person's body.

For measuring temperature, clinical thermometers generally use a scale called the Celsius scale. On this scale, the unit of temperature is degree Celsius, and it is denoted by the symbol °C.

Earlier, mercury thermometers were used for measuring body temperature. But mercury is an extremely toxic substance, and it is very difficult to dispose of safely if the thermometer breaks accidentally. That is why mercury thermometers are not commonly used anymore.

Digital thermometers pose no such risk, and also the numbers in their display are easier to read. Therefore, mercury thermometers are being replaced by digital thermometers. The temperature in a digital thermometer is determined with the help of heat sensors.

During the COVID-19 pandemic, some special thermometers were used which could measure the temperature of a person from a distance. Do you know what those were called? They are called non-contact thermometers, also known as infrared thermometers. Such thermometers can measure temperature without touching a person's body, and thus they help reduce the risk of spreading diseases. This was very useful during the pandemic when people needed to check their temperature without coming in contact with others.

Now, let's do Activity 7.2 together. Let us use a digital clinical thermometer to measure body temperature. You may measure your own temperature as well as the temperature of some of your friends, but make sure to talk to your friends first and find out who would be willing to get their body temperatures measured by you.

First, wash your hands and the tip of the digital thermometer with soap and water. Then, reset the thermometer by pressing the reset button. Now, place the thermometer under your tongue and close your mouth. Wait till the thermometer makes a beeping sound or flashes a light. Then, take it out from the mouth and read the temperature on the digital display. Record the temperature in Table 7.1 in your notebook. After that, clean the tip of the thermometer with soap and water and dry it. Repeat the above steps for measuring the temperatures of your friends.

Here are some precautions that you must take while using a digital clinical thermometer. First, always read the instruction manual of the thermometer before using it. Second, wash the tip of the thermometer with soap and water before and after use. Third, while washing, be careful to keep the digital portion, such as the display, out of water. Fourth, do not hold the thermometer by the tip; hold it by the handle instead.

Now, let's think about what we observed. The normal temperature of a healthy human body is taken to be 37.0°C. But in this activity, did you find that the normal temperature of every person was exactly 37.0°C? Or did you find the temperature slightly higher or lower for some people?

The temperature of every person may not be exactly 37.0°C. What we call normal temperature is actually the average body temperature of a large number of healthy people. A perfectly healthy person may, therefore, have a normal temperature slightly different from 37.0°C. The body temperature is influenced by several factors, such as age, time of the day, and activity level. For example, body temperature is usually lower in the early morning and higher in the afternoon. Also, after physical activity, body temperature may increase slightly.

You may try measuring your own temperature at different times of the day and on different days. Record the thermometer readings in your notebook. After a month, analyze your temperature record and see if there are any variations. If yes, try to think about what might be the reasons for that.

The temperature of human beings does not normally go below 35°C or above 42°C. If someone's temperature goes below 35°C or above 42°C, it is a matter of serious concern and they need medical attention.

For measuring the body temperature of small children or old people, the digital thermometer can also be placed in the armpit. The temperature measured this way is about 0.5°C to 1°C lower than the actual body temperature. So, if you measure temperature in the armpit, you need to add about 0.5°C to 1°C to get the actual body temperature.

Now, here is an interesting question. I have seen a friend of mine using a digital thermometer that reads temperature on a different scale. It shows the normal temperature of a healthy human body as 98.6°F. What is the reason for this difference?

There is another scale of temperature known as the Fahrenheit scale. On this scale, the unit of temperature is degree Fahrenheit, and it is denoted by °F. A temperature measured as 37.0°C on the Celsius scale is equivalent to 98.6°F on the Fahrenheit scale. The Fahrenheit scale is not used in most scientific studies anymore, but it is still commonly used in some countries like the United States.

In scientific work, there is another scale of temperature known as the Kelvin scale. On this scale, the unit for temperature is kelvin, and it is denoted by the symbol K. The SI unit of temperature is kelvin. The Kelvin scale is used in many scientific experiments and calculations.

