Matter in Our Surroundings

Class 9 – NCERT Science Solutions

Intext Questions 1

Question 1

Which of the following are matter?
Chair, air, love, smell, hate, almonds, thought, cold, lemon water, the smell of perfume.

Answer

The following substances are matter:

• Chair
• Air
• Almonds
• Lemon water
• Smell of perfume (due to the presence of some volatile substances in air that occupy space & have mass.)

Question 2

Give reasons for the following observation:

The smell of hot sizzling food reaches you several meters away, but to get the smell from cold food, you have to go close.

Answer

Particles of matter are continuously moving as they possess kinetic energy. An increase in temperature increases the kinetic energy of particles hence they start moving faster and can move several meters quickly. Therefore, the smell of hot sizzling food reaches a person even at a distance of several meters but to get the smell from cold food, we have to go close.

Question 3

A diver is able to cut through water in a swimming pool. Which property of matter does this observation show?

Answer

The observation that a diver can cut through water in a swimming pool indicates that the particles of liquid matter have spaces between them, and the intermolecular forces of attraction are relatively weak. Consequently, the force exerted by the diver is sufficient to overcome these forces of attraction, allowing the diver to move through the water.

Question 4

What are the characteristics of the particles of matter?

Answer

The characteristics of the particles of matter are:

• Particles of matter have space between them.
• Particles of matter are always in random motion.
• Particles of matter attract each other.

Intext Questions 2

Question 1

The mass per unit volume of a substance is called density. (Density = Mass/Volume ). Arrange the following in order of increasing density – air, exhaust from the chimneys, honey, water, chalk, cotton and iron.

Answer

The following substances are arranged in increasing density:

Air < exhaust from chimney < cotton < water < honey < chalk < iron

Question 2

(a) Tabulate the differences in the characteristics of states of matter.

(b) Comment upon the following:

rigidity, compressibility, fluidity, filling a gas container, shape, kinetic energy and density.

Answer

(a) The difference in the characteristics of the three states of matter are:

Characteristics	Solid	Liquid	Gas
Shape	Fixed shape	No Fixed shape	No Fixed shape
Volume	Fixed volume	Fixed volume	No Fixed volume
Intermolecular force	Maximum	Less than solids	Very less
Intermolecular space	Very less	More than solids	Maximum
Compressibility	Negligible	Compressible	Highly compressible

(b) (i) Rigidity — It is the property of matter to continue to remain in its shape when treated with an external force.

(ii) Compressibility — It is the property of the particles of matter to contract their intermolecular space when exposed to an external force, thereby increasing its density.

(iii) Fluidity — It is the ability of a substance to flow or move about freely.

(iv) Filling a gas container — The particles of gas fill the entire container as the intermolecular spaces between the gas molecules is very large and hence the particles are free to move about in all directions.

(v) Shape — It is the definite structure of an object within an external boundary

(vi) Kinetic energy — Motion allows particles to possess energy which is referred to as kinetic energy.

K.E = 1221​ mv², where

‘m’ is the mass and

‘v’ is the velocity of the particle.

(vii) Density — It is the mass of a unit volume of a substance.

d = mvvm​, where

d is the density,

m is the mass and

v is the volume of the substance

Question 3

Give reasons

(a) A gas fills completely the vessel in which it is kept.

(b) A gas exerts pressure on the walls of the container.

(c) A wooden table should be called a solid.

(d) We can easily move our hand in the air, but to do the same through a solid block of wood, we need a karate expert.

Answer

(a) Gas is a state of matter in which intermolecular attraction is very weak and intermolecular space is so large that the particles become completely free to move randomly in the entire available space. Hence, it fills completely the vessel in which it is kept.

(b) Gas molecules have negligible intermolecular forces and possess high kinetic energy, enabling them to move rapidly in all directions. This random motion leads to collisions among particles and with the container walls, resulting in the exertion of pressure by gas particles on the container.

(c) A solid is characterized by a fixed shape and volume, and a wooden table also has these properties with its definite shape and volume, minimal intermolecular spaces, and rigidity. Therefore, it is appropriately called as a solid.

(d) Air particles have very weak intermolecular force of attraction and hence very large intermolecular spaces. As a result, we can move our hands freely in the air. The particles in a solid block, on the other hand, are bound together by a strong force of attraction. As a result, there is no space between them. Hence, we will require a karate expert.

Question 4

Liquids generally have a lower density than solids. But you must have observed that ice floats on water. Find out why.

Answer

Water shows unique behaviour as it is one of the few substances that expands on freezing resulting in a decrease in its density. Therefore, the ice formed is less dense than the liquid water, causing it to float on water.

Intext Questions 3

Question 1

Convert the following temperature to Celsius scale:

(a) 300 K

(b) 573 K

Answer

(a) °C = Kelvin – 273 = 300 – 273 = 27°C

(b) °C = Kelvin – 273 = 573 – 273 = 300°C

Question 2

What is the physical state of water at?

(a) 250°C

(b) 100°C

Answer

(a) At 250°C water is in gaseous state since it is beyond its boiling point.

(b) At 100°C water is in the transition state as the water is at its boiling point. Hence it would be present in both liquid and gaseous states.

