Frank Brothers Solutions for ICSE Class 9 Physics Chapter 5.3 Heat Thermometry

Page No : 209

Question 1. Define temperature.
Answer: Temperature is the quantity that tells about the thermal state of a body i.e. the degree of hotness or coldness of a body.
In simple words: Temperature is a measure of how hot or cold an object is compared to a standard scale.

📝 Teacher's Note: Distinguish between heat and temperature in class. Use the analogy that heat is like the total amount of water in a tank, while temperature is the water level.

🎯 Exam Tip: The phrase "degree of hotness or coldness" is a keyword examiners look for in this definition.

Question 2. Which instrument is used for measuring temperature?
Answer: Thermometer is used for measuring the temperature of a body.
In simple words: A thermometer is a tool we use to find out the exact temperature of a person or an object.

📝 Teacher's Note: Bring different types of thermometers (digital, clinical, laboratory) to class to show students the variations in design for different purposes.

🎯 Exam Tip: Ensure you spell "thermometer" correctly; it is a common spelling error in science papers.

Question 3. State the principle on which a thermometer works.
Answer: Thermometer works on the principal that substances expand when heated and contract on cooling.
In simple words: When things get hot, they need more room and grow bigger (expand). When they get cold, they shrink (contract). Thermometers use this to show temperature changes.

📝 Teacher's Note: Demonstrate thermal expansion with a ball and ring experiment to make this abstract principle more concrete for students.

🎯 Exam Tip: Mention both "expansion on heating" and "contraction on cooling" to provide a complete answer.

Question 4. What is meant by the range of a thermometer?
Answer: Range of thermometer is the range of temperature which can be measured by thermometer.
In simple words: The range is the lowest and highest temperatures a thermometer is able to show us.

📝 Teacher's Note: Compare the ranges of a clinical thermometer (\( 35^\circ \text{C} \) to \( 42^\circ \text{C} \)) and a laboratory thermometer (\( -10^\circ \text{C} \) to \( 110^\circ \text{C} \)).

🎯 Exam Tip: Use the words "minimum" and "maximum" when describing range in a more detailed response.

Question 5. Name the liquid used in a clinical thermometer.
Answer: Mercury is the liquid used in a clinical thermometer
In simple words: Mercury is that shiny silver liquid inside old-fashioned thermometers that moves up and down.

📝 Teacher's Note: Mention that mercury is chosen because it stays liquid over a wide range and is a good conductor of heat.

🎯 Exam Tip: If asked for a reason, mention that mercury does not stick to the walls of the glass tube.

Question 6. What is the usual range of temperature marked on a clinical thermometer?
Answer: The usual range of temperature marked on clinical thermometer is \( 95^\circ \text{F} \) to \( 110^\circ \text{F} \).
In simple words: A doctor's thermometer only shows temperatures close to a human's normal body heat, from \( 95^\circ \) to \( 110^\circ \text{F} \).

📝 Teacher's Note: Point out that this range is narrow because human life is only possible within these limited body temperatures.

🎯 Exam Tip: Be careful with units—ensure you specify \( ^\circ \text{F} \) if using these values, or \( 35^\circ \text{C} \) to \( 42^\circ \text{C} \) for Celsius.

Question 7. Who designed the first thermometer?
Answer: Fahrenheit designed the first thermometer.
In simple words: A scientist named Gabriel Fahrenheit was the one who invented the first modern thermometer.

📝 Teacher's Note: While Galileo invented the thermoscope, Fahrenheit is credited with the first reliable mercury thermometer with a standard scale.

🎯 Exam Tip: This is a common "Who is who" fact-based question; memorize the name carefully.

Question 8. What are the steps taken before constructing a thermometer?
Answer: Before constructing a thermometer, we determine lower fixed point and upper fixed point and divide the whole range of thermometer into specific number of equal divisions to provide a scale for measuring the temperatures within a range.
In simple words: To make a thermometer, we find the freezing and boiling points of water first. Then we mark the space between them with equal little lines, like a ruler.

📝 Teacher's Note: Explain "calibration" as the process of marking a scale on an instrument to ensure it reads correctly.

