9th Class Physics Notes 2026 – Chapter 7
Class 9 Physics Notes – Chapter 7: Thermal Properties of Matter
Punjab Board | 2026
A. Multiple Choice Questions
7.1. How do the molecules in a solid behave?
(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
Correct Answer: (b) Vibrate about their mean positions
7.2. What type of motion is of the molecules in a gas?
(a) Linear motion
(b) Random motion
(c) Vibratory motion
(d) Rotatory motion
Correct Answer: (b) Random motion
7.3. Temperature of a substance is:
(a) The total amount of heat contained in it
(b) The total number of molecules in it
(c) Degree of hotness or coldness
(d) Dependent upon the intermolecular distance
Correct Answer: (c) Degree of hotness or coldness
7.4. Heat is the:
(a) Total kinetic energy of the molecules
(b) The internal energy
(c) Work done by the molecules
(d) The energy in transit
Correct Answer: (d) The energy in transit
7.5. In Kelvin scale, the temperature corresponding to the melting point of ice is:
(a) Zero
(b) 32
(c) −273
(d) +273
Correct Answer: (d) +273
7.6. The temperature which has the same value on Celsius and Fahrenheit scales is:
(a) −40
(b) +40
(c) +45
(d) −45
Correct Answer: (a) −40
7.7. Which one is a better choice for a liquid-in-glass thermometer?
(a) Is colourless
(b) Is a bad conductor
(c) Expands linearly
(d) Wets glass
Correct Answer: (c) Expands linearly
7.8. One disadvantage of using alcohol in a liquid-in-glass thermometer is:
(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
Correct Answer: (c) It wets the glass tube
7.9. Water is not used as a thermometric liquid mainly due to:
(a) Being colourless
(b) Being a bad conductor of heat
(c) Non-linear expansion
(d) A low boiling point (100°C)
Correct Answer: (c) Non-linear expansion
7.10. A thermometer has a narrow capillary tube so that it:
(a) Quickly responds to temperature changes
(b) Can read the maximum temperature
(c) Gives a large change for a given temperature rise
(d) Can measure a large range of temperature
Correct Answer: (c) Gives a large change for a given temperature rise
7.11. Which thermometer is most suitable for recording rapidly varying temperature?
(a) Thermocouple thermometer
(b) Mercury-in-glass laboratory thermometer
(c) Alcohol-in-glass thermometer
(d) Mercury-in-glass clinical thermometer
Correct Answer: (a) Thermocouple thermometer
B. Short Answer Questions
7.1. Why do solids have a fixed volume and shape according to the particle theory of matter?
Answer:
Molecules in solids are held together by strong intermolecular forces, locking them in relatively fixed positions where they vibrate about their mean positions.
7.2. What are the reasons that gases have neither a fixed volume nor a fixed shape?
Answer:
Intermolecular forces in gases are extremely weak, and molecules move at high speeds in all directions, filling any available container.
7.3. Compare the spacing of molecules in the solid, liquid, and gaseous states.
Answer:
Solids: Molecules are tightly packed with minimum spacing.
Liquids: Molecules are slightly further apart than in solids.
Gases: Molecules are very far apart, with large empty spaces between them.
7.4. What is the effect of raising the temperature of a liquid?
Answer:
The average kinetic energy of the molecules increases, their motion becomes more vigorous, and the liquid generally expands.
7.5. What is meant by the temperature of a body?
Answer:
Temperature is the physical quantity that determines the hotness or coldness of a body and the direction of heat flow.
7.6. Define heat as “energy in transit.”
Answer:
Heat is energy transferred from a hotter body to a colder body because of a temperature difference. It is energy in transit rather than a quantity stored within a body.
7.7. What is meant by the thermometric property of a substance? Enlist some thermometric properties.
Answer:
A thermometric property is a physical property that changes with temperature and can be used to measure it.
Examples include:
Volume of a liquid
Electrical resistance
Pressure of a gas at constant volume
7.8. State the main scales used for the measurement of temperature.
Answer:
Celsius scale (°C)
Fahrenheit scale (°F)
Kelvin absolute scale (K)
7.9. What is meant by the sensitivity of a thermometer?
Answer:
Sensitivity is the ability of a thermometer to detect small temperature changes. A more sensitive liquid-in-glass thermometer shows a larger movement of the liquid column for a given temperature change.
7.10. What do you mean by the linearity of a thermometer?
Answer:
Linearity means that equal changes in temperature produce equal changes in the length of the thermometric liquid column.
