PROPER TIES OF MATTER CHAPTER# 07 PHYSICS 9TH - Short / Detailed Question Answers

PHYSICS 9TH

PHYSICS 9TH - Short / Detailed Question Answers
PROPERTIES OF MATTER
CHAPTER# 07

Q.1: Define matter.

Ans: Anything which occupies space and having mass is called matter.

Q.2: How many states of matter are there?

Ans: There are three states of matter. These states are solid, liquid, and gas. All the material objects around us belong to any one of these states.

Q.3: Water is the best example of three states of matter. Explain.

Ans: In nature, water exists in all three states of matter.

• The solid-state of water is ice.
  • Ice exists in many forms like ice cubes, snow, glaciers, and icebergs.
• The liquid state is water itself.
  • Water is found in oceans, rivers, and underground deposits.
• The gaseous state of water is steam.
• The “white smoke” that we see in the figure is a small cloud formed by water vapours in the air, above the cup.

Q.4: Describe the properties of three states of matter in terms of their shapes, volume, density, and compressibility.

Ans: These states have different properties, which are listed in the following table:

State of Matter	Shape	Volume	Density	Compressibility
Solid	Fixed	Fixed	High	Incompressible
Liquid	Not Fixed	Not Fixed	High	Incompressible
Gas	Not Fixed	Not Fixed	Low	Compressible

Q.5: How can a matter change its state?

Ans: The addition or removal of a certain amount of energy can change the state of a matter. Conversion of matter between three states involves physical changes, not chemical changes.

Q.6: In terms of changes in the state of matter, define melting, boiling, condensing, freezing, and evaporation.

Ans: The terms for changes in the state of matter are:

• Melting: Conversion from solid to liquid.
• Boiling: Conversion from liquid to gas.
• Condensing: Conversion from gas to liquid.
• Freezing: Conversion from liquid to solid.
• Evaporation: Conversion from liquid to gas.

Q.7: Define evaporation.

Ans: Evaporation:
Evaporation is a process by which a liquid becomes a gas at a temperature below the boiling point.
Drying wet clothes and drying wet floors, etc., are examples of evaporation.
Evaporation is different from boiling.

Q.8: Describe the Kinetic Molecular Model or Theory of Matter.

Ans: Kinetic Molecular Model/Theory of Matter:

• The kinetic molecular theory of matter explains how particles are able to move about in three states of matter. It also explains the arrangement of the particles in these states.

• Atoms:
  Matter is made up of tiny particles called atoms.

• Molecules:
  A group of atoms is called a molecule. These molecules are always in continuous random motion. The evidence of molecular motion is Brownian motion.

  • According to this model, particles are in continuous motion. Thus, an alternative name for the model is "The Particle Model of Matter."
  • The kinetic molecular theory explains the physical properties of solids, liquids, and gases by considering the position and motion of molecules.

Q.9: Write down the properties of solids.

Ans: Properties of Solids:

The particles in solids have the following features:

1. The molecules are closely packed together and occupy minimum space.
2. The molecules are usually arranged in a regular pattern called a lattice.
3. There are a large number of particles per unit volume. That is why solids have the highest densities.

The movements of particles in solids have the following features:

1. The forces of attraction between particles are very strong.
2. The particles are not able to change position.
3. The particles vibrate about fixed positions, thus are not entirely stationary. This explains why solids have fixed shapes and volumes.

Q.10: Write down the properties of liquids.

Ans: Properties of Liquids:

The particles in a liquid have the following features:

1. The molecules are slightly further apart as compared to those of solids.
2. The molecules occur in clusters.
3. There are slightly fewer particles per unit volume compared to solids. This is why liquids have relatively high densities.

The movements of particles in liquids have the following features:

1. The forces of attraction between particles are strong.
2. The particles are free to move about within the liquid.

These features explain why liquids have fixed volumes but take the shape of the container.

Q.11: Write down the properties of gases.

Ans: Properties of Gases:

The particles in gases have the following features:

1. The molecules are very far apart.
2. The molecules are arranged randomly and are free to move at very high speeds.
3. There is a small number of particles per unit volume.

The movement of particles in gases has the following features:

1. The force of attraction between particles is negligible.
2. The particles are able to move freely in random directions at very high speeds.
3. The particles occupy any available space.

Q.12: Define Brownian motion.

Ans: Brownian Motion:
The evidence of molecular motion was first discovered by botanist Robert Brown in 1827. He observed the irregular motion of pollen grains suspended in water and deduced that the water molecules were in constant, random motion. This irregular motion caused by water molecules is called "Brownian motion," named after the scientist.