🎯 Exam Tip: Use the terms "Lower Fixed Point" and "Upper Fixed Point" as they are standard scientific terminology.

Question 9. What is a clinical thermometer?
Answer: The clinical thermometer is specially designed thermometer used to measure the temperature of a human body easily and as accurately as possible.
In simple words: This is a special thermometer made just for checking if a person has a fever.

📝 Teacher's Note: Show the "kink" or constriction in a clinical thermometer and explain how it prevents the mercury from falling back immediately.

🎯 Exam Tip: Mention the "constriction" or "kink" as the main feature that distinguishes it from a lab thermometer.

Question 10. List three properties that make a liquid suitable for use in a thermometer.
Answer: Three properties of a liquid which make it suitable to be used in a thermometer are:

• The substance should have high coefficient of expansion so that it is sensitive to the smallest change in temperature
• The substance should have uniform expansion all over its entire volume
• The substance should have minimum specific heat so that it absorbs minimum heat from the body under measurement.

In simple words: A good thermometer liquid should grow even with a tiny bit of heat, grow at the same rate every time, and not take too much heat away from what you are measuring.

📝 Teacher's Note: Use mercury as an example for all three points to explain why it is the preferred choice for many years.

🎯 Exam Tip: If asked for just two points, the "high coefficient of expansion" and "uniform expansion" are the easiest to remember and explain.

Question 11. State two disadvantages of using mercury as a thermometric liquid.
Answer: Two disadvantages of using mercury as a thermometric liquid:

• It does not have uniform expansion.
• Mercury is less sensitive than alcohol as its coefficient of expansion is less than alcohol.

In simple words: Mercury can be tricky because it doesn't grow perfectly evenly, and it doesn't react as much as alcohol when the temperature changes just a little.

📝 Teacher's Note: While mercury is generally considered uniform, certain high-precision contexts show non-uniformity. Also, emphasize that mercury is toxic, which is a major real-world disadvantage.

🎯 Exam Tip: Comparing mercury and alcohol is a classic question; remember that alcohol is more sensitive but has a lower boiling point.

Question 12. State three advantages of using mercury as a thermometric liquid.
Answer: Three advantages of using mercury as a thermometric liquid:

• Mercury is good conductors of heat.
• Mercury have high coefficient of expansion thus is sensitive to the smallest change in temperature.
• Freezing points is very low and boiling point is high.

In simple words: Mercury is great because it heats up fast, grows a lot when hot, and stays liquid from very cold temperatures to very hot ones.

📝 Teacher's Note: Point out that its high boiling point (\( 357^\circ \text{C} \)) allows it to measure temperatures far higher than boiling water.

🎯 Exam Tip: The "high boiling point" and "low freezing point" are critical for explaining the wide range of a mercury thermometer.

Question 13. Why is water not used as a thermometric liquid?
Answer: Water is not used as a thermometric liquid because it has low coefficient of expansion so it is less sensitive to temperature changes. Moreover, it is transparent thus making it difficult to read the thermometer and water evaporates with time thus producing error and also the freezing and boiling points are also low.
In simple words: Water is hard to see because it's clear, it doesn't expand much when heated, it evaporates easily, and it freezes or boils at common temperatures.

📝 Teacher's Note: Remind students about the anomalous expansion of water between \( 0^\circ \text{C} \) and \( 4^\circ \text{C} \)—it would actually shrink when slightly heated from \( 0^\circ \text{C} \)!

🎯 Exam Tip: Mentioning that "water sticks to the glass" is another valid and easy-to-remember reason.

Question 14. Convert \( 0^\circ \text{C} \) into Kelvin.
Answer: Temperature in Kelvin \( \text{To K} = \text{Temperature in Celsius To C} + 273 \).
\( \text{To K} = 0^\circ \text{C} + 273 \)
\( \implies \text{To K} = 273 \text{ K} \).
In simple words: To change Celsius to Kelvin, you just add 273. So, freezing water is \( 0^\circ \text{C} \), which is \( 273 \text{ K} \).

📝 Teacher's Note: Explain that Kelvin is the absolute scale and doesn't use the degree (\( ^\circ \)) symbol.

