Heat and Temperature — Exercise Solutions

Samacheer Kalvi Tamil Nadu State Board Class 7 Science Term 2 Chapter 1 exercise solutions. Covers MCQs, fill-in-the-blanks, true/false, match the following, assertion and reason, very short, short, detailed answers, numerical problems, and HOTS questions.

I. Choose the Correct Answer (MCQ)

Q1. International unit of measuring temperature is: (a) Kelvin (b) Fahrenheit (c) Celsius (d) Joule Answer: (a) Kelvin ✓ — Kelvin (K) is the SI unit of temperature.

Q2. In thermometer when bulb contacts a hot object, liquid inside it: (a) expands (b) contracts (c) remains same (d) none of above Answer: (a) expands ✓ — Liquids expand on heating and contract on cooling; this principle is used in thermometers.

Q3. Body temperature of a healthy man is: (a) 0°C (b) 37°C (c) 98°C (d) 100°C Answer: (b) 37°C ✓ — Normal human body temperature is 37°C (98.6°F).

Q4. Mercury is often used in laboratory thermometers because it: (a) is a harmless liquid (b) is silvery and attractive (c) Expands uniformly (d) is a low cost liquid Answer: (c) Expands uniformly ✓ — Uniform expansion ensures accurate temperature readings.

Additional MCQs

Q5. Heat energy is the total of the particles that make up a substance: (a) potential energy (b) substance (c) temperature (d) none Answer: (b) substance ✓ — Heat energy is the total kinetic energy of all particles in a substance.

Q6. Heat energy is always transferred from ___ to ___ temperatures: (a) lower, higher (b) higher, lower (c) conductor, insulator (d) both a and b Answer: (b) higher, lower ✓ — Heat always flows from a hotter body to a colder body.

Q7. A clinical thermometer is calibrated from ___ to ___: (a) 10°C to 100°C (b) 32°C to 110°C (c) 0°C to 100°C (d) 35°C to 42°C Answer: (d) 35°C to 42°C ✓ — The clinical thermometer range covers the range of human body temperature.

Q8. The thermometer ranging from -10°C to 110°C is: (a) clinical thermometer (b) digital thermometer (c) laboratory thermometer (d) All of these Answer: (c) laboratory thermometer ✓ — A wider range is needed for laboratory use.

Q9. Which scale has lower fixed point at 0°C? (a) Kelvin (b) Fahrenheit (c) Celsius (d) All Answer: (c) Celsius ✓ — On the Celsius scale, 0°C is the melting point of ice (lower fixed point).

Q10. The lower fixed point on the Celsius scale is: (a) melting point of mercury (b) melting point of ice (c) boiling point of water (d) none Answer: (b) melting point of ice ✓ — 0°C corresponds to the melting/freezing point of ice.

Q11. The measure of degree of hotness or coldness of a body is called: (a) heat energy (b) Celsius (c) kelvin (d) temperature Answer: (d) temperature ✓ — Temperature measures how hot or cold a body is.

Q12. Digital thermometer is widely used due to: (a) Analog display (b) Mercury (c) High accuracy (d) Low accuracy Answer: (c) High accuracy ✓ — Digital thermometers provide accurate readings without mercury.

Q13. Temperature of boiling water cannot be measured by a ___ thermometer: (a) laboratory (b) digital (c) clinical (d) both b and c Answer: (d) both b and c ✓ — A clinical thermometer (35-42°C) and most digital thermometers cannot measure boiling water temperature (100°C).

Q14. When a substance is heated, its temperature: (a) rises (b) falls (c) remains same (d) none Answer: (a) rises ✓ — Heating increases the kinetic energy of particles, raising the temperature.

II. Fill in the Blanks

1. Doctor uses clinical thermometer to measure the human body temperature. 2. At room temperature mercury is in liquid state. 3. Heat energy transfer from hotter object to colder object. 4. -7°C temperature is less than 0°C temperature. 5. The common laboratory thermometer is a mercury thermometer.

