9th Class Physics: Chapter 7 Properties of Matter Short Questions Answers

Kinetic molecular model is used to explain the three states of matter-solid, liquid and gas. Kinetic molecular model of the three states of natter can be shown in the given figure.

Kinetic theory has some important features.

• Matter is made up of particles called molecules.
• The molecules remain in continuous motion.
• Molecules attract each other.

1. Solids 2. Liquid 3. Gas

• Solids: Solids have following characteristics according to kinetic molecular model.
• Fixed shape and volume: Solids have fixed shapes and volume.
• Strong forces of attraction: Molecules of solids are help close together by strong forces of attraction as shown in the given figure.
• Vibratory motion: The molecules of solids vibrate about their fixed mean position but do not move from place to place/
• Examples: Stone, metal, spoon and pencil are some examples of solids.
• Liquids: Liquids have following characteristics according to kinetic molecular model.
• Fixed volume but not fixed shape: The volume of a certain amount of liquid remains the same but because of flow property it attains the shape of the container.
• Weak attractive forces: The distances between the molecules of liquid are more than in solids. Thus attractive forces between them are weaker as shown in the given figure.
• Vibratory motion: Like solids, the molecules of liquids also vibrate about their mean positions but are not rigidly held with each other.
• Property of flow: Due to the weaker attractive forces between the molecules of liquids, they can slide over each other. Thus the liquids have property to flow.

Examples: Water, oil, honey and petrol are the examples of liquids.

• Gases: Gases have following characteristics according to the kinetic molecular model.
• No fixed shape or volume: Gases have no fixed shape or volume. They can be filled in any container of any shape.
• Weakest force of attraction: In gases, the molecules are much farther apart than solids or liquids as shown in the figure.
• Gases are less denser: Gases are much less denser than solids and liquids. They can be squeezed into smaller volumes.
• Random motion: Molecules of gases have random motion and move with very high velocities.
• Pressure of gases: The molecules of gases are constantly striking the walls of a container as they move with very high velocities. Thus, a gas exerts pressure on the walls of the container.

Yes, there exist a fourth stateof matter that is called plasma.

At very high temperature, the matter assumer the state of ions and electrons this is called plasma.

Density of a substance is defined as the mass per unit volume.

Density = mass/volume

d = m/v

The SI unit of density is kilogramme per cubic metre (kgm^-3)

Hydrometer is a device which is used to measure the density of fluid. As the hydrometer is a glass tube with a scale marked on its stem and heavy weight in the bottom. It is partially immersed in the milk that is also a fluid, the density of which is to be measured, hence we can use hydrometer to measure the density of milk.

The force acting normally on unit area at the surface of a body is called pressure.

Pressure = force/area

P = F/A

In SI, the unit of pressure is newton per square meter (Nm^-2).

Atmosphere and Atmospheric pressure:

The Earth is surrounded by a cover of air called atmosphere. It extends to a few hundred kilometers above sea level. Just as certain sea creatures live at the bottom of ocean, the humans live at the bottom of huge ocean of air. The pressure of atmosphere is called atmospheric pressure.

Air: Air is mixture of gases.

Direction of atmospheric pressure:

Atmospheric pressure acts in all directions.

Relation of height with atmospheric pressure:

The atmospheric pressure decreases continuously as we go up. Thus knowing the atmospheric pressure of a place, its altitude can be determined.

Example No 1.

The formation of spherical shape, soap bubbles is an example of which represents that atmospheric pressure acts on it equally in all directions.

A soap bubble can expand till the pressure of air in it is equal to the atmospheric pressure.

Example No 2.

A balloon expands as it is filled with air. Air exerts a pressure inside the balloon which expands till it is equal to the atmospheric pressure.

Explanation by Experiment:

Atmospheric pressure can easily be understood by a simple experiment. The steps of experiment are as following.

1. Take an empty tin can with a lid.
2. Open its cap and put some water in it.
3. Place it over flame.
4. Wait till water begins to boil and the steam expels the air out of the can.
5. Remove it from the flame.
6. Close the can firmly by its cap.
7. Place the can under tap water. The can will squeeze due to atmospheric pressure.

Conclusion: The experiment shows that atmosphere exerts pressure in all directions:

Reason of collapsing the can:

When the can is cooled by tap water, the steam in it condenses.