🎯 Exam Tip: Always show the formula \( \text{K} = \text{C} + 273 \) before plugging in the numbers to get full marks for the method.

Question 15. Convert the body temperature of a healthy person (\( 98.4^\circ \text{F} \)) into Celsius.
Answer: Body temperature of a healthy person is \( 98.4^\circ \text{F} \).
We know \( \frac{\text{C}}{100} = \frac{\text{F} - 32}{180} \).
\( \text{C} = \frac{5}{9}(\text{F} - 32) \)
\( \implies \text{C} = \frac{5}{9}(98.4 - 32) \)
\( \implies \text{C} = \frac{5}{9} \times 66.4 \)
\( \implies \text{C} = 36.88^\circ \text{C} \)
Temperature of body of healthy man is \( 36.88^\circ \text{C} \).
In simple words: A normal person's temperature is about \( 98.4 \) degrees on the Fahrenheit scale, which is about \( 37 \) degrees on the Celsius scale.

📝 Teacher's Note: This is a common conversion. It's helpful to remember that "normal" body temperature is roughly \( 37^\circ \text{C} \).

🎯 Exam Tip: Practice the fraction multiplication carefully; \( 5/9 \) is approximately \( 0.555 \), but using the fraction is more accurate.

Question 16. What is the absolute scale of temperature and how does a \( 1^\circ \text{C} \) rise compare to it?
Answer: Absolute scale of temperature is Kelvin scale.
Conversion of temperature from Celsius to Kelvin scale is
Temperature in Kelvin \( \text{To K} = \text{Temperature in Celsius To C} + 273 \).
So a rise of temperature of \( 1^\circ \text{C} \) in Celsius scale is equal to rise of \( 1 \text{ K} \) in Kelvin scale.
In simple words: The Kelvin scale is the "absolute" way scientists measure heat. Even though they start at different numbers, one step on the Celsius ruler is exactly the same size as one step on the Kelvin ruler.

📝 Teacher's Note: This is a crucial point—the size of the degree is identical in Celsius and Kelvin, only the starting point (zero) is different.

🎯 Exam Tip: If a question asks about a temperature *difference*, remember that \( \Delta \text{C} = \Delta \text{K} \).

Question 17. Which temperature scale is used in the SI system?
Answer: Kelvin temperature scale is used in SI system.
In simple words: Scientists all over the world use the Kelvin scale for their official measurements.

📝 Teacher's Note: SI stands for International System of Units. Just as meters are used for length, Kelvin is used for temperature.

🎯 Exam Tip: This is a very common one-mark objective question.

Question 18. Describe the fixed points and divisions on the Celsius scale.
Answer: In Celsius scale there are two fixed points namely lower fixed point and upper fixed point at \( 0^\circ \text{C} \) and \( 100^\circ \text{C} \) respectively. This range is divided into \( 100 \) equal divisions and each part gives \( 1^\circ \text{C} \).
In simple words: On the Celsius scale, water freezes at \( 0 \) and boils at \( 100 \). There are \( 100 \) small steps between these two points.

📝 Teacher's Note: The Celsius scale is also called the "centigrade" scale because it has \( 100 \) steps (centi = hundred, grade = steps).

🎯 Exam Tip: Always mention that the fixed points are based on pure water at standard atmospheric pressure.

Question 19. Convert \( -15^\circ \text{C} \) into Fahrenheit.
Answer: We know \( \frac{\text{C}}{100} = \frac{\text{F} - 32}{180} \).
\( \text{C} = \frac{5}{9}(\text{F} - 32) \)
\( \frac{9}{5}\text{C} + 32 = \text{F} \)
\( \text{F} = \frac{9}{5}\text{C} + 32 \).
Temperature given in Celsius \( = -15^\circ \text{C} \).
\( \text{F} = \frac{9}{5} \times (-15) + 32 \)
\( \implies \text{F} = -27 + 32 \)
\( \implies \text{F} = 5^\circ \text{F} \).
In simple words: When it is a freezing \( -15 \) degrees on the Celsius scale, it is only \( 5 \) degrees on the Fahrenheit scale.

📝 Teacher's Note: Be careful with negative numbers in these formulas. A negative Celsius value will often lead to a lower Fahrenheit value.