Question 3

For any substance, why does the temperature remain constant during the change of state?

Answer

The temperature remains constant during a change of state because the heat energy supplied or released is used to overcome the intermolecular forces between particles rather than increasing the kinetic energy of the particles. This heat energy that is supplied or released without showing any change in temperature is known as the latent heat.

Question 4

Suggest a method to liquify atmospheric gases.

Answer

In order to liquify atmospheric gases, the intermolecular spaces between their molecules should be reduced. This can be achieved in two ways:

1. By increasing the pressure of the gas.
2. By decreasing the temperature of the gas.

Intext Questions 4

Question 1

Why does a desert cooler cool better on a hot dry day?

Answer

A dessert cooler works on the principle of evaporation. In hot and dry conditions, the air has low humidity, which means it can absorb more water vapour leading to more efficient evaporation and, consequently, better cooling effects compared to a humid day.

Question 2

How does the water kept in an earthen pot (matka) become cool during summer?

Answer

An earthen pot is porous in nature. Water gets evaporated through the pores of pot by absorbing heat from the water present inside the pot leading to cooling.

Question 3

Why does our palm feel cold when we put some acetone or petrol, or perfume on it?

Answer

Acetone, petrol, and perfume are volatile substances. These particles gain energy from our palm or surrounding and evaporate causing the palm to feel cool.

Question 4

Why are we able to sip hot tea or milk faster from a saucer rather than a cup?

Answer

Evaporation is a surface phenomenon. If the surface area is increased, the rate of evaporation increases. The surface area of a saucer is more than a cup, hence rate of evaporation is greater in a saucer than in a cup. As evaporation causes cooling, so hot tea or milk gets cooled quickly in a saucer than in a cup enabling us to sip it faster.

Question 5

What type of clothes should we wear in summer?

Answer

During summer we perspire more. During evaporation of sweat the particles at the surface gain energy from the surroundings or body surface and change into vapour. The heat energy equal to the latent heat of vaporization is absorbed leaving the body cool. Cotton being a good absorber of water helps in absorbing the sweat and exposing it to the atmosphere for easy evaporation. Hence, one should wear cotton clothes in summer.

Exercises

Question 1

Convert the following temperatures to Celsius scale.

(a) 293 K

(b) 470 K

Answer

(a) °C = Kelvin – 273 = 293 – 273 = 20°C

(b) °C = Kelvin – 273 = 470 – 273 = 197°C

Question 2

Convert the following temperatures to the Kelvin scale.

(a) 25°C

(b) 373°C

Answer

(a) Kelvin = (°C + 273) K = 25 + 273 = 298 K

(b) Kelvin = (°C + 273) K = 373 + 273 = 646 K

Question 3

Give reason for the following observations:

(a) Naphthalene balls disappear with time without leaving any solid.

(b) We can get the smell of perfume sitting several metres away.

Answer

(a) Naphthalene balls when left open, directly change into vapour without heating due to sublimation. Hence, with time they become small in size and finally disappear.

(b) Perfumes comprise substances that are volatile, and diffuse rapidly into the air. Hence, we are able to smell perfume sitting several metres away.

Question 4

Arrange the following in increasing order of forces of attraction between the particles –

water, sugar, oxygen.

Answer

Oxygen (gas) < water (liquid) < sugar (solid)

Question 5

What is the physical state of water at?

(a) 25°C

(b) 0°C

(c) 100°C?

Answer

(a) At 25°C (normal room temperature) — liquid state

(b) At 0°C (freezing point) — Both solid and liquid phases are observed

(c) At 100°C (boiling point) — Both liquid and gaseous phases are observed

Question 6

Give two reasons to justify –

(a) water at room temperature is a liquid.

(b) an iron almirah is a solid at room temperature.

Answer

(a) Water at room temperature is a liquid because of the following reasons:

1. At room temperature, water does not have a fixed shape and flows to fit the container’s shape.
2. A fixed volume is occupied by a fixed mass of water.

(b) An iron almirah is a solid at room temperature because of the following reasons:

1. It has a definite shape and volume at room temperature.
2. It is rigid.

Question 7

Why is ice at 273 K more effective in cooling than water at the same temperature?

Answer

The latent heat of fusion of ice is 336 J g^-1 so it absorbs 336 J of heat and changes to water at 0°C and then water at 0°C absorbs heat and temperature is raised. Therefore, ice at 0°C absorbs extra heat in comparison to water at 0°C (due to latent heat of fusion). Hence, ice at 0°C has greater cooling effect than water at 0°C.

Question 8

What produces more severe burns, boiling water or steam?

Answer

Steam has a higher heat content on account of high specific latent heat of condensation that is 2268 J g^-1. Hence, steam at 100°C carries more heat than water. Therefore, burns caused by steam are more severe than burns caused by boiling water.

Question 9

Name A, B, C, D, E and F in the following diagram showing a change in its state.

Answer

(A) Solid to Liquid — Melting or liquefaction

(B) Liquid to Gas — Evaporation or vaporization

(C) Gas to liquid — Condensation

(D) Liquid to Solid — Solidification

(E) Solid to Gas — Sublimation

(F) Gas to Solid — Deposition