All three temperature scales — Celsius, Fahrenheit, and Kelvin — are named in honour of the scientists who developed these scales. Anders Celsius developed the Celsius scale, Daniel Gabriel Fahrenheit developed the Fahrenheit scale, and William Thomson, also known as Lord Kelvin, developed the Kelvin scale.

We can easily convert the temperature from the Celsius scale to the Kelvin scale by using this formula: Temperature in Kelvin scale equals temperature in Celsius scale plus 273.15. So, for example, if the temperature is 37°C, then in Kelvin it would be 37 plus 273.15, which is 310.15 K.

Now, let me tell you some more important points about writing temperature. The names of temperature scales — Celsius scale, Fahrenheit scale, and Kelvin scale — start with a capital letter. For the units for temperature, degree Celsius and degree Fahrenheit, the word "degree" starts with a lowercase letter while Celsius and Fahrenheit start with a capital letter. The unit "kelvin" starts with a lowercase letter. The symbols of all units — °C, °F, and K — are capital letters. Note that the degree sign (°) is not written with K. Also, a full stop is not written after the symbol, except at the end of a sentence. While writing the temperature, a space is left between the number and the unit. For temperatures more than one degree, use the plural of "degree," that is, "degrees," while writing the full form of the unit. For example, we write "37 degrees Celsius" and not "37 degree Celsius."

Now, here is an important question: Can a clinical thermometer be used for measuring the temperature of boiling water? Or for measuring the temperature of ice? The answer is no. The temperatures of boiling water and ice are outside the range of a clinical thermometer. A clinical thermometer is designed specifically for measuring human body temperature, which is usually between 35°C and 42°C. Boiling water has a temperature of about 100°C, and ice can have a temperature below 0°C, which is much lower than what a clinical thermometer can measure.

Now, the question is: how can we measure temperatures beyond the range of a clinical thermometer? The answer is by using a laboratory thermometer, which we will learn about next.

Let's now learn about the laboratory thermometer. There are many types of laboratory thermometers, but the one that might be available in your school laboratory may look like the one shown in Figure 7.3a in your book. It consists of a long, narrow, uniform glass tube which is sealed. At one end of the tube is a bulb which contains a liquid. Outside the bulb, in the tube, a narrow column of liquid can be seen. There is a Celsius scale marked along the tube. The liquid column rises or falls with change in temperature. The mark of the Celsius scale with which the top level of the liquid column coincides is the temperature reading.

The liquid used in the laboratory thermometer is generally alcohol, which is coloured red to make it easily seen, or mercury. Mercury is also used in some thermometers, but as we learned earlier, it is toxic and dangerous.

Now, let's do Activity 7.3. Let us try to find the temperature range of a given laboratory thermometer. Take a laboratory thermometer and observe it carefully. Note down the following: What is the lowest temperature it can measure? And what is the highest temperature it can measure? The difference between these two values gives us the range of the thermometer.

Now, look at the thermometer shown in Figure 7.3a in your book carefully. Can you tell its range? Its range is from –10°C to 110°C. That means it can measure temperatures as low as 10 degrees below zero (which is below the freezing point of water) and as high as 110 degrees Celsius (which is above the boiling point of water).

Now, here are some precautions to be taken while using a laboratory thermometer. First, handle it with care. If it hits against some hard object, it can break. Second, do not hold it by the bulb; hold it by the stem. The bulb is the part that contains the liquid, and it is very delicate.

Now, let's do Activity 7.4. Let us try to find the smallest value that a given laboratory thermometer can read. Again, take the same laboratory thermometer which you used in Activity 7.3 and observe it carefully. Note down the following: How much is the temperature difference indicated between the two bigger marks? How many divisions, shown by smaller marks, are there between these two bigger marks? How much temperature does one small division indicate? So, the smallest value that the thermometer can read is the temperature difference between two big marks divided by the number of small divisions between them.

