CBSE Important Questions Class 8 Science Chapter 7 – Particulate Nature of Matter

Class 8 Science Chapter 7 Particulate Nature of Matter is part of the new NCERT Curiosity textbook. It explains that all matter is made of tiny particles, that these particles have space between them, keep moving and attract one another. Students learn how these ideas explain the three states of matter, diffusion, evaporation, melting and boiling, and why gases can be compressed while solids and liquids cannot.

These CBSE important questions for Class 8 Science Chapter 7 cover a case study on evaporation from four beakers, the effect of wind and humidity, reading a heating curve to find melting and boiling points, how fast different liquids flow, compressibility of gases, the properties of solids, liquids and gases, and an assertion-reason question on the shape and volume of liquids. Every question has a clear step-by-step answer in simple English with figures, so students can practise on their own, parents can check the answers, and teachers can use them for class tests.

Q1. Case Study: A student took four glass beakers, namely P, Q, R and S. He poured 100 mL of water in each of them as shown in the figure. Beakers P, R and S were of the same size whereas beaker Q was smaller. Beaker Q was covered, and beakers Q and S were placed under a fan. Answer the following questions:
(i) Name the process involved when liquid water transforms into vapour.
(ii) Will the water level be the same in beaker P and S after an hour?
(iii) What happens to the water level in beaker Q and beaker R after an hour?

Four beakers P, Q, R and S with water, two placed under a fan

Answer: (i) The change of a liquid into vapour is called evaporation.
(ii) No. The water in beaker S will evaporate more than in beaker P, because the fan blows away the air just above the water surface in S. The water vapour is carried away with the moving air, so less vapour stays above the surface and the rate of evaporation increases.
(iii) The water level in beaker Q stays almost the same even though it is under the fan, because it is covered from the top and the vapour cannot escape. The water in beaker R decreases only a little, because although there is no fan, the vapour can still escape into the surroundings.

Q2. Piyush took water in two different beakers. Beaker A is kept near the window and beaker B is kept in front of a ceiling fan.
(a) In which beaker will the rate of evaporation be faster?
(b) What will happen if beaker B is placed in a humid environment?

One beaker near a window and another in front of a fan

Answer: (a) Evaporation is faster in beaker B. When the wind speed increases, the water vapour is carried away with the moving air, so more water can evaporate.
(b) If the air already holds a lot of water vapour (high humidity), it cannot take in much more. So the rate of evaporation would decrease. Evaporation is inversely related to the humidity of the air.

Q3. The graph shows the heating curve of a substance 'P'.
(a) Identify the physical states of the substance at points A, C and E.
(b) What physical process is taking place from B to C?
(c) Substance 'P' is water. How can you judge this from the graph?

Heating curve showing temperature against time with points A to E

Answer: (a) Point A: solid. Point C: liquid (just after melting is complete). Point E: gas (vapour).
(b) From B to C the substance is melting, that is changing from solid to liquid. The temperature stays constant because the heat supplied is used as latent heat.
(c) From the graph, the melting point of 'P' is 273 K and its boiling point is 373 K. These are exactly the melting and boiling points of water, so 'P' must be water.

Q4. Ananya took four different liquids in four different beakers.
A: Water   B: Juice   C: Cooking oil   D: Honey
Arrange the liquids in decreasing order of the speed with which they will flow when poured on a smooth floor.
(a) Water > Juice > Cooking oil > Honey
(b) Water > Cooking oil > Juice > Honey
(c) Honey > Cooking oil > Juice > Water
(d) Water > Juice > Honey > Cooking oil

Answer: (a) Water > Juice > Cooking oil > Honey
The speed of flow depends on how strongly the particles of the liquid attract each other. Water has the weakest attraction, so it flows fastest. Juice is slightly thicker, cooking oil has still stronger attraction, and honey has the strongest, so it flows the slowest.

Q5. If we apply pressure on different substances: milk, fluorine gas, sand, water. Which of the following substances is/are compressed by a piston?
(a) Milk
(b) Fluorine
(c) Water
(d) Sand

A gas inside a cylinder being compressed by a piston

Answer: (b) Fluorine
Gas particles are very far apart with a lot of empty space between them. When the piston pushes down, the particles come closer, so gases can be compressed easily. Milk and water are liquids and sand is a solid; in these the particles are already close together, so they cannot be compressed.

Q6. There are three substances X, Y and Z.
X: Indefinite shape, fixed volume
Y: High density, less diffusibility
Z: High kinetic energy, high fluidity
Based on this, which will represent the properties of a table, hydrogen and milk? Select the correct option.
(a) X: Milk, Y: Table, Z: Hydrogen
(b) X: Hydrogen, Y: Table, Z: Milk
(c) X: Milk, Y: Hydrogen, Z: Table
(d) X: Hydrogen, Y: Milk, Z: Table

Answer: (a) X: Milk, Y: Table, Z: Hydrogen
X has no fixed shape but a fixed volume, which is the property of a liquid, so X is milk. Y is dense and its particles hardly spread, which is the property of a solid, so Y is the table. Z has very high kinetic energy and flows freely in all directions, which is the property of a gas, so Z is hydrogen.