Additional Fill in the Blanks

6. Digital thermometers do not use mercury. 7. In a thermometer, when liquid gets heat, it expands and when cooled, it contracts. 8. The SI unit of temperature is kelvin. 9. A small change in the temperature causes change in volume of a liquid. 10. The freezing point of alcohol is less than -100°C. 11. All clinical thermometers have a kink that prevents mercury from flowing back into the bulb. 12. A clinical thermometer indicates temperatures from minimum 94°F to maximum 108°F. 13. Before use, the mercury level in clinical thermometer should be below 35°C or 94°F. 14. The coldest natural temperature ever recorded on earth is 178.45 K. 15. The hottest natural temperature ever recorded on earth is 392.85 K.

III. True or False (with corrections)

Q1. Temperature is related to how fast the atoms within a substance are moving. Answer: True.

Q2. Mercury is a poor conductor of heat. Answer: False. Mercury is a good conductor of heat.

Q3. Fahrenheit scale is more sensitive than the Celsius scale. Answer: True. — Each Fahrenheit degree is smaller than each Celsius degree, making it more sensitive.

Q4. In humans, the average internal temperature is 98.60°C. Answer: False. In humans, the average internal temperature is 98.60°F (37°C).

Q5. Digital thermometers are mainly used to take the body temperature. Answer: True.

Q6. The glass of a clinical thermometer will burst due to excessive pressure created by contraction of mercury. Answer: False. The glass bursts due to excessive pressure created by expansion of mercury.

Q7. In Greek, Centium means 100 and Gradus means steps, both words make Centigrade and later Celsius. Answer: True.

Q8. Kelvin is written as K, also known as absolute scale as it starts from absolute zero temperature. Answer: True.

Q9. The temperatures in Celsius and in Fahrenheit will be same at -40. Answer: True. — At -40, both Celsius and Fahrenheit scales show the same value.

IV. Match the Following

V. Assertion and Reason

Q1. Assertion (A): Heat energy is transferred from one body to another due to a temperature difference. Reason (R): Heating a substance causes a rise in temperature. Answer: (b) Both A and R are true but R is not the correct explanation. ✓ — The correct reason: heat travels from hotter to colder objects because of the temperature difference, not merely because heating causes a temperature rise.

Q2. Assertion (A): When very hot liquid is poured into a thick glass tumbler, it cracks. Reason (R): Unequal expansion of inner and outer glass walls causes cracking. Answer: (a) Both A and R are true and R is the correct explanation. ✓ — The inner wall expands more than the outer wall, creating stress that cracks the glass.

VI. Very Short Answer Questions

Q1. Temperature of Srinagar (J and K) is -4°C and Kodaikanal is 3°C. Which has greater temperature? What is the difference between these two temperatures? Answer: Kodaikanal has the greater temperature. Difference = 3 - (-4) = 7°C.

Q2. Jyothi was prepared to measure hot water temperature with a clinical thermometer. Is it right or wrong? Why? Answer: Wrong. A clinical thermometer has a small range (35°C to 42°C). Used on hot water, the mercury would expand beyond the scale, potentially cracking the glass.

Q3. A clinical thermometer is not used to measure air temperature. Why? Answer: The range of a clinical thermometer (35-42°C) is far less than what is needed to measure the wider range of air temperatures, which can go below 0°C or above 50°C.

Q4. What is the use of kink in clinical thermometer? Answer: The kink prevents mercury from flowing back into the bulb when the thermometer is removed from the patient's mouth, allowing the temperature to be read conveniently.

Q5. Why do we jerk a clinical thermometer before measuring body temperature? Answer: The jerk allows mercury to flow back into the bulb so the level is below normal body temperature, ensuring an accurate new reading.

Additional Very Short Answers

Q6. Mention the three units used to measure temperature. Answer: Degree Celsius (°C), Fahrenheit (°F), and Kelvin (K).

Q7. What is the boiling point of mercury? Answer: 357°C

Q8. What is the freezing point of alcohol? Answer: Less than -100°C

Q9. Mention the use of laboratory thermometers. Answer: To measure temperature in laboratories for scientific research and in industry.

Q10. What is the average internal temperature of the human body? Answer: 37°C (98.6°F)

Q11. What is the freezing point of water in Fahrenheit? Answer: 32°F

Q12. At what temperature will the value be same in Celsius and Fahrenheit? Answer: -40°

Q13. Name the principle used in thermometers. Answer: Liquids expand on heating and contract on cooling.