Figure 7.3b shows a close-up of a part of the thermometer shown in Figure 7.3a. Can you now find the smallest value that this thermometer can read? For the thermometer shown in Figure 7.3b, the temperature difference indicated between 0°C and 10°C, or between 10°C and 20°C, is 10°C. And the number of divisions between these marks are 10 divisions. So, one small division can read 10 divided by 10, which is 1°C. That is, the smallest value that this thermometer can read is 1°C.

Your school laboratory may have thermometers for which the range and the value of the smallest division may be different. It is, therefore, always necessary to look carefully at the thermometer you are about to use before you start measuring temperature.

Now, let's learn the correct way of measuring temperature using a laboratory thermometer. There are some important steps you need to follow:

First, when the thermometer is immersed in water, its bulb should not touch the bottom or the sides of the beaker. If the bulb touches the bottom or the sides, it will measure the temperature of the beaker instead of the water.

Second, the thermometer should be held vertically. It should not be tilted. If it is tilted, the reading may not be accurate.

Third, the temperature must be read while the thermometer is immersed in water. If you take it out to read, the temperature will change quickly.

Fourth, while reading the thermometer, the eye should be directly in line with the level of the liquid column to be read. If you look from above or below, you will get a wrong reading due to parallax error. This is shown in Figure 7.4 in your book.

Now, let's do Activity 7.5. Take some warm water in a beaker. Dip the thermometer in water so that the bulb is immersed in water. Observe the rise of liquid column in the thermometer. Wait till the column stops rising and note the temperature. But do not wait too long, otherwise the water itself will start to cool down.

What is the temperature of water measured by you? Compare it with the readings of your friends. Do you notice that, as soon as you take the thermometer out of the water, the level of liquid column begins to fall? This means that the temperature must be read while the thermometer is immersed in water.

Now, here is a question: Can we use a laboratory thermometer for measuring body temperature of a person? The answer is no, because the range of a laboratory thermometer is usually from –10°C to 110°C, which is much larger than the range of human body temperature. Also, a laboratory thermometer is not designed to be placed in the human body. It is meant for measuring the temperature of substances in a laboratory.

Now, let's talk about an activity that your teacher might arrange in the class. There are two experimental setups: one for measuring the temperature of ice and another for measuring the temperature of boiling water. One by one, as per your turn, go closer to the setups and observe the readings of both the thermometers. What are the temperatures of ice and boiling water?

Caution: This activity should be performed strictly only under the supervision of your teacher. Do not touch the experimental setups.

Read the temperatures of ice and boiling water again after some time. Are the temperatures the same or have they changed? You may have noticed that the temperature of water remains constant while it is boiling. Also, the temperature of ice remains constant while it is melting. This is a very important observation. When ice is melting, its temperature remains at 0°C, and when water is boiling, its temperature remains at 100°C, as long as the pressure is normal. This is because the heat energy is used for changing the state of the substance, not for increasing its temperature.

Now, let's do Activity 7.6. Phiban's Science teacher arranged the experimental setup for measuring temperature of boiling water. The temperature readings of the boiling water taken by Phiban and her classmates in Shillong are given in Table 7.2 in your book. Phiban recorded 97.8°C, Shemphang recorded 98.0°C, Onestar recorded 97.9°C, Kloi recorded 98.0°C, and Bandarisha recorded 98.1°C.

Now, compare the temperatures of boiling water recorded by different students. Why are there differences in their readings? Discuss the possible reasons amongst yourselves. Maybe, the correct way of reading temperature was not followed by all the students. Some students might have taken the thermometer out of the water before reading, some might have tilted the thermometer, some might not have waited for the liquid column to stop rising, or some might have read the temperature from an angle. All these factors can lead to errors in measurement.

Now, let's learn about air temperature. You might have seen thermometers, such as the one shown in Figure 7.5, hung on the walls of your school laboratory, doctor's clinic, and hospitals. These give an approximate idea of the room temperature. These are called room thermometers or wall thermometers.

