Get the complete Chapter 7 Solved Exercise of Thermal Properties of Matter for Class 9th. Perfect for All Punjab Boards, this guide includes MCQs, short and long questions with detailed explanations.
MCQs on Temperature and Heat
Q1: How do the molecules in a solid behave?
Statement: Molecules in a solid
Options:
(a) Move randomly
(b) Vibrate about their mean positions
(c) Rotate and vibrate randomly at their own positions
(d) Move in a straight line from hot to cold ends
Answer: (b) Vibrate about their mean positions
Explanation: In solids, molecules are tightly packed and can only vibrate around fixed positions due to strong intermolecular forces.
Tip: In solids, particles do not have translational motion but only vibrational motion.
Q2: What type of motion is exhibited by gas molecules?
Statement: The motion of molecules in a gas is mostly
Options:
(a) Linear motion
(b) Random motion
(c) Vibratory motion
(d) Rotatory motion
Answer: (b) Random motion
Explanation: Gas molecules move freely and collide randomly in all directions, leading to chaotic or random motion.
Tip: Gas molecules have the highest kinetic energy and move in all directions.
Q3: What does temperature measure?
Statement: Temperature of a substance is
Options:
(a) The total amount of heat contained in it
(b) The total number of molecules in it
(c) The degree of hotness or coldness
(d) Dependent upon the intermolecular distance
Answer: (c) The degree of hotness or coldness
Explanation: Temperature is a measure of the average kinetic energy of molecules, which determines how hot or cold a substance is.
Tip: More kinetic energy means a higher temperature.
Q4: What is heat?
Statement: Heat is
Options:
(a) The total kinetic energy of the molecules
(b) The internal energy
(c) Work done by the molecules
(d) The energy in transit
Answer: (d) The energy in transit
Explanation: Heat is a form of energy that flows from a hotter body to a cooler body until thermal equilibrium is reached.
Tip: Heat is always transferred from high to low temperature.
Q5: What is the melting point of ice in Kelvin?
Statement: In Kelvin scale, the temperature corresponding to the melting point of ice is
Options:
(a) Zero
(b) 32
(c) –273
(d) 273
Answer: (d) 273
Explanation: The melting point of ice in Celsius is 0°C. Since Kelvin = Celsius + 273, we get 273 K.
Tip: Always add 273 to convert Celsius to Kelvin.
Q6: Which thermometer can measure a large range of temperatures?
Statement: A thermometer that measures a large range of temperature is
Options:
(a) Mercury-in-glass thermometer
(b) Alcohol-in-glass thermometer
(c) Clinical thermometer
(d) Digital thermometer
Answer: (a) Mercury-in-glass thermometer
Explanation: Mercury has a wide operating temperature range (-39°C to 356°C), making it ideal for measuring high temperatures.
Tip: Alcohol thermometers are used for extremely low temperatures, while clinical thermometers are limited to body temperature ranges.
Q7: What is a disadvantage of using alcohol in thermometers?
Statement: One disadvantage of alcohol-in-glass thermometers is
Options:
(a) It has large expansivity
(b) It has a low freezing point (-112°C)
(c) It wets the glass tube
(d) Its expansion is linear
Answer: (c) It wets the glass tube
Explanation: Alcohol adheres to the glass, making readings difficult. Mercury does not wet glass, so it is preferred for precise readings.
Tip: Alcohol is used in cold regions due to its lower freezing point.
Here are well-explained answers in simple language for Class 9 students based on the given image.
Short Answer Questions
Q1: Why do solids have a fixed volume and shape according to the particle theory of matter?
Answer:
Solids have a fixed shape and volume because their particles are closely packed together in a fixed pattern. The strong forces of attraction between the particles keep them in place, allowing only vibrations but no free movement.
Q2: Why do gases have neither a fixed volume nor a fixed shape?
Answer:
Gases do not have a fixed shape or volume because their particles are far apart and move freely in all directions. They take the shape of their container and expand to fill any available space.
Q3: Compare the spacing of molecules in solid, liquid, and gaseous states.
Answer:
- Solid: Particles are very close together and arranged in a fixed pattern.
- Liquid: Particles are close but can move past each other, allowing the liquid to flow.