7.11. What makes the scale reading of a thermometer accurate?
Answer:
Accurate calibration using standard reference points, such as the ice point (0°C) and steam point (100°C), helps ensure reliable thermometer readings.
7.12. What determines the direction of heat flow?
Answer:
The temperature difference between two bodies determines the direction of heat flow. Heat flows spontaneously from a higher temperature to a lower temperature.
7.13. Distinguish between heat and internal energy.
Answer:
Internal energy: The total microscopic kinetic and potential energy of the particles within a body.
Heat: Energy transferred between bodies or systems because of a temperature difference.
7.14. 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 hand to the cold surface because your hand is at a higher temperature. Heat flows from the warmer hand to the colder object.
7.15. Can you feel your fever by touching your own forehead? Explain.
Answer:
It is difficult to judge your own fever by touching your forehead because your hand and forehead may be at similar temperatures. A thermometer provides a more reliable measurement.
C. Constructed Response Questions
7.1. Is the kinetic molecular theory of matter applicable to the plasma state of matter? Describe briefly.
Answer:
Plasma consists of ionized atoms and free electrons. The kinetic molecular theory helps explain the motion and energy of its particles, but electromagnetic interactions between charged particles must also be considered.
7.2. Why is mercury usually preferred to alcohol as a thermometric liquid?
Answer:
Mercury has a wide liquid range, from approximately −39°C to 357°C. It does not wet glass, is easily visible, and expands approximately uniformly over a useful temperature range.
7.3. Why is water not suitable for use in thermometers? Without calculations, guess what is the equivalent temperature of 373 K on Celsius and Fahrenheit scales.
Answer:
Water is unsuitable because it expands non-linearly over its temperature range and wets glass. Its usable temperature range is also limited by its freezing and boiling points under normal atmospheric pressure.
A temperature of 373 K is approximately equivalent to 100°C and 212°F.
7.4. Mention two ways in which the design of a liquid-in-glass thermometer may be altered to increase its sensitivity.
Answer:
Use a bulb containing a larger volume of thermometric liquid.
Use a narrower capillary tube.
7.5. One litre of water is heated by a stove and its temperature rises by 2°C. If two litres of water are heated on the same stove for the same time, what will be the rise in temperature?
Answer:
Assuming the stove supplies the same amount of heat in both cases and heat losses are negligible, the temperature rise will be 1°C because the mass of water is doubled.
Using:
For the same heat supplied \(Q\), doubling the mass \(m\) halves the temperature rise \(\Delta T\).
7.6. Why are there no negative numbers on the Kelvin scale?
Answer:
The Kelvin scale is an absolute temperature scale. Its zero point, 0 K, is absolute zero, the lowest possible temperature in thermodynamics. Therefore, ordinary thermodynamic temperatures are not negative on this scale.
7.7. Comment on the statement: “A thermometer measures its own temperature.”
Answer:
The statement is correct. A thermometer measures its own temperature, and when it reaches thermal equilibrium with an object, its temperature becomes equal to that of the object. Its reading then indicates the object’s temperature.
7.8. There are various objects made of cotton, wood, plastic, metals, etc. On a winter night, compare their temperatures with the air temperature by touching them with your hand.
Answer:
If the objects have been in the same room long enough, they will generally be at approximately the same temperature as the surrounding air. Metals feel colder because they conduct heat away from the hand more quickly than materials such as wood, cotton and plastic.
7.9. Which is greater: an increase in temperature of 1°C or one of 1°F?
Answer:
An increase of 1°C is greater because:
Therefore, a temperature increase of 1°C corresponds to an increase of 1.8°F.
7.10. Why would you not expect all the molecules in a gas to have the same speed?
Answer:
Gas molecules move randomly and collide with one another. These collisions continuously change their speeds and directions, producing a distribution of molecular speeds rather than one common speed.
7.11. Does it make sense to talk about the temperature of a vacuum?
Answer:
An ideal vacuum contains no particles whose average kinetic energy can define a thermodynamic temperature in the usual way. However, radiation passing through a vacuum can have a temperature associated with its spectrum, and real vacuums may contain residual particles.
7.12. Comment on the statement: “A hot body does not contain heat.”
Answer:
The statement is correct in the scientific sense. A hot body contains internal energy, while heat refers to energy transferred between bodies because of a temperature difference.
7.13. Discuss whether the Sun is matter.
Answer:
Yes. The Sun is matter because it has mass and occupies space. It is composed mainly of extremely hot plasma containing ionized hydrogen and helium.