Have you seen weather reports in newspapers, TV news, or on the internet? These reports also mention the maximum and minimum air temperature of the day. This information helps people plan their activities. For example, if the maximum temperature is very high, people might avoid going out in the afternoon. If the minimum temperature is very low, they might wear warm clothes.

Now, let's do Activity 7.7. Read or listen to the weather reports for a place for 10 successive days. Record the maximum and minimum air temperature for each day in Table 7.3 in your notebook. Analyze the data in Table 7.3. Does the maximum and minimum temperature stay at the same level during these days? Because weather depends on several factors, these temperatures usually vary every day. Generally, as we approach the summer season, the temperature rises, and during the winter season, it falls.

There are many techniques for measuring air temperature. Air temperature is an important weather parameter and is monitored at weather stations all over the world. The data gathered on air temperature, along with various other parameters, are used for making weather forecasts. This helps us prepare for extreme weather conditions like heatwaves or cold waves.

Now, let me tell you about a famous Indian scientist. Her name is Anna Mani. She was an Indian scientist who lived from 1918 to 2001. She is also known as the "Weather Woman of India." She invented and built a large number of weather measurement instruments. This reduced the reliance of India on other nations for such instruments. She also explored the possibilities of using wind and solar energy in India. Her work helped India to become one of the global leaders in renewable energy. She is truly an inspiration for all of us.

Now, let's summarize what we have learned so far. The temperature of a body tells us how hot or cold it is. The three most-used scales of temperature are: the Celsius scale, the Fahrenheit scale, and the Kelvin scale. The units of temperature in these scales are: degree Celsius, denoted by °C; degree Fahrenheit, denoted by °F; and kelvin, denoted by K. The SI unit for temperature is kelvin. A clinical thermometer is used for measuring body temperature. The normal temperature of a healthy human adult is taken to be 37.0°C or 98.6°F. Laboratory thermometers typically have a temperature range from –10°C to 110°C.

Now, let's move on to the exercises at the end of the chapter. I want you to understand each and every question thoroughly, so pay attention.

Question 1: The normal temperature of a healthy human being is close to __________. The options are: (i) 98.6°C, (ii) 37.0°C, (iii) 32.0°C, and (iv) 27.0°C. The correct answer is 37.0°C. So, the answer is option (ii). Remember, 98.6°F is the normal temperature on the Fahrenheit scale, not Celsius.

Question 2: 37°C is the same temperature as __________. The options are: (i) 97.4°F, (ii) 97.6°F, (iii) 98.4°F, and (iv) 98.6°F. The correct answer is 98.6°F. So, the answer is option (iv). This is the temperature that is commonly shown on digital thermometers in India.

Question 3: Fill in the blanks. (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. Let me explain: For (i), temperature determines how hot or cold a body is. For (ii), clinical thermometers have a limited range, usually from 35°C to 42°C, so they cannot measure temperatures below 35°C, such as ice-cold water which is at 0°C. For (iii), degree Celsius is a unit of temperature on the Celsius scale.

Question 4: The range of a laboratory thermometer is usually __________. The options are: (i) 10°C to 100°C, (ii) –10°C to 110°C, (iii) 32°C to 45°C, and (iv) 35°C to 42°C. The correct answer is –10°C to 110°C. So, the answer is option (ii). This range allows us to measure temperatures below the freezing point and above the boiling point of water.

Question 5: Four students used a laboratory thermometer to measure the temperature of water as shown in Figure 7.6. The question asks who followed the correct way for measuring temperature. Based on what we learned, the correct way is: the bulb should not touch the bottom or sides of the beaker, the thermometer should be held vertically, the temperature should be read while the thermometer is immersed in water, and the eye should be directly in line with the level of the liquid column. Without the figure, I cannot tell exactly which student is correct, but typically, the student who has the thermometer properly immersed in water without touching the sides or bottom, and is reading it at eye level, would be following the correct method.

