Class 9 Science Chapter 7 Temperature and its Measurement: Complete Important Questions & Answers

Chapter 7 appears consistently in CBSE Science papers with 8–10 marks allocated across Sections A, B, and C. The board tests your understanding in three key areas: (1) conceptual clarity on temperature vs. heat, (2) practical knowledge of thermometer types and their correct usage, and (3) numerical problem-solving using the Celsius scale and temperature conversion. In recent board papers, examiners have shifted toward application-based questions — for example, 'Why is a clinical thermometer used only for body temperature measurement?' or 'Explain how a mercury thermometer works based on thermal expansion.' Short-answer questions (2 marks) often ask you to differentiate between clinical and lab thermometers, while 5-mark questions demand detailed explanations of heat transfer mechanisms (conduction, convection, radiation) with everyday examples. Understanding the structure of important questions helps you allocate study time efficiently: spend 40% on concept clarity, 30% on definition-based short answers, and 30% on application and heat transfer scenarios. This page organizes all question types you're likely to encounter, so you can practice with the exact rigor your examiner expects.

Multiple-choice questions in Section A are designed to test your instant recall of definitions, facts, and basic principles. You'll have limited time (usually 20–30 seconds per question), so familiarity with common distractors is crucial. **Question 1:** The SI unit of temperature is: (a) Fahrenheit (b) Kelvin (c) Celsius (d) Centigrade **Answer:** (b) Kelvin. Although Celsius and Fahrenheit are widely used, Kelvin (K) is the SI unit. In CBSE exams, one question almost always asks this. **Question 2:** A clinical thermometer is designed to measure temperature in the range: (a) −10 °C to 110 °C (b) 35 °C to 42 °C (c) 0 °C to 100 °C (d) 50 °C to 150 °C **Answer:** (b) 35 °C to 42 °C. This narrow range is specific to body temperature measurement. Lab thermometers (−10 °C to 110 °C) have a wider range. **Question 3:** The phenomenon by which a liquid in a thermometer rises when heated is called: (a) Thermal conduction (b) Thermal convection (c) Thermal expansion (d) Thermal radiation **Answer:** (c) Thermal expansion. Mercury and alcohol expand when heated, pushing the liquid column up the tube. **Question 4:** Which of the following is NOT a method of heat transfer? (a) Conduction (b) Convection (c) Radiation (d) Absorption **Answer:** (d) Absorption. The three mechanisms of heat transfer are conduction, convection, and radiation. Absorption is a property, not a transfer method. **Question 5:** An ideal thermometer should have: (a) High specific heat capacity (b) Low thermal conductivity (c) High coefficient of expansion (d) Low boiling point **Answer:** (c) High coefficient of expansion. A liquid with a high expansion coefficient (like mercury or alcohol) allows even small temperature changes to produce visible movement in the tube, making the thermometer sensitive and accurate.

Two-mark questions require clear, concise answers (usually 2–3 sentences) that define a concept or compare two ideas. These are the workhorses of CBSE papers. **Question 1:** Differentiate between a clinical thermometer and a lab thermometer. **Answer:** A clinical thermometer measures body temperature (35–42 °C range), has a kink in the tube that prevents mercury from falling back when removed from the body, and is graduated in 0.1 °C divisions for precision. A lab thermometer measures a wider range (−10 to 110 °C), has no kink, and is graduated in 1 °C divisions, making it suitable for various laboratory experiments. **Question 2:** What is meant by the term 'thermal expansion'? Give one example. **Answer:** Thermal expansion is the increase in volume of a substance when its temperature increases. Example: A mercury thermometer works because mercury expands when heated, rising in the tube to indicate higher temperatures. **Question 3:** Explain why a thermometer should not be shaken vigorously. **Answer:** Shaking a thermometer vigorously can cause the glass bulb to crack or break, damaging the instrument. Additionally, if you shake it too hard during measurement, it may give an incorrect reading or cause the liquid to separate from the tube. **Question 4:** Define heat and temperature. Are they the same? **Answer:** Temperature is the measure of the average kinetic energy of particles in a substance, measured in degrees Celsius or Kelvin. Heat is the energy transferred from one body to another due to a temperature difference. They are not the same: temperature is a property of a substance, while heat is energy in motion. **Question 5:** Why does a digital thermometer give faster readings than a mercury thermometer? **Answer:** A digital thermometer uses an electronic sensor (like a thermistor) that detects temperature changes instantly through electrical resistance changes, displaying the result immediately. A mercury thermometer relies on the slower process of thermal expansion of mercury, which takes more time to reach equilibrium with the body or object being measured.