🎯 Exam Tip: Check your signs carefully when multiplying \( 9/5 \) by a negative number.

Question 20. What is absolute zero and what is its value on the Celsius scale?
Answer: Absolute zero of temperature is \( 0 \text{ K} \).
Temperature in Kelvin \( \text{To K} = \text{Temperature in Celsius To C} + 273 \).
\( 0 \text{ K} = \text{To C} + 273 \).
\( \implies \text{To C} = -273^\circ \text{C} \).
Absolute zero of temperature on Celsius scale is \( -273^\circ \text{C} \).
In simple words: Absolute zero is the coldest temperature possible where nothing can get any colder. It is \( -273 \) degrees Celsius.

📝 Teacher's Note: Absolute zero is the temperature at which all molecular motion theoretically stops.

🎯 Exam Tip: Memorize the value \( -273^\circ \text{C} \) as it is a fundamental constant in physics.

Question 21. If the difference between two bodies is \( 1^\circ \text{C} \), what is the difference in Fahrenheit?
Answer: Difference of temperature of two bodies in Celsius scale \( = 1^\circ \text{C} \).
We know \( \frac{\text{C}}{100} = \frac{\text{F} - 32}{180} \).
\( \text{C} = \frac{5}{9}(\text{F} - 32) \)
\( \frac{9}{5}\text{C} + 32 = \text{F} \)
\( \text{F} = \frac{9}{5}\text{C} + 32 \).
So, difference of \( 1^\circ \) in Celsius scale is equal to the difference of \( 9/5^\circ \) in Fahrenheit scale.
Thus, Difference of \( 1^\circ \text{C} \) of temperature of two bodies in Celsius scale is equal to difference of \( 1.8^\circ \) in Fahrenheit scale.
In simple words: Because Fahrenheit degrees are smaller, if something warms up by \( 1 \) degree Celsius, it's the same as warming up by \( 1.8 \) degrees Fahrenheit.

📝 Teacher's Note: This is about the *interval*, not the specific point. There are \( 180 \) Fahrenheit divisions for every \( 100 \) Celsius divisions, so \( 180/100 = 1.8 \).

🎯 Exam Tip: Don't add \( 32 \) when calculating temperature *differences*. Only use the ratio \( 1.8 \).

Question 22. Who invented the Celsius scale of temperature?
Answer: Celsius invented the Celsius scale of temperature.
In simple words: A Swedish astronomer named Anders Celsius invented this scale, which is why it bears his name.

📝 Teacher's Note: Originally, Celsius set \( 0 \) as boiling and \( 100 \) as freezing, but it was reversed after his death to the system we use today.

🎯 Exam Tip: This is a basic trivia question; ensure you spell "Celsius" correctly.

Question 23. Who invented the Fahrenheit scale of temperature?
Answer: Fahrenheit invented the Fahrenheit scale of temperature.
In simple words: Gabriel Fahrenheit created this scale, which is still very popular in the United States today.

📝 Teacher's Note: Fahrenheit was also the first to use mercury as the thermometric liquid instead of alcohol/water mixtures.

🎯 Exam Tip: Like the Celsius question, this is straightforward—the scale is named after its inventor.

Question 24. Which liquids are commonly used in thermometers?
Answer: Mercury, alcohol are commonly used in thermometers.
In simple words: Most thermometers use either mercury (the silver one) or alcohol (usually dyed red or blue) to show the temperature.

📝 Teacher's Note: Alcohol is often used in very cold climates because it doesn't freeze until \( -114^\circ \text{C} \), while mercury freezes at \( -39^\circ \text{C} \).

🎯 Exam Tip: List both for a complete answer.

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Question 25. Which animals can vary their body temperature according to the seasons?
Answer: Camel and are two animals which are able to increase their body temperature in summers and decrease their body temperature in winters.
In simple words: Some animals, like camels, can let their bodies get hotter in the summer and cooler in the winter to save energy and water.

📝 Teacher's Note: This is called being "heterothermic." It allows camels to go longer without drinking water in the desert heat.

