Class 6 Science Chapter 7: A Complete Guide to Temperature and its Measurement

1. Introduction: Can You Trust Your Touch? 🤔

Have you ever felt like you might have a fever, so you ask a friend or parent to touch your forehead? That’s exactly what happened to a student named Lambok one day after school. He complained of feeling feverish, so his sister Phiban touched his forehead and agreed he felt warm. Thankfully, she used a thermometer to check and found that his temperature was perfectly normal.

This leads us to the central question posed by our science chapter: "Can it always be correctly judged, that a person has fever, only by touching the person?" As we're about to discover, our sense of touch is not always the most reliable tool. It can easily be tricked! To know for sure how hot or cold something is, we need a more scientific and reliable method. Let's dive into the world of temperature and how we measure it accurately.

2. What We'll Learn in This Chapter 📝

In this guide, we'll cover all the essential concepts about temperature from your chapter. Here’s what you can expect to learn:

• Why our sense of touch isn't always reliable.
• What temperature really is.
• The different types of thermometers and what they're used for.
• How to read and use thermometers correctly.
• The different scales for measuring temperature (Celsius, Fahrenheit, and Kelvin).

3. The Big Misconception: Hot or Cold? 🌡️

Our Sense of Touch Can Fool Us!

We use our sense of touch every day. We know that in the summer, tap water feels hotter than cool water from a matka (earthen pot). But can we always trust this feeling? Let's look at a simple experiment.

Imagine you have three containers:

• Container A: Warm water
• Container B: Regular tap water
• Container C: Ice-cold water

If you place your right hand in the warm water (A) and your left hand in the ice-cold water (C) for a minute, and then place both hands into the tap water (B) at the same time, something strange happens. Your right hand will feel that the tap water is cool, while your left hand will feel that the same water is warm!

This experiment proves a very important point: We cannot always rely upon our sense of touch to decide correctly whether a body is hot or cold. The sensation is relative to what our hands were feeling just before.

4. What is Temperature?

So, if we can't trust our touch, what can we trust? We need a reliable, scientific way to measure how hot or cold something is.

A reliable measure of hotness (or coldness) of a body is its temperature.

It’s that simple! A hotter body has a higher temperature than a colder body. The device we use to measure this is called a thermometer. The two most common types you will encounter are the clinical thermometer (for people) and the laboratory thermometer (for science experiments).

5. Measuring Body Temperature: The Clinical Thermometer 🩺

Clinical thermometers are designed specifically for measuring human body temperature.

5.1. Types of Clinical Thermometers

• Digital Thermometers: These are the most common type used today. Also known as digital clinical thermometers, they run on batteries and use heat sensors to measure temperature, showing the final reading on a digital screen.
• Non-Contact (Infrared) Thermometers: You probably saw these a lot during the COVID-19 pandemic. They are special thermometers that can measure a person's temperature from a distance without any physical contact, which helps reduce the spread of germs.
• Mercury Thermometers: These older glass thermometers were used for many years but are being replaced. This is because they contain mercury, which is an extremely toxic substance. If one of these thermometers breaks, the mercury is very difficult and dangerous to dispose of. Digital thermometers are much safer!

5.2. How to Use a Digital Clinical Thermometer

Here are the simple steps to measure body temperature correctly, just like in Activity 7.2 from your book:

1. Wash the tip of the thermometer with soap and water.
2. Press the reset button to turn it on and prepare it for a new reading.
3. Place the tip of the thermometer under the tongue and close your mouth.
4. Wait patiently until the thermometer makes a beeping sound or flashes a light. This means it's finished measuring.
5. Take the thermometer out and read the temperature on the digital display.

⚠️ Precautions to be taken

• Always read the instruction manual before using a new thermometer.
• Wash the tip with soap and water before and after every use.
• Be careful not to get the digital display or battery compartment wet when washing.
• Do not hold the thermometer by its tip, as the heat from your fingers could affect the reading.

5.3. Understanding Body Temperature Readings

• The normal temperature of a healthy human body is considered to be 37.0 °C.
• 💡 It's important to remember that this is an average temperature. A perfectly healthy person’s temperature might be slightly higher or lower, and it can be influenced by factors like age, the time of day, and activity level.
• The temperature of a human body does not normally go below 35 °C or above 42 °C. This is why clinical thermometers are designed for this specific, narrow range.
• For small children or elderly people, the thermometer can be placed in the armpit. However, the temperature measured this way will be about 0.5 °C to 1 °C lower than the actual body temperature.

6. The Scales of Temperature: °C, °F, and K

Temperature can be measured using different scales. Here are the three you should know:

• Celsius Scale (°C): This is the most common scale used in clinical thermometers and in most parts of the world. Its unit is degree Celsius.
• Fahrenheit Scale (°F): This is another scale you might see. The normal human body temperature on this scale is 98.6 °F, which is equivalent to 37.0 °C. It is not used in most scientific studies anymore.
• Kelvin Scale (K): This scale is very important for scientific work. The SI unit of temperature is kelvin. An important rule to remember is that the degree sign (°) is not written with K.

7. For Everything Else: The Laboratory Thermometer 🧪

When we need to measure the temperature of things in a science lab, like boiling water or melting ice, we need a different tool. Remember how we learned that a clinical thermometer's range is only from 35 °C to 42 °C? Well, water boils at 100 °C! That's far beyond what a clinical thermometer can handle and would break it. That's why for jobs like this, we need a laboratory thermometer.