As the steam changes into water, it leaves an empty space behind it. This lowers the pressure inside the can as compared to the atmospheric pressure outside the can. This will cause the can to collapse from all directions.

Collapsing of Plastic bottle: Atmosphere exerts pressure in all directions can be demonstrated by collapsing of an empty plastic bottle when air is sucked out of it.

It is very difficult to remove air from a glass bottle because air pressure in the bottle is less than atmospheric pressure.

The instrument that measures atmospheric pressure is called barometer. One of the simple barometer is mercury barometer.

As we know that mercury is 13.6 times more dense than water, Atmospheric pressure can hold vertical column of water about 13.6 times greater than the height of mercury column at that place. Thus, at sea level vertical height of water column would be 0.76m x 13.6 =10.34m. Thus, we would need a glass tube more than 10m long or approximately 11m to make a water barometer that is too long, therefore water is not suitable to be used in barometer.

A wall sucker sticks to wall because of the difference in pressure between wall and rubber sucker which causes adhesion. This sucker sticks on the smooth wall or in other words the pressure on the outside becomes greater than the pressure on the inside.

As we know Earth’s atmosphere extends upward about a few hundred kilometers with continuously decreasing density, and the pressure is directly proportional to the density, therefore as we going up, the atmospheric air becomes thinner and thinner, so, density also reduces. Even at a height of about 30km, the atmospheric pressure becomes only 7mm of Hg, which is approximately 1000 Pa.  It would become zero at an altitude where there is no air.

A sudden fall in atmospheric pressure often followed by a storm rain and typhoon to occur in few hours time.

If the barometer reading shows a sudden increase or a rapid increase in atmospheric pressure, means that it will soon be followed by a decrease in the atmospheric pressure indicating poor weather ahead.

Pressure applied at any point of a liquid enclosed in a container, is transmitted without loss to all other parts of liquid.

Working of Hydraulic Press:

In the hydraulic press the object which has to be compressed place over the piston of large cross-sectional area A, the force F₁ is applied on piston of small cross-sectional area (a.)

The pressure P, produced by small piston is transmitted equally to the larger piston and a force F­₂ acts on A, which is much larger than F₁.

Pressure on piston of small area: Pressure on piston of small area (a) is given by;

P = F₁/a ———– (1)

Pressure on piston of large area: According to pascal’s law, the pressure on large piston of area A will be the same as on the small piston.

P = F₂/A ———– (2)

By comparing eq (1) and (2) we get.

F₂/A = F₁/a

F₂ =  x A

F₂ =  xF₁

Since the ration A/a is greater than 1.

Hence, the force (F₂) that acts on the larger piston is greater than the force

Elasticity is the property of matter by virtue of which matter resists any force which tries to change its length, shape or volume.

When an object is wholly or partially immersed in a liquid, it loses its weight equal to the weight of the liquid displaced. This known as Archimedes principle.

Upthrust is an upward force which acts on an object kept inside a liquid. It apparently makes the object to lose weight.

Principle of floatation:

A floatation objects displaces a fluid having weight equal to the weight of the object.

Explanation: Principle of floatation can be explained by the process of sinking and floating of any object.

Process of sinking of an object: An object sinks if its weight is greater than the upthrust acting on it.

Process of floating of an object: An object floats if its weight is equal or less than the upthrust. When an object floats in a fluid, the upthrust acting on it is equal it is equal to the weight of the object.

In case of floating object, the object may be partially immersed.

The upthrust is always equal to the weight if the fluid displaces by the object.

This is the principle of floatation.

Importance of principle:

Principle of floatation is applicable on liquids as well as gases.

There are numerous applications of this principle in our daily life.

A submarine can travel over as well as under water. It works on the principle of floatation. It has a system of tanks which can be filled with and emptied from seawater. Submarine floats over water when the weight of the water equal to its volume is greater than its weight Under this condition, it is similar to a ship and remains partially above water level.

When the tanks of submarine are filled, the weight of the submarine increases.

As soon as its weight becomes greater than the upthrust, it dives into water and remains under water.

Ships and boats float on water. It is because the weight of an equal volume of water is greater than the weight of ships and boats.

A stone sinks in water. It is because the weight of an equal volume of water is smaller than the weight of stone.

Hooke’s Law: The strain produced in a body by the stress applied to it is directly proportional to the stress within the elastic limit of the body os called Hooke’s Law.