D. Comprehensive Questions
7.1. Describe the main points of the particle theory of matter which differentiate solids, liquids, and gases.
Answer:
Solids: Particles are closely packed, intermolecular forces are strong, and particles vibrate about fixed mean positions. Solids generally have a fixed shape and volume.
Liquids: Particles are close together but can move past one another. Liquids have a fixed volume but take the shape of their container.
Gases: Particles are widely separated, intermolecular forces are weak, and particles move rapidly in random directions. Gases have neither a fixed shape nor a fixed volume.
7.2. What is temperature? How is it measured? Describe briefly the construction of a mercury-in-glass thermometer.
Answer:
Temperature is a physical quantity that indicates the hotness or coldness of a body and determines the direction of heat flow.
It is measured using a thermometer calibrated against known reference temperatures.
A mercury-in-glass thermometer consists of:
Glass bulb: Contains the mercury.
Narrow capillary tube: Allows small changes in mercury volume to produce a visible change in the length of the column.
Mercury: Acts as the thermometric liquid.
Graduated scale: Indicates the temperature.
Sealed upper end: Prevents mercury from escaping and, in a conventional design, contains a space above the mercury column.
The thermometer is calibrated using reference points such as the ice point and steam point.
7.3. Compare the three scales used for measuring temperature.
Answer:
Celsius scale: The ice point is 0°C and the steam point is 100°C at standard atmospheric pressure.
Fahrenheit scale: The ice point is 32°F and the steam point is 212°F at standard atmospheric pressure.
Kelvin scale: The corresponding temperatures are 273.15 K and 373.15 K.
Temperature conversion formulas:
Celsius to Fahrenheit:
Fahrenheit to Celsius:
Celsius to Kelvin:
7.4. What is meant by the sensitivity, range, and linearity of thermometers? Explain with examples.
Answer:
Sensitivity: The amount of change in a thermometer’s indication for a given temperature change. In a liquid-in-glass thermometer, a larger liquid-column movement per degree indicates greater sensitivity.
Range: The minimum and maximum temperatures a thermometer can measure. For example, a clinical thermometer has a narrower range than many laboratory thermometers.
Linearity: The property by which equal temperature changes produce equal changes in the thermometric property. In a liquid-in-glass thermometer, this means equal temperature intervals correspond to equal changes in the liquid-column length.
7.5. Explain how the parameters mentioned in Question 7.4 are improved in the structure of a liquid-in-glass thermometer.
Answer:
Sensitivity: Increased by using a larger bulb and a narrower capillary tube.
Range: Increased by selecting a thermometric liquid with a suitable freezing point and boiling point and designing the capillary and scale accordingly.
Linearity: Improved by using a liquid whose expansion is approximately uniform over the required temperature range.
E. Numerical Problems
7.1. The temperature of a normal human body on the Fahrenheit scale is 98.6°F. Convert it into Celsius and Kelvin scales.
Solution:
Step 1: Given
Step 2: Convert Fahrenheit to Celsius
Use the formula:
Substitute the values:
Step 3: Convert Celsius to Kelvin
Use the formula:
Answer:
7.2. At what temperature would the Celsius and Fahrenheit thermometer readings be the same?
Solution:
Step 1: Let the required temperature be \(x\).
Step 2: Use the conversion relation:
Step 3: Substitute \(C=F=x\).
Step 4: Cross-multiply.
Step 5: Expand.
Step 6: Rearrange.
Step 7: Solve for \(x\).
Answer:
7.3. Convert 5°F to Celsius and Kelvin scales.
Solution:
Step 1: Given
Step 2: Convert to Celsius
Step 3: Convert to Kelvin
Answer:
7.4. What is the equivalent temperature of 25°C on Fahrenheit and Kelvin scales?
Solution:
Step 1: Given
Step 2: Convert to Fahrenheit
Step 3: Convert to Kelvin
Answer:
7.5. The ice and steam points on an ungraduated thermometer are found to be 192 mm apart. What temperature will be on the Celsius scale if the length of the mercury thread is at 67.2 mm above the ice point mark?
Solution:
Step 1: Given
Length at ice point:
Length at steam point:
Length at the required temperature:
Step 2: Use the calibration formula
Step 3: Substitute the values
Answer:
7.6. The length between the fixed points of a liquid-in-glass thermometer is 20 cm. If the mercury level is 4.5 cm above the lower mark, what is the temperature on the Fahrenheit scale?
Solution:
Step 1: Given
Fundamental interval:
Length above the lower mark:
Step 2: Calculate the Celsius temperature
Step 3: Convert Celsius to Fahrenheit
Answer:
Notes by Sir Dilawar
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