Three-mark questions blend definition, explanation, and reasoning. They test whether you can link concepts together and apply knowledge to simple scenarios. **Question 1:** Explain the structure and working of a mercury thermometer. Why is mercury preferred over water? **Answer:** A mercury thermometer consists of a thin glass tube with a bulb at one end containing mercury, sealed at the top with a vacuum or inert gas, and marked with a scale. When the bulb is in contact with a hot object, mercury absorbs heat, expands, and rises up the tube; the height of the column indicates temperature. Mercury is preferred over water because: (1) it has a higher boiling point (357 °C), allowing measurement of higher temperatures, (2) it has a larger coefficient of expansion, making small temperature changes visible, (3) it does not wet glass, so readings are clear and unambiguous, and (4) it remains in liquid state over a wider temperature range than water. **Question 2:** Describe the three methods of heat transfer with one example each. **Answer:** (1) Conduction: Heat transfer through direct contact without movement of particles. Example: A metal spoon placed in hot water heats up because heat travels through the spoon material from hot to cold end. (2) Convection: Heat transfer through movement of fluids (liquids or gases). Example: In a room heated by a radiator, warm air rises and cold air sinks, creating circulation that distributes heat throughout the room. (3) Radiation: Heat transfer through electromagnetic waves without needing a medium. Example: The Earth receives heat from the Sun through the vacuum of space via radiation. **Question 3:** A clinical thermometer reads 39.2 °C. What does this reading tell you? Why should it be used only for measuring body temperature? **Answer:** A reading of 39.2 °C indicates a fever, as normal human body temperature is approximately 37 °C (37 ± 0.5 °C). A clinical thermometer should be used only for body temperature because: (1) its narrow range (35–42 °C) is designed specifically for human body temperatures, (2) the kink in the tube prevents mercury from falling back, allowing safe removal from the body without loss of reading, (3) its fine graduations (0.1 °C) provide precision needed for medical diagnosis, and (4) its short length and design make it safe and convenient for oral or axillary (underarm) use. **Question 4:** Explain why the Celsius scale is more convenient for everyday use than the Kelvin scale. **Answer:** The Celsius scale uses the freezing point of water (0 °C) and boiling point of water (100 °C) as reference points, which are familiar and reproducible phenomena accessible in everyday life. Most people encounter water freezing and boiling, making Celsius readings intuitive: 0 °C means freezing, 100 °C means boiling, and room temperature is around 25 °C. In contrast, the Kelvin scale has its zero point at absolute zero (−273.15 °C), which is a theoretical extreme not experienced in daily life. Additionally, Celsius uses smaller numbers for everyday temperatures (e.g., 25 °C vs. 298 K for room temperature), making it more practical for common use. However, scientists prefer Kelvin because it avoids negative values and is the SI unit for thermodynamic calculations.

Five-mark questions demand comprehensive explanations, multi-step reasoning, or extended descriptions. These often appear in Section C and test your ability to synthesize knowledge. **Question 1:** Explain the principle behind the working of a thermometer. Include the role of thermal expansion, and discuss why the glass container must have a uniform bore. **Answer:** Principle: A thermometer works on the principle of thermal expansion — substances expand when heated and contract when cooled. The bulb contains a liquid (usually mercury or alcohol) that expands uniformly with temperature changes. Working: When the thermometer bulb touches a hot object, the liquid absorbs heat and expands. This increased volume forces the liquid to rise in the narrow tube above the bulb. The height of the liquid column is marked on a calibrated scale that indicates the temperature. As temperature decreases, the liquid contracts and the column falls, giving a lower reading. Uniform Bore: The glass tube must have a uniform bore (constant cross-sectional area) to ensure linear and accurate readings. If the bore is irregular, the same volume of liquid expansion might produce different heights in different sections, making the scale non-linear and readings inaccurate. For instance, if the tube is wider in one section, 1 °C expansion might cause the column to rise 0.5 mm in that section but 2 mm in a narrower section, causing confusion. A uniform bore ensures that each degree of temperature change produces a proportional and consistent rise in the liquid column, making the thermometer reliable and easy to read. **Question 2:** A student recorded the following temperatures at different times: 6 AM: 12 °C, 12 PM: 28 °C, 6 PM: 22 °C, 12 AM: 15 °C. Explain the changes using heat transfer concepts and suggest how each temperature change occurs in nature. **Answer:** The temperature progression reflects diurnal (daily) temperature variations driven by solar radiation and heat transfer mechanisms. 6 AM to 12 PM (12 °C → 28 °C, increase of 16 °C): After sunrise, the Sun's radiation heats the Earth's surface through direct radiation. Heat is then transferred from the ground to the air through conduction (direct contact) and convection (warm air rises, cold air sinks). This produces the morning-to-noon temperature rise. The increase is rapid because solar intensity increases as the sun climbs higher in the sky. 12 PM to 6 PM (28 °C → 22 °C, decrease of 6 °C): Even though the Sun is still above the horizon at 6 PM, solar intensity begins to decrease as the Sun approaches the horizon. Additionally, heat stored in the ground during midday starts to be re-radiated into space. The atmosphere cools more slowly than the ground because air has low thermal conductivity; it receives heat primarily through radiation from the ground and convection, not directly from the Sun. 6 PM to 12 AM (22 °C → 15 °C, decrease of 7 °C): After sunset, the Sun no longer radiates energy to this location. The Earth's surface and atmosphere continue to lose heat through radiation into space (infrared radiation escaping to the atmosphere and beyond). This cooling accelerates after dark. By midnight, the minimum temperature is reached. Key Insight: The daily cycle is driven by solar radiation (energy input) and heat loss through radiation (energy output), modulated by conduction and convection. **Question 3:** A child accidentally touches a hot pan and immediately pulls their hand away. Explain the pathway of heat transfer involved, the reason for the rapid heat transfer, and one precautionary measure to prevent such accidents. **Answer:** Heat Transfer Pathway: When the child's hand (cooler, ~37 °C) contacts the hot pan (heated, ~150–200 °C), heat flows from the pan to the hand through conduction. Conduction occurs because the metal pan is in direct contact with the skin. The metal atoms in the pan vibrate vigorously due to their high temperature; they collide with adjacent atoms, transferring kinetic energy progressively through the metal and then from the metal surface to the skin cells. This energy transfer ultimately stimulates pain receptors in the skin, triggering the reflex action of pulling the hand away. Why Rapid Heat Transfer: Metals are excellent conductors of heat because they contain free electrons that move easily and transfer energy rapidly. Additionally, the large temperature difference between the pan and the hand (ΔT ≈ 150 °C) drives heat flow quickly according to Fourier's law: the rate of heat transfer is proportional to the temperature difference. The small distance between contact surfaces also facilitates rapid energy transfer. Precautionary Measure: Use insulated handles, oven gloves, or tongs when handling hot cookware. These materials (cotton, rubber, or silicone) are poor conductors of heat and thermal insulators. They trap air pockets that slow heat conduction, preventing the hand from feeling intense heat. Alternatively, always be aware of hot objects, avoid touching them carelessly, and teach children about heat safety early.