Stress ∝ stain

Elastic limit: Elastic limit can be defined as limit within which a body recovers its original length, volume or shape after the deforming force is removed.

Take a rubber band. Construct a balance of your own using a rubber band. Check its accuracy by weighing various objects.

I took a rubber band. And constructed a balance of mine using a rubber band. Then I measured the weight of various objects to check its accuracy. First of all I took a lead pencil and measured its weight, then I put a small eraser on it and measured its weight.

Both the objects have less weight than the elastic limit of rubber band. That’s why they could be measured easily.

I took a heavy stone and placed it in the balance.

But this mass was so heavy as compared to the elastic limit of the rubber band. Rubber band was broken and weight of the stone could not be measured.

• Kinetic theory has some important features.
• Matter is made up of particles called molecules.
• The molecules remain in continuous motion.
• Molecules attract each other.

Plasma: At very high temperature, the matter assumes the state of ions and electrons. This state is called plasma.

Formation of Plasma: The kinetic energy of gas molecules goes on increasing if a gas is heated continuously. This causes the gas molecules to move faster and faster. The collisions between atoms and molecules of the gas become so strong that they tear off the atoms. Atoms lose their electrons and become positive ions. This ionic state of matter is called plasma.

Yes, there exist a fourth state of matter that is called plasma.

At very high temperature, the matter assumes the state of ions and electrons this is called plasma.

The mass of 200cm³ of stone is 500g, find its density.

m = 500g

v = 200 cm³

Density = Mass/Volume

Density = 500g/200cm³ = 2.5 gcm³

Thus the density of stone is 2.5gcm³.

Hydrometer is a device which is used to measured the density of fluid. As the hydrometer is a glass tube with a scale marked on its stem and heavy weight in the bottom. It is  partially immersed in the milk that is also a fluid, the density of which is  to be measured, hence we can use hydrometer to measure the density of milk.

Density: Density of a substance is defined as the mass per unit volume.

Formula: density = mass/volume

d = m/v

Unit: The SI unit of density is kilogramme per cubic meter. (Kg m-³).

The force acting normally on unit area at the surface of a body is called pressure.

Pressure = force/area

P = F/A

In SI, the unit of pressure is newton per square meter (Nm^-2).

The unit of pressure is pascal (Pa).

Pascal: Pressure is said to be one pascal if a force of one Newton is applied on an area of 1m².

Earth’s atmosphere extends upward about a few hundred kilometers with continuously decreasing density. Nearly half of its mass is between sea level and 10 km. Up to 30 km from sea level contains about 99% of the mass of the atmosphere.
The air becomes thinner and thinner as we go up.

Atmospheric pressure: The Earth is surrounded by a cover of air called atmosphere. It extends to a few hundred kilometers above sea level. The pressure of atmosphere is called atmospheric pressure.

Pascal’s law finds numerous applications in our daily life. For example Automobiles, hydraulic brake system, hydraulic jack, hydraulic press etc.

Archimedes Principle: When an object is wholly or partially immersed in a liquid, it loses its weight equal to the weight of the liquid displaced. This is known as Archimedes principle.

Principle of floatation:

A floatation objects displaces a fluid having weight equal to the weight of the object.

Elasticity:

Elasticity is the property of matter by virtue of which matter resists any force which tries to change its length, shape or volume.

Hooke’s Law: The strain produced in a body by the stress applied to it is directly proportional to the stress within the elastic limit of the body os called Hooke’s Law.

Stress ∝ stain

Elastic limit: Elastic limit can be defined as limit within which a body recovers its original length, volume or shape after the deforming force is removed.

Young’s Modulus:

According to Hook’s law, the ratio of stress to tensile strain is constant within the elastic limit of the body. “The ratio of stress to tensile strain is called as young’s modulus”.

Stress:

Stress is defined as the force acting on unit area at the surface of a body. Stress is related to the force producing deformation.

Strain:

When stress acts on a body, it may change its length, volume, or shape. A comparison of such a change caused by the stress with original length, volume or shape is called as strain.

Hooke’s Law: The strain produced in a body by the stress applied to it is directly proportional to the stress within the elastic limit of the body os called Hooke’s Law.

Stress ∝ stain

Equation: Stress/Strain = Constant