🎯 Exam Tip: If the text mentions "Camel and [blank]," it usually refers to another desert animal like an oryx, but stick to what is in your notes.

Question 26. Define the CGS unit of heat and its relation to temperature.
Answer: CGS unit of heat is Joule denoted by J.
\( 1 \text{ J} \) is amount of heat required to raise the temperature of a body by \( 1/4.12^\circ \text{C} \) of temperature.
In simple words: A Joule is a tiny bit of heat energy. One Joule can heat an object up by a small fraction of a degree.

📝 Teacher's Note: Verbatim note: Technically, the CGS unit of heat is the 'calorie'. The Joule is the SI unit. The value \( 4.12 \) (often \( 4.186 \)) is the mechanical equivalent of heat.

🎯 Exam Tip: In this context, remember that \( 1 \text{ calorie} \approx 4.18 \text{ Joules} \).

Question 27. Show the relationship between Celsius and Fahrenheit as a graph with Celsius on the Y-axis.
Answer: We know \( \frac{\text{C}}{100} = \frac{\text{F} - 32}{180} \).
\( \text{C} = \frac{5}{9}(\text{F} - 32) \)
\( \text{C} = \frac{5}{9}\text{F} - 17.77 \).
Graph between \( \text{T}^\circ \text{C} \) and \( \text{T}^\circ \text{F} \) is straight line having slope of \( 5/9 \) and intercept on \( \text{Y} \) axis is \( -17.77 \).
In simple words: This graph shows how Celsius and Fahrenheit temperatures go up together in a straight line. If you know one, you can find the other.

📝 Teacher's Note: The intercept \( -17.77 \) is where \( 0^\circ \text{F} \) would be on the Celsius scale (\( 0 \text{ F} \approx -17.8 \text{ C} \)).

🎯 Exam Tip: Note that the intercept is on the negative part of the vertical axis.

Question 28. Show the relationship between Fahrenheit and Celsius as a graph with Fahrenheit on the Y-axis.
Answer: We know \( \frac{\text{C}}{100} = \frac{\text{F} - 32}{180} \).
\( \text{C} = \frac{5}{9}(\text{F} - 32) \)
\( \frac{9}{5}\text{C} + 32 = \text{F} \)
\( \text{F} = \frac{9}{5}\text{C} + 32 \).
Graph between \( \text{T}^\circ \text{F} \) and \( \text{T}^\circ \text{C} \) is straight line having slope of \( 9/5 \) and intercept on \( \text{Y} \) axis is \( 32 \).
In simple words: This graph shows that when Celsius is zero, Fahrenheit starts at \( 32 \). Then, they both increase in a straight line.

📝 Teacher's Note: The intercept is at \( 32 \) because \( 0^\circ \text{C} = 32^\circ \text{F} \). The line is steeper than the previous graph because the slope \( 1.8 \) is greater than \( 1 \).

🎯 Exam Tip: Label the axes clearly with their units and the intercept value \( 32 \) to get full marks.

Question 29. State the formula relating Celsius and Fahrenheit scales.
Answer: Relation between Celsius and Fahrenheit scales of temperature is
\( \frac{\text{C}}{100} = \frac{\text{F} - 32}{180} \).
In simple words: This math formula is like a translation guide that helps you change a Celsius temperature into a Fahrenheit one.

📝 Teacher's Note: You can simplify this formula to \( \frac{\text{C}}{5} = \frac{\text{F} - 32}{9} \) for easier calculations during exams.

🎯 Exam Tip: Memorize this formula; it's the most common calculation required in this chapter.

Question 30. What are the ice and steam points on the Fahrenheit scale?
Answer: Temperature of ice point on Fahrenheit scale \( = 32^\circ \text{F} \).
Temperature of steam point on Fahrenheit scale \( = 212^\circ \).
In simple words: On the Fahrenheit scale, water turns to ice at \( 32 \) degrees and boils into steam at \( 212 \) degrees.

📝 Teacher's Note: Point out that the difference between these two points is exactly \( 180 \) divisions (\( 212 - 32 = 180 \)).

🎯 Exam Tip: These values are the "Fixed Points" of the Fahrenheit scale. Always include the \( ^\circ \text{F} \) unit.