Q7. X: A solid that is deformed by force but regains its shape after the force is removed.
Y: A solid which can be compressed easily.
Identify X and Y.
(a) X: Water, Y: Sponge
(b) X: Clay, Y: Ball
(c) X: Cotton, Y: Pen
(d) X: Spring, Y: Sponge

Answer: (d) X: Spring, Y: Sponge
A spring can be stretched or squeezed by a force but springs back to its original shape, so it is X. A sponge has many tiny air pores; when it is pressed the air escapes, so it appears easily compressible even though solids in general are not. So Y is the sponge.

Q8. A diver can cut through water in a swimming pool. Which of the following can be concluded from this?
(I) Particles of liquids have space in them.
(II) Particles of liquids possess kinetic energy.
(III) Particles of liquids have weak attractive forces.
(IV) Particles of liquids have lower kinetic energy.
(a) II and III
(b) I and III
(c) I, II and IV
(d) I, II and III

Answer: (d) I, II and III
The diver can pass through water because the water particles have space between them, are moving (they have kinetic energy) and are held by only weak forces, so they move aside easily. Statement IV is wrong: liquid particles have more kinetic energy than solids, though less than gases.

Q9. Match the applications of the properties of states of matter in Column A with the associated property in Column B.

Column A Column B
a) Iron block sinks in water. i) Compressibility
b) Oxygen gas can be filled in gas cylinders. ii) No fixed shape
c) Chlorine gas fills the container completely. iii) High density
d) Milk takes the shape of the container. iv) Negligible force of attraction

(a) (a)-(i); (b)-(iii); (c)-(iv); (d)-(ii)
(b) (a)-(ii); (b)-(i); (c)-(iv); (d)-(iii)
(c) (a)-(iii); (b)-(i); (c)-(iv); (d)-(ii)
(d) (a)-(i); (b)-(iv); (c)-(iii); (d)-(ii)

Answer: (c) (a)-(iii); (b)-(i); (c)-(iv); (d)-(ii)
An iron block sinks because of its high density. Oxygen can be packed into a cylinder because gases are compressible. Chlorine fills the whole container because the force of attraction between gas particles is negligible. Milk takes the shape of its container because a liquid has no fixed shape.

Q10. Which of the following statements is INCORRECT about gases?
(a) Liquids are less dense than gases.
(b) Solids and liquids are heavier than gases.
(c) Gases fill a container completely.
(d) Particles of gases move at high speed.

Answer: (a) Liquids are less dense than gases.
This is the wrong statement. Gas particles are spread far apart, so a gas is much less dense than a liquid, not more. The other three statements are correct.

Q11. The kinetic energy of gas particles is ______, and the forces of attraction are ________.
(a) high, strong
(b) low, strong
(c) low, weak
(d) high, negligible

Answer: (d) high, negligible
Gas particles move very fast in all directions, so their kinetic energy is high. They are so far apart that the forces of attraction between them are almost absent, which is why a gas spreads out and fills its whole container.

Q12. Read the Assertion (A) and Reason (R) and choose the correct option.
Assertion (A): Water takes the shape of the vessel in which it is poured.
Reason (R): Liquids do not have a fixed volume.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.

Answer: (c) A is true, but R is false.
Water does take the shape of its vessel, so A is correct. But the reason is wrong: liquids do have a fixed volume. They only lack a fixed shape, which is why they take the shape of the container.

Related Study Material on Extramarks

Q.1 Define force of friction. How is the force of friction disadvantageous ?

Marks:5
Ans

When two surfaces slide over each other, the force, which opposes the motion of one body over the other, is called force of friction. Disadvantage of frictional forces are seen as:
(i)Wear and tear of objects coming in contact. Example: soles of shoes and tyres of vehicles.
(ii)Loss of energy as heat. Example: Machine parts get heated up and lead to wear and tear.
(iii)Loss of energy to overcome friction. Example: Wastage of fuel in machinery.

Q.2 State and explain different types of friction.

Marks:3
Ans

There are three types of friction:

(i)Static friction: The friction that exists between the surfaces in contact when there is no relative motion is called static friction.

(ii) Dynamic or sliding friction: The friction that exists between the surfaces in contact when a body slides on the surface of another body is known as the sliding friction.

(iii)Rolling friction: The friction that exists between the surfaces in contact when a body rolls over the surface of another body is known as the rolling friction.

For any two surfaces in contact during motion:

Rolling friction < Sliding friction < Static friction.

Q.3 Mention three disadvantages of friction between the parts of a machine. How does (a) oiling and (b) using ball bearings help reduce friction?

Marks:5
Ans

Three disadvantages of friction between the parts of a machine are
1. Lot of heat is produced
2. Lot of energy is wasted
3. There is wear and tear of different parts
Friction can be reduced by
(a)By oiling: Oil act as a lubricant. It fills up the dents (irregularities) of the surfaces that move against each other in a machine. It also prevents direct contact between the surfaces by forming a film between the surfaces.

(b)By using ball bearings: Use of ball bearings converts sliding friction into rolling friction. Rolling friction is less than sliding friction. The ball bearing s used between the hubs and the axles of bicycle, and fans.

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