Higher Order Thinking Skills (HOTS) questions require analysis, evaluation, and creative application of concepts. Case-study questions present realistic scenarios and ask you to solve them step-by-step. **Case-Study Question:** A scientist is designing a thermometer to measure temperatures from −50 °C to +200 °C for use in laboratories. She must choose between mercury and alcohol as the working liquid. Based on the properties listed below, recommend which liquid is more suitable and justify your answer. Properties: - Mercury: boiling point 357 °C, freezing point −39 °C, coefficient of expansion 0.00018 per °C, does not wet glass - Alcohol (ethanol): boiling point 78 °C, freezing point −117 °C, coefficient of expansion 0.0011 per °C, slightly wets glass **Solution with Steps:** Step 1: Identify the requirements. The thermometer must measure from −50 °C to +200 °C, a range of 250 °C. Both extremes must remain within the liquid's freezing and boiling points. Step 2: Check freezing point compatibility. - Mercury freezes at −39 °C, which is higher than the required −50 °C minimum. If the thermometer reaches −50 °C, mercury will freeze and become unusable. - Alcohol freezes at −117 °C, well below −50 °C. It remains liquid throughout the required range. **Decision: Alcohol is superior for low-temperature measurements.** Step 3: Check boiling point compatibility. - Mercury boils at 357 °C, which safely exceeds the +200 °C maximum. Mercury will remain liquid throughout the range. - Alcohol boils at 78 °C, far below the +200 °C maximum. If the thermometer reaches 100 °C, alcohol will boil and become gas, making the instrument non-functional. **Decision: Mercury is superior for high-temperature measurements.** Step 4: Evaluate expansion coefficient. - Mercury (0.00018 per °C): Lower expansion coefficient means a 1 °C change causes smaller displacement in the tube. Over a 250 °C range, the total displacement is 250 × 0.00018 = 0.045 cm of expansion, requiring a long tube but allowing precise gradations. - Alcohol (0.0011 per °C): Higher expansion coefficient means 1 °C change causes larger displacement. Over 250 °C, total displacement is 250 × 0.0011 = 0.275 cm, making readings more visible and easier to read without a very long tube. Step 5: Consider wetting behavior. - Mercury does not wet glass, producing clear, sharp meniscus readings. - Alcohol slightly wets glass, which can make the meniscus less sharp, leading to reading ambiguity. **Recommendation & Justification:** Neither liquid is perfect for this extreme range (−50 to +200 °C). However, **alcohol is the better choice** because: (1) it remains liquid across the entire range (most critical requirement), (2) its higher expansion coefficient makes small temperature changes visible without an excessively long tube, and (3) for the high-temperature end, the scientist can design the thermometer with a narrower upper section or design it to measure only up to 78 °C (alcohol's boiling point) and use a separate mercury thermometer for higher temperatures. Alternatively, she could use a **mixture of alcohol and other liquids** or a **gas thermometer** (which can handle extreme ranges) as an advanced solution, but if restricted to simple liquid thermometers, alcohol is the practical recommendation with the caveat that its range is −117 °C to 78 °C, not the full 200 °C.

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