Question 6: Colour to show the red column on the drawings of thermometers as per the temperatures written below: 14°C, 17°C, and 7.5°C. This is a drawing question, so I cannot colour it for you, but I can explain how to do it. For a thermometer, the level of the red liquid column indicates the temperature. So, for 14°C, the column should be at the 14 mark. For 17°C, the column should be at the 17 mark. For 7.5°C, the column should be halfway between the 7 and 8 marks, since each division is 1°C and 7.5 is exactly in the middle.

Question 7: Observe the part of thermometer shown in Figure 7.8 and answer the following questions. (i) What type of thermometer is it? (ii) What is the reading of the thermometer? (iii) What is the smallest value that this thermometer can measure? Without the figure, I cannot give you the exact answers, but I can tell you how to find them. (i) Look at the scale and the design to determine if it is a clinical thermometer or a laboratory thermometer. (ii) Read the temperature where the liquid column meets the scale. (iii) Find the difference between two big marks and divide by the number of small divisions between them.

Question 8: A laboratory thermometer is not used to measure our body temperature. Give a reason. The reason is that a laboratory thermometer has a wide range, usually from –10°C to 110°C, but it is not designed to measure the precise temperature of the human body, which is around 37°C. Also, it is not safe to put a laboratory thermometer in the mouth or under the armpit. Clinical thermometers are specifically designed for measuring body temperature and have a narrow range that includes normal body temperature.

Question 9: Vaishnavi has not gone to school as she is ill. Her mother has kept a record of her body temperature for three days as shown in Table 7.4. (i) What was Vaishnavi's highest recorded temperature? (ii) On which day and at what time was Vaishnavi's highest temperature recorded? (iii) On which day did Vaishnavi's temperature return to normal? Let's analyze the table. Looking at the temperatures, the highest temperature recorded is 40.0°C on Day One at 7pm. So, the answer to (i) is 40.0°C. The answer to (ii) is Day One at 7pm. For (iii), we need to find when her temperature returned to normal. Normal body temperature is around 37.0°C. Looking at the table, on Day Three, her temperature at 4pm is 37.0°C and at 7pm it is 36.8°C, which are both around normal. So, her temperature returned to normal on Day Three.

Question 10: If you have to measure the temperature 22.5°C, which of the following three thermometers will you use? Explain. The answer depends on the range and the smallest division of the thermometers. You need to choose a thermometer that can measure 22.5°C accurately. This means the thermometer should have a range that includes 22.5°C, and its smallest division should be small enough to measure 22.5°C precisely. If one thermometer has divisions of 1°C, it cannot measure 22.5°C accurately. If another has divisions of 0.5°C, it can measure 22.5°C exactly. So, you should choose the thermometer with the smallest division of 0.5°C or less.

Question 11: The temperature shown by the thermometer in Figure 7.10 is (i) 28.0°C, (ii) 27.5°C, (iii) 26.5°C, or (iv) 25.3°C. Without the figure, I cannot give you the exact answer, but I can tell you how to find it. Look at where the liquid column meets the scale. If it is exactly on a big mark, read that number. If it is between two big marks, count the small divisions and add them to the lower big mark.

Question 12: A laboratory thermometer has 50 divisions between 0°C and 100°C. What does each division of this thermometer measure? The temperature difference between 0°C and 100°C is 100°C. If there are 50 divisions between them, then each division measures 100 divided by 50, which is 2°C. So, each division measures 2°C.

Question 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. To draw this, you need to show big marks at every 10°C, 11°C, 12°C, and so on up to 20°C. Between each big mark, there should be smaller divisions. Since the smallest division reads 0.5°C, there should be two small divisions between each big mark. For example, between 10°C and 11°C, there should be one small mark at 10.5°C.

Question 14: Komal tells you that she has a fever of 101 degrees. Does she mean it on the Celsius scale or Fahrenheit scale? This is a tricky question. In India, we commonly use the Celsius scale, so 101°C would be impossible because the human body cannot survive at such a high temperature. Therefore, Komal most likely means 101°F, which is a common fever temperature on the Fahrenheit scale. On the Celsius scale, a fever of 101°F is about 38.3°C, which is a moderate fever.