- Gas: Particles are far apart and move randomly in all directions.
Q4: What is the effect of raising the temperature of a liquid?
Answer:
When the temperature of a liquid increases, its particles move faster and spread further apart. If enough heat is added, the liquid can turn into a gas (evaporation or boiling).
Q5: What is meant by the temperature of a body?
Answer:
Temperature is the measure of how hot or cold an object is. It depends on the average kinetic energy (motion) of the particles in the object.
Q6: Define heat as ‘energy in transit.’
Answer:
Heat is the transfer of thermal energy from a hotter object to a cooler one. It always flows from a high-temperature area to a low-temperature area.
Q7: What is meant by the thermometric property of a substance?
Answer:
A thermometric property is a physical property of a substance that changes with temperature. Examples include the expansion of mercury in a thermometer or the change in electrical resistance of metals.
Q8: Describe the main scales used for the measurement of temperature. How are they related?
Answer:
The three main temperature scales are:
- Celsius (°C) – Water freezes at 0°C and boils at 100°C.
- Fahrenheit (°F) – Water freezes at 32°F and boils at 212°F.
- Kelvin (K) – Water freezes at 273 K and boils at 373 K.
The relationship between Celsius and Kelvin is:
K=°C+273K = °C + 273
Q9: What is meant by the sensitivity of a thermometer?
Answer:
Sensitivity of a thermometer refers to how quickly and accurately it detects small changes in temperature. A thermometer with a thinner tube or more responsive liquid is more sensitive.
Q10: What do you mean by the linearity of a thermometer?
Answer:
Linearity means that the liquid inside the thermometer expands uniformly with temperature change. If the liquid does not expand evenly, the thermometer will not be accurate.
Q11: What makes the scale reading of a thermometer accurate?
Answer:
A thermometer’s scale is accurate if:
- The liquid expands uniformly.
- The tube is narrow for better precision.
- It has clear, evenly spaced markings.
Q12: What does determine the direction of heat flow?
Answer:
Heat always flows from a hotter object to a cooler one until both reach the same temperature.
Q13: Distinguish between heat and internal energy.
Answer:
- Heat: Energy in transit that moves from a hot object to a cold one.
- Internal Energy: The total energy (kinetic + potential) of all particles in an object.
Q14: When you touch a cold surface, does cold travel from the surface to your hand, or does energy travel from your hand to the cold surface?
Answer:
Energy travels from your warm hand to the cold surface. Heat always moves from a warmer object to a cooler one.
Q15: Can you feel your fever by touching your own forehead? Explain.
Answer:
No, because your hand and forehead are at the same temperature. To measure fever accurately, you need a thermometer.
Constructed Response Questions
Q1: Is kinetic molecular theory of matter applicable to the plasma state of matter?
Answer:
Yes, the kinetic molecular theory explains the motion of particles in solids, liquids, and gases. Plasma is a state of matter with freely moving charged particles, which also follow the principles of kinetic theory.
Q2: Why is mercury usually preferred to alcohol as a thermometric liquid?
Answer:
Mercury is preferred because:
- It does not stick to glass.
- It expands uniformly.
- It is easy to see due to its shiny appearance.
- It has a wider temperature range (-39°C to 356°C).
Q3: Why is water not suitable for use in thermometers? Without calculations, guess what is an equivalent temperature of 373 K on Celsius and Fahrenheit scales?
Answer:
Water is not used because:
- It does not expand uniformly.
- It evaporates quickly.
- It wets the glass, making readings difficult.
373 K in Celsius and Fahrenheit:
- Celsius: 373−273=100°C373 – 273 = 100°C
- Fahrenheit: 100°C=212°F100°C = 212°F
Q4: Mention two ways in which the design of a liquid-in-glass thermometer may be altered to increase its sensitivity.
Answer:
- Making the capillary tube narrower.
- Using a liquid that expands more with temperature changes.
Q5: One liter of water is heated by a stove, and its temperature rises by 2°C. If one liter of water is heated on the same stove for the same time, what will be the rise in temperature?
Answer:
The temperature rise will also be 2°C because the same amount of heat is applied to the same amount of water.