VII. Short Answer Questions

Q1. Why do we use mercury in thermometers? Can water be used instead? What are the problems? Answer: Mercury remains in liquid form across a wide temperature range. A small temperature change causes a measurable volume change, making it useful. Water cannot be used because:

• It does not work below 0°C or above 100°C
• It is transparent, making scale readings difficult
• It wets the glass tube
• Its expansion is not uniform

These constraints make water unsuitable as a thermometric liquid.

Q2. Swathi kept a laboratory thermometer in hot water and took it out to read the temperature. Ramani said it was wrong. Do you agree? Answer: Yes, Ramani is correct. A laboratory thermometer has no kink, so when removed from the water, the mercury level falls back immediately. The reading should be taken while the bulb is still surrounded by the hot water.

Q3. Srinath's body temperature is 99°F. Is he suffering from fever? Answer: Yes, because the normal body temperature is 98.6°F. A temperature of 99°F is above normal, indicating a mild fever.

Additional Short Answers

Q4. Mention two precautions while using a clinical thermometer. Answer: 1. Wash before and after use, preferably with antiseptic solution. 2. Jerk the thermometer a few times to bring the mercury level down before use.

Q5. What are the uses of laboratory thermometers? Answer: They are used in school and other laboratories for scientific research, and in industry as they can measure higher temperatures than clinical thermometers.

Q6. Mention two properties of alcohol. Answer: 1. Its freezing point is below -100°C, so it can measure very low temperatures. 2. Its expansion per degree Celsius rise is very large, making it sensitive to small temperature changes.

Q7. Why should we avoid keeping a clinical thermometer in the sun or near a flame? Answer: It has a small temperature range. The glass can crack or burst due to excessive pressure from mercury expansion beyond its scale limits.

VIII. Detailed Answer Questions

Q1. State similarities and differences between laboratory thermometer and clinical thermometer.

Similarities: Both have long, narrow, uniform glass tubes; both bulbs contain mercury; both have Celsius scales.

Differences:

Q2. Explain the properties of mercury. Answer: 1. Uniform expansion — Equal heat produces equal expansion, ensuring accurate readings. 2. Opaque and shining — Easy to see the mercury level in the glass tube. 3. Does not stick to glass — Mercury does not wet the sides of the glass tube. 4. Good conductor of heat — Quickly takes the temperature of the surrounding substance. 5. Wide range — High boiling point (357°C) and low freezing point (-39°C) enable measurement of a wide range of temperatures.

Q3. Explain the construction and working of a clinical thermometer. Answer: Clinical thermometers measure human body temperature at home, clinics, and hospitals. Key features:

• Contains a kink that prevents mercury from flowing back into the bulb when removed from the patient's mouth
• Temperature scales on both sides — Celsius and Fahrenheit
• Since the Fahrenheit scale is more sensitive, body temperature is measured in °F
• Range: 35°C (94°F) minimum to 42°C (108°F) maximum

Q4. Explain precautions while using a clinical thermometer. Answer: 1. Wash before and after use, preferably with antiseptic solution 2. Jerk a few times to bring the mercury level down before use 3. Before use, mercury level should be below 35°C or 94°F 4. Do not hold the thermometer by its bulb 5. Keep the mercury level along your line of sight when reading 6. Handle with care — it may break if it hits a hard object 7. Do not place in a hot flame or in the hot sun

Q5. Write a note on laboratory thermometer and precautions. Answer: (a) Laboratory thermometer: Used in school and other laboratories for scientific research and in industry for temperatures beyond the clinical range. The stem and bulb are longer than those of a clinical thermometer. There is no kink. The scale ranges from -10°C to 110°C in Celsius only.

(b) Precautions: 1. Do not tilt the thermometer — place it upright 2. Note the reading only when the bulb has been surrounded by the substance from all sides

Q6. Explain how to determine the temperature of hot water using a laboratory thermometer. Answer: 1. Take water in a beaker 2. Immerse the bulb end vertically in water, ensuring the entire bulb is submerged but not touching the bottom or sides 3. Observe the mercury rise; when it stabilizes, take the reading 4. Repeat with hot water and note that reading

Q7. Write a note on digital thermometer. Answer: Mercury is toxic and difficult to dispose of if a thermometer breaks. Digital thermometers do not use mercury; instead they have a sensor that measures heat coming from the body directly to determine temperature. They are mainly used for body temperature. Usage steps: 1. Wash the tip with warm (not hot), soapy water 2. Press the "ON" button 3. Insert the tip into the mouth, bottom, or under the armpit 4. Hold in place until it beeps (about 30 seconds) 5. Read the display 6. Turn off, rinse with water, and store safely

Precautions: Do not use a clinical thermometer for objects other than the human body. Avoid keeping it in the sun or near a flame.