Now, let's look at the "Learning Further" section. These are additional activities and questions for you to explore.

First, gather information from the internet and find out how the body temperature of animals, such as a cat, dog, horse, camel, cow, and buffalo, is measured. If there is any veterinary hospital in your vicinity, you may visit to see the body temperature of animals being measured. Different animals have different normal body temperatures. For example, a dog's normal temperature is around 38.3°C to 39.2°C, a cat's is around 38.0°C to 39.0°C, a cow's is around 38.0°C to 39.3°C, and so on. The method of measuring temperature in animals is similar to that in humans, but sometimes special thermometers are used.

Second, find out which places in India are usually regarded to be the coldest and hottest. Also, find out the minimum and maximum temperatures recorded for these places. The coldest places in India are usually in the northern Himalayas, like Leh, Ladakh, and some parts of Jammu and Kashmir. The hottest places are in the desert regions of Rajasthan, like Jodhpur, Jaisalmer, and Bikaner. You can search for the specific records online.

Third, various planets in our Solar System are at different distances from the Sun. Search the internet and make a table with the planets, their distances from the Sun in increasing order, and their temperatures written. Does the average temperature of planets decrease as their distance from the Sun increases? If it is not true for any planet, find out for which planet and why. Generally, planets closer to the Sun are hotter, and planets farther from the Sun are colder. However, Venus is an exception because it has a thick atmosphere that traps heat, making it hotter than Mercury, even though Mercury is closer to the Sun.

Fourth, hang a room thermometer in your classroom. Set up the apparatus, as shown in Figure 7.11, near the thermometer hung on the wall. Take the readings of thermometers 1 and 2 three times during the day, say, first period, lunch break, and last period. Record your readings. Compare the readings and draw your conclusions. Repeat this for two weeks. This activity will help you understand how temperature changes throughout the day and how different thermometers might give slightly different readings.

Now, let me tell you some interesting facts. The temperature at the core of the Sun reaches as high as 15 million degrees Celsius. Are there objects in the sky that have even higher temperatures? There is no limit on the highest temperature that can exist. However, as per scientific understanding, there is a limit to the lowest temperature that can be achieved. It is close to –273.15°C, which is 0 kelvin, and is called absolute zero. At absolute zero, all molecular motion stops. Scientists have never been able to achieve absolute zero in practice, but they have come very close to it.

Now, let's do a quick recap of everything we have learned in this chapter.

We started with the story of Lambok and Phiban, and we learned that we cannot always correctly judge if a person has fever just by touching them. Our sense of touch is not reliable for measuring temperature accurately.

Then, we learned that temperature is a reliable measure of the hotness or coldness of a body. A hotter body has a higher temperature than a colder body.

We learned about the three temperature scales: Celsius, Fahrenheit, and Kelvin. The Celsius scale is commonly used in India. The normal body temperature is 37.0°C or 98.6°F. The SI unit of temperature is kelvin. We can convert Celsius to Kelvin by adding 273.15.

We learned about clinical thermometers, which are used to measure body temperature. They are usually digital now. We learned how to use a clinical thermometer properly and the precautions to take while using it.

We learned about laboratory thermometers, which are used for measuring temperatures in experiments. They have a wider range, usually from –10°C to 110°C. We learned how to find the range and the smallest division of a laboratory thermometer. We also learned the correct way of measuring temperature using a laboratory thermometer.

We learned about air temperature and how it is reported in weather reports. We learned about the maximum and minimum temperatures of the day.

We learned about Anna Mani, the "Weather Woman of India," who made significant contributions to weather measurement instruments in India.

We also solved all the exercises in the "Let us enhance our learning" section and discussed the activities in the "Learning Further" section.

So, students, this is everything you need to know about Chapter 7: Temperature and its Measurement. I hope you have understood all the concepts clearly. Remember, temperature is a very important physical quantity, and knowing how to measure it correctly is a useful skill in science and in daily life.

Thank you for listening attentively. Keep learning, keep exploring, and never stop asking questions. See you in the next lesson!