Q.13: Define lattice.

Ans: Lattice:
The molecules are usually arranged in a regular pattern called a lattice.

Q.14: Why some materials are solids and liquids while others are gases at room conditions?

Ans:
Forces between the molecules are responsible for the different states of matter as well as for the physical properties. According to the kinetic molecular model, molecules of gases have large kinetic energy. As a result, there are no forces of attraction between them, so gas molecules can move freely and go farther apart. This is why gases can occupy any available space and can be compressed easily. The boiling and melting points of gases are also very low because of this reason.

• The molecules of liquids have less kinetic energy compared to gases, so intermolecular forces come into play. This makes the molecules of liquids very close to each other but still free to move. Therefore, liquids do not have a fixed shape but have a fixed volume. The melting and boiling points of liquids are also higher compared to gases.

• The molecules of solids have extremely low kinetic energies and experience strong attractive forces. They cannot move freely and only have small vibrations about mean positions. This gives solids a fixed shape and volume. The densities, melting, and boiling points of solids are very high. For example, we can convert water into ice, ice into ice cream, and natural gas into compressed natural gas (CNG).

Q.15: The state of a substance can be changed either by heating or by cooling it. Explain.

Ans:
The state of a substance can be changed either by heating or by cooling it. When a solid substance is heated, the molecules start to vibrate more and more strongly. Eventually, the molecules vibrate more violently, and intermolecular forces become weak. As a result, the material becomes a liquid. If the process of heating continues, the molecules have sufficient energy to overcome all of the attractive forces, and as a result, the substance becomes a gas.

• HEATING

  • SOLID → LIQUID → GAS

• COOLING

  • GAS → LIQUID → SOLID

When a gas is cooled, the molecules move slowly and collide with one another, may stick together, and the force of attraction between molecules increases. Keep cooling the gas, and eventually, all of the molecules stick together to form a liquid. Further cooling will cause all the molecules to stick together to form a solid.

The table below shows the boiling and melting points of some pure substances:

Substance	Melting Point (°C)	Boiling Point (°C)
Helium	−272	−269
Oxygen	−218	−183
Nitrogen	−191	−177
Mercury	−39	257
Water	0	100
Iron	2080	3570
Diamond (Carbon)	4100	5400
Tungsten	3920	6500

Helium has the lowest boiling and melting points compared to other substances. It solidifies only when it is cooled and compressed. Mercury is the only metal that is not solid at room temperature.

Q.16: What is the name of the process in which a liquid changes into a solid?

Ans:
Freezing is a phase transition in which a liquid turns into a solid when its temperature is lowered to its freezing point.

Q.17: What is the name of the temperature at which a liquid changes into a solid?

Ans:
The name of the temperature at which a liquid changes into a solid is the freezing point. It is the temperature at which a liquid becomes a solid at normal atmospheric pressure.

Q.18: Describe the properties and behavior of gases in the light of the kinetic theory.

Ans: Behavior of Gases:
The kinetic molecular theory clearly describes the properties and behavior of gases. Hot air balloons are a practical application of this.

1. The molecules in gases have relatively large distances between them.
2. The molecules in gases move about very quickly.
3. A gas molecule moves in a straight line.
4. It changes its direction only when:
   • (a) It collides with another gas molecule or
   • (b) With the walls of its container. After the collision, it moves away in a new direction.
5. Since gas molecules collide many times each second, the motion of molecules is constant and random.

The behavior of a gas can be described completely by its pressure, volume, and temperature.

Q.19: Define pressure. Describe the pressure of gases.

Ans: Pressure:
Pressure is defined as the force per unit area.

The Pressure of Gases:
All gases exert pressure on the walls of their container. This pressure is the total force exerted per unit area by the gas molecules during collisions. Gas molecules exert pressure only when they collide with the walls. The number of collisions is proportional to the number of molecules. If the number of molecules is doubled, the number of collisions will also be doubled. Hence, the pressure is also doubled.

Blowing up a balloon is an example of pressure. If more air is pushed into the balloon, it will be inflated more because air molecules apply pressure on the rubber walls of the balloon, hence it gets inflated.

The pressure of a gas can also be increased by compressing it. This is done by reducing the size of the gas container. The gas molecules have been compressed into a smaller volume, so they will collide more frequently with the walls of the container and create more pressure. If the gas is compressed to half its original volume, its pressure will be doubled.