Q6: Why are there no negative numbers on the Kelvin scale?
Answer:
Kelvin scale starts at absolute zero (0 K), the lowest possible temperature where all molecular motion stops. Since temperature cannot be lower than absolute zero, there are no negative Kelvin temperatures.
Q7: Comment on the statement, “A thermometer measures its own temperature.”
Answer:
This statement means that a thermometer must reach thermal equilibrium with the object being measured. The reading it shows is its own temperature, which matches the temperature of the object.
Short Answer Questions
7.8. There are various objects made of cotton, wood, plastic, metals, etc., in a winter night. Compare their temperatures with the air temperature by touching them with your hand.
Answer: The temperature of all objects will be the same as the air temperature. However, they feel different when touched because of their thermal conductivity. Metals feel colder as they conduct heat away from your hand quickly, whereas materials like cotton and wood are poor conductors and feel warmer.
7.9. Which is greater: an increase in temperature by 1°C or one 1°F?
Answer: An increase of 1°C is greater than 1°F because 1°C is equivalent to 1.8°F.
7.10. Why would you not expect all the molecules in a gas to have the same speed?
Answer: In a gas, molecules move randomly and collide with each other. Due to these collisions and variations in kinetic energy, different molecules have different speeds. Some move faster while others move slower.
7.11. Does it make sense to talk about the temperature of a vacuum?
Answer: No, because temperature is a measure of the average kinetic energy of particles. In a vacuum, there are no particles, so the concept of temperature does not apply.
7.12. Comment on the statement: “A hot body does not contain heat.”
Answer: The statement means that a hot body contains internal energy due to molecular motion. Heat, on the other hand, is energy in transit that flows from a hotter object to a cooler one.
7.13. Discuss whether the Sun is matter.
Answer: The Sun is mainly composed of plasma, which is a high-energy state of matter where atoms are ionized into charged particles. While plasma is a form of matter, the Sun itself also emits radiation (light and heat), which is not matter. Therefore, while part of the Sun is matter, its radiation is not.
Comprehensive Questions
7.1. Describe the main points of the particle theory of matter which differentiate solids, liquids, and gases.
Answer: The particle theory of matter states:
- Matter is made up of tiny particles.
- These particles are in constant motion.
- There are forces of attraction between particles.
- The spaces between particles differ: solids have the least, gases have the most.
- The energy of particles increases from solids to gases.
These points explain the differences in the properties of solids, liquids, and gases.
7.2. What is temperature? How is it measured? Describe briefly the construction of a mercury-in-glass thermometer.
Answer: Temperature is the measure of the average kinetic energy of particles in a substance. It is measured using thermometers.
A mercury-in-glass thermometer consists of:
- A thin glass tube with a mercury reservoir at the bottom.
- A temperature scale marked on the glass.
- When temperature rises, mercury expands and moves up the tube, indicating the temperature.
7.3. Compare the three scales used for measuring temperature.
Answer: The three main temperature scales are:
- Celsius (°C): Water freezes at 0°C and boils at 100°C.
- Fahrenheit (°F): Water freezes at 32°F and boils at 212°F.
- Kelvin (K): Starts at absolute zero (0 K = -273.15°C) and is used in scientific calculations.
Kelvin is the most fundamental scale as it does not have negative values.
7.4. What is meant by sensitivity, range, and linearity of thermometers? Explain with examples.
Answer:
- Sensitivity: The ability to detect small temperature changes. A thermometer with a thin tube and alcohol as a liquid is more sensitive.
- Range: The temperature limits a thermometer can measure. Mercury thermometers have a high range, while alcohol thermometers are used for very low temperatures.
- Linearity: How evenly the liquid expands with temperature. Mercury expands uniformly, making it highly accurate.
7.5. Explain how the parameters mentioned in question 7.4 are improved in the structure of a glass-in-glass thermometer.
Answer:
- Sensitivity is improved by using a narrow capillary tube and a liquid that expands more, such as alcohol.
- Range is increased by choosing different liquids. Alcohol works at very low temperatures, while mercury is used at high temperatures.
- Linearity is ensured by using uniform liquid expansion, such as mercury, which expands evenly across different temperatures.