IX. Numerical Problems

Formulae:

• °F = (9/5 × °C) + 32
• °C = (°F - 32) × 5/9
• K = °C + 273.15

Q1. Convert 45°C to °F. Answer: °F = (9/5 × 45) + 32 = 81 + 32 = 113°F

Q2. Convert 20°C to °F. Answer: °F = (9/5 × 20) + 32 = 36 + 32 = 68°F

Q3. Convert 68°F to °C. Answer: °C = (68 - 32) × 5/9 = 36 × 5/9 = 20°C

Q4. Convert 185°F to °C. Answer: °C = (185 - 32) × 5/9 = 153 × 5/9 = 85°C

Q5. Convert 0°C to K. Answer: K = 0 + 273.15 = 273.15 K

Q6. Convert -20°C to K. Answer: K = -20 + 273.15 = 253.15 K

Q7. Convert 272.15 K to °C. Answer: °C = 272.15 - 273.15 = -1°C

Additional Numerical Problems

Q8. Convert 40°C to °F. Answer: °F = (9/5 × 40) + 32 = 72 + 32 = 104°F

Q9. Convert 0 K to °C. Answer: °C = 0 - 273.15 = -273.15°C

Q10. Convert -30°C to K. Answer: K = -30 + 273.15 = 243.15 K

Q11. Convert 150°F to °C. Answer: °C = (150 - 32) × 5/9 = 118 × 5/9 = 65.6°C

X. Higher Order Thinking Skills (HOTS)

Q1. What must be the temperature in Fahrenheit so that it will be twice its value in Celsius? Answer: Using F = (9C/5) + 32 and F = 2C: 2C = (9/5)C + 32 2C - (9/5)C = 32 (1/5)C = 32 C = 160 F = 2 × 160 = 320°F

Q2. Is there a maximum possible temperature like the minimum of 0 K? Answer: The minimum possible temperature is 0 K (absolute zero). There is no defined limit to the maximum temperature. Theoretically, temperature can increase without bound as particles gain more kinetic energy.

Common Mistakes to Avoid

1. Confusing heat and temperature — Heat is a form of energy (measured in joules); temperature measures how hot or cold a body is (measured in °C, °F, or K). 2. Reading a laboratory thermometer after removing it — Unlike a clinical thermometer, it has no kink, so mercury falls back immediately. Always read while the bulb is in the substance. 3. Using °C symbol for Kelvin — Kelvin uses only "K" without a degree symbol (e.g., 273 K, not 273°K). 4. Forgetting to add 32 in Fahrenheit conversion — The formula is F = (9/5 × C) + 32, not just (9/5 × C). 5. Saying mercury expands on cooling — Mercury expands on heating and contracts on cooling. 6. Confusing clinical and laboratory thermometer ranges — Clinical: 35-42°C; Laboratory: -10°C to 110°C. 7. Thinking water can replace mercury in thermometers — Water freezes at 0°C, boils at 100°C, is transparent, and wets glass — making it unsuitable.

Exam Checklist

• [ ] Can define heat and temperature and distinguish between them
• [ ] Know the three temperature scales (Celsius, Fahrenheit, Kelvin) and their conversions
• [ ] Can list the properties of mercury that make it suitable for thermometers
• [ ] Can explain the construction and working of a clinical thermometer
• [ ] Can differentiate between clinical and laboratory thermometers
• [ ] Can explain the purpose and function of the kink in a clinical thermometer
• [ ] Can state precautions for using both clinical and laboratory thermometers
• [ ] Can solve temperature conversion problems (Celsius-Fahrenheit-Kelvin)
• [ ] Understand why digital thermometers are preferred over mercury thermometers
• [ ] Can explain why water cannot replace mercury as a thermometric liquid