Q.20: Define volume and describe the volume of a gas.

Ans: Volume:
The space occupied by a substance is known as volume.

The Volume of a Gas:
A gas has no definite volume because the molecules of the gas are far apart from each other and can move freely at high speeds. Therefore, gas always takes up the shape and volume of its container.

For example, the smell of perfume quickly spreads through the room as soon as you spray it at your body or clothes because the molecules move freely and randomly at high speeds throughout the room.

The volume of a gas can also be increased by decreasing pressure. This could be done by reducing the load on the piston of the gas container. As the gas molecules are in random motion, they quickly cover the whole space, and the volume increases. If the gas is compressed to half its original volume, its pressure will be doubled.

Q.21: Describe the temperature of a gas. OR What is the effect of temperature on the average translational kinetic energy of molecules?

Ans: Temperature of a Gas:
The temperature of a gas is determined by the average translational kinetic energy of its molecules. If a gas is heated, the average translational kinetic energy of its molecules increases, and the temperature of the gas rises. If a gas is cooled down, the average translational kinetic energy of its molecules decreases, and the temperature of the gas falls.

Q.22: Explain the pressure-volume relationship in gases. OR Describe Boyle’s Law.

Ans: Pressure-volume relationship in gases / Boyle’s Law:
Robert Boyle, an English physicist and chemist in 1662, studied the relationship between pressure and volume of a gas.

Boyle’s Law:
The result of Boyle’s experiment is shown below:

$$P \propto \frac{1}{V} \text{ or } V \propto \frac{1}{P}$$

This means that the pressure of a gas is inversely proportional to its volume when the temperature is kept constant.

1. If the pressure of the gas is doubled, the volume becomes half. If pressure increases by three times, then the volume becomes one-third, and so on.

2. The graph between "p" and "V" and between "p" and $\frac{1}{V}$ is shown below:

   • (a) Shows the inverse relationship between pressure $p$ and volume $V$.
   • (b) Shows a direct relationship between pressure $p$ and $\frac{1}{V}$.

3. The graph between "p" and "V" shows that if pressure increases, then volume decreases and vice versa.

4. The graph between "p" and $\frac{1}{V}$ shows a straight line passing through the origin.

5. At constant temperature, the product of pressure and volume is constant, i.e.,

   $$pV = \text{constant}$$

6. Using the above result, at a constant temperature, we can write:

   $$\text{initial pressure} \times \text{initial volume} = \text{final pressure} \times \text{final volume}$$ $$p_1 V_1 = p_2 V_2$$

Thus Robert Boyle concluded his law, known as "Boyle's Law," which states that:

Statement of Boyle's Law:
The volume of a fixed mass of gas is inversely proportional to its pressure, provided its temperature remains constant.

Q.23: Write down the application of Boyle’s Law.

Ans: Applications of (p-V) relationship of gas / Boyle’s Law:
Some applications of the pressure-volume (p-V) relationship of a gas, i.e., Boyle’s Law, are given below:

• Aerosols, such as spray paints, use Boyle’s Law in their working mechanism.
• When we breathe, our diaphragm moves downward, increasing the volume of the lungs. This causes the pressure inside the lungs to be less than the outside pressure, so air rushes in.

A bicycle pump is a good example of Boyle’s Law.
As the volume of the air trapped in the pump is reduced, its pressure goes up, and the air is forced into the tire.

When fluids are drawn into a syringe, the volume inside the syringe is increased, and the pressure decreases on the inside. The pressure on the outside of the syringe is greater; therefore, fluids are forced into the syringe.

Q.24: Draw diagrams of the molecules in a gas to explain the effect of pressure change on volume.

Ans:

1. Low Pressure:

   • $P = 1 \, \text{atm}$
   • $V = 1 \, \text{L}$
   • $T = 298 \, \text{K}$

2. High Pressure:

   • $P = 2 \, \text{atm}$
   • $V = 0.50 \, \text{L}$
   • $T = 298 \, \text{K}$

3. Higher Pressure:

   • $P = 4 \, \text{atm}$
   • $V = 0.25 \, \text{L}$
   • $T = 298 \, \text{K}$

Q.25: What is meant by the subscripts 1 and 2 in the equation, $p_1 V_1 = p_2 V_2$?

Ans:
In the above equation of Boyle’s Law:

• The subscript ‘1’ means initial pressure and volume.
• The subscript ‘2’ means final pressure and volume.