7.1. What Does It Look Like?

A typical laboratory thermometer is a long, narrow, uniform glass tube with a bulb at one end. This bulb contains a liquid—usually alcohol (coloured red to make it easily seen) or sometimes mercury—that expands when heated and contracts when cooled.

7.2. Understanding Its Range and Scale

• The range of a laboratory thermometer found in a school lab is typically from –10 °C to 110 °C. This wide range allows it to measure things that are freezing and things that are boiling.
• To get a precise reading, you need to find the smallest value the thermometer can read. This smallest value is called the thermometer's least count—it tells you the most precise measurement you can make. Let's see how to find it: if the markings for 10 °C and 20 °C are separated by 10 small divisions, then each small division represents 1 °C.

7.3. How to Use a Laboratory Thermometer Correctly

Using a lab thermometer requires a bit more care than a clinical one. Follow these steps for an accurate reading:

1. Hold the thermometer vertically, not tilted.
2. Immerse the bulb completely in the substance you are measuring.
3. Make sure the bulb does not touch the bottom or the sides of the container. You want to measure the temperature of the substance, not the container!
4. Read the temperature while the bulb is still immersed in the substance. If you take it out, the reading will immediately start to change.
5. Keep your eye level with the top of the liquid column to avoid errors in reading the scale.

⚠️ Precautions to be taken

• Handle the thermometer with care. It's made of glass and can break easily if it hits a hard object.
• Do not hold the thermometer by the bulb, as the heat from your hand will affect the measurement.

8. Important Laws & Key Points to Remember ⭐

• Temperature is the reliable measure of hotness or coldness. A thermometer is the device used to measure it.
• Our sense of touch is not a reliable way to judge temperature because the sensation is relative.
• Clinical thermometers are for measuring body temperature and have a narrow range (35°C to 42°C).
• Laboratory thermometers are for scientific experiments and have a wide range (e.g., -10°C to 110°C).
• The normal human body temperature is an average of 37.0 °C (or 98.6 °F).
• The temperature of a substance remains constant during a change of state, like the melting of ice (0°C) or the boiling of water (100°C at sea level).

9. Formula Corner 🧮

There is a simple formula to convert a temperature from the Celsius scale to the Kelvin scale, which is used in many scientific calculations.

Temperature in Kelvin scale = Temperature in Celsius scale + 273.15

10. Did You Know? ✨

• 🤔 Ever wonder who the temperature scales are named after? They're all named in honour of the scientists who developed them! (Celsius, Fahrenheit, and Kelvin).
• ❤️ How did people check for fever before thermometers were common? They sometimes checked a person's pulse rate! A faster pulse can be a sign of fever, but since other things also affect pulse rate, this method isn't very reliable.
• 🇮🇳 Did you know we have a famous Indian scientist known as the ‘Weather Woman of India’? Her name is Anna Mani, and she invented many important weather instruments!

11. Frequently Asked Questions (FAQs) 🙋‍♀️

Q1: Why can't I just use my hand to tell if I have a fever?

• Answer: Your sense of touch is relative and can be misleading. The experiment with the three bowls of water shows this perfectly: your left hand will feel that the tap water is warm, while your right hand will feel that the very same water is cool. A thermometer gives an objective, reliable number.

Q2: What is the real difference between a clinical and a laboratory thermometer?

• Answer: There are two main differences. First is their purpose: a clinical thermometer is only for measuring body temperature, while a laboratory thermometer is for general science experiments. Second is their temperature range: a clinical thermometer has a very narrow range (35°C to 42°C), while a lab thermometer has a very wide range (like -10°C to 110°C).

Q3: Why can't I use the thermometer from my first-aid kit to measure boiling water?

• Answer: The thermometer in your first-aid kit is a clinical thermometer with a maximum reading of around 42°C. Water boils at 100°C, which is far beyond its range. Trying to measure boiling water with it would cause the liquid inside to expand too much and would break the thermometer.

Q4: Why are the old glass thermometers with the silver liquid not used much anymore?

• Answer: Those thermometers contained a liquid metal called mercury. Mercury is an extremely toxic substance. If the thermometer broke, the spilled mercury would be very dangerous and difficult to clean up safely. Modern digital thermometers don't have this risk.

Q5: Is everyone's normal body temperature exactly 37.0°C?

• Answer: No, 37.0°C is an average. A healthy person's normal temperature can be slightly different. It can also change a little bit based on your age, what you've been doing (your activity level), and even the time of day.

12. Conclusion & Top Exam Tips 🏆

So, the next time you think someone has a warm forehead, you'll know exactly what to do. Like Phiban, you'll reach for a thermometer—not just because it's the right tool, but because you understand the science behind it. You've moved from simply "feeling" to scientifically "knowing," and that's what being a great scientist is all about!

Exam Tips

• Memorize the key temperature values: Normal body temp (37.0 °C), freezing point of water (0 °C), and boiling point of water (100 °C).
• Be able to list at least two differences between a clinical and a laboratory thermometer (purpose and range are the big ones!).
• Practice the steps for using both types of thermometers, especially the most important rule for lab thermometers: "read it while it's still in the liquid!"
• Remember the conversion formula from Celsius to Kelvin (in shorthand: K = °C + 273.15).