NCERT Solutions Class 9 Science Chapter 1 Matter in Our Surroundings

NCERT Solutions Class 9 Science 

The NCERT Solutions in English Language for Class 9 Science Chapter – 1 (Matter in Our Surroundings) has been provided here to help the students in solving the questions from this exercise.

Chapter – 1 (Matter in Our Surroundings)

Questions

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 and The smell of perfume.

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 – The smell of hot sizzling food reaches you several meters away, but to get the smell from cold food, we have to go close because the kinetic energy of particles of matter increases with the increase in temperature

3. A diver is able to cut through water in a swimming pool. Which property of matter does this observation show?
Answer – The diver is able to easily cut through the water in the swimming pool because of the weak forces of attraction between water molecules. It is this property of water that attributes to easy diving.

4. What are the characteristics of the particles of matter?
Answer –  The characteristics of particles of matter are as follows:

• Presence of intermolecular spaces between particles.
• Particles are in constant motion.
• They attract each other.
• All matter is composed of very small particles which can exist independently.

Questions

1. The mass per unit volume of a substance is called density.
(density=mass/volume).
Arrange the following in the 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 and Iron.

2. Answer the following.
(a) Tabulate the differences in the characteristics of matter.
Answer – The difference in the characteristics of the three states of matter.

(b) Comment upon the following: rigidity, compressibility, fluidity, filling a gas container, shape, kinetic energy and density.

• Rigidity – Rigidity is described as a material’s tendency to resist change in shape.
• Compressibility – When force is applied, compressibility is described as the ability to be compressed to a smaller volume.
• Fluidity – The capacity to flow is defined as fluidity.
• Filling the gas container – Filling a gas container refers to the gas taking on the shape of the container.
• Shape – It establishes a clear barrier.
• Kinetic energy – Kinetic energy is the energy possessed by a particle as a result of its motion.
• Density – Density is defined as the mass per unit volume.

3. Give reasons
(a) A gas fills completely the vessel in which it is kept.
Answer – Gases have a negligible intermolecular force between the gas molecules and for this reason the gas particles randomly move in all directions and so the vessel is filled with gas.

(b) A gas exerts pressure on the walls of the container.
Answer – The moving particles in a gas collide with each other and also with the walls of the container and due to these collisions, Gases exert pressure on the walls of the container.

(c) A wooden table should be called a solid.
Answer – A wooden table has a definite shape and volume. It is very rigid and cannot be compressed i.e., it has the characteristics of a solid. Hence, 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 – Molecules in the gas have a very low intermolecular attraction, but in a solid block of wood the molecules are tightly packed with strong intermolecular force, hence this happens. 

4. Liquids generally have a lower density than solids. But you must have observed that ice floats on water. Find out why.
Answer – The density of ice is less than the density of water. The low density of ice can be attributed to the small pores it has which allows it to trap air hence ice floats on water.

Questions

1. Convert the following temperature to Celsius scale:
a. 300K
b. 573K
Answer –
(a) 300K
0°C = 273K
300K= (300 – 273)°C = 27°C

(b) 573K
573K = (573 – 273)°C
= 300°C

2. What is the physical state of water at:
a. 250°C
b. 100°C ?
Answer –
(a) At 250°C – Physical state of water at 250°C is gaseous state because the boiling point of water is 100°C. Therefore at a temperature higher than its boiling point. It exists as gas. 

(b) At 100°C – Both liquid and gaseous states are present. These are in a state of equilibrium. So at 100°C both liquid water and vapour are present.  

3. For any substance, why does the temperature remain constant during the change of state?
Answer – The temperature remains constant during the change of state because the heat supplied during the change is used up in overcoming the intermolecular forces between the particles of the state.

4. Suggest a method to liquify atmospheric gases.
Answer – In order to liquefy a gas, the constituent particles or molecules have to be brought closer. The atmospheric gases can be liquefied either by increasing pressure or by decreasing temperature.

Questions

1. Why does a desert cooler cool better on a hot dry day?
Answer – It is because the temperature is high and less humid on a hot dry day, enabling better evaporation. High levels of this evaporation provide better cooling effects.

2. How does the water kept in an earthen pot (matka) become cool during summer?
Answer – An earthen pot (matka) has many small pores. Water seeps out through them and evaporates from the surface of the pot. The energy needed for evaporation is taken from the water kept in the earthen pot. As a result, water kept in earthen pot becomes cool.

3.Why does our palm feel cold when we put on some acetone or petrol, or perfume on it?
Answer –  Both acetone and perfume are low boiling liquids. When they are poured on the palm, they evaporate readily and for this change of state they take the energy from the palm and we get a cooling sensation.

4. Why are we able to sip hot tea or milk faster from a saucer rather than a cup?
Answer – We are able to sip hot tea faster from a saucer rather than a cup because a saucer has a greater surface area. As a result, rate of evaporation increases.

5. What type of clothes should we wear in summer?
Answer – In summer, it is preferred to wear light-coloured cotton clothes because light colour reflects heat and cotton materials have pores that absorb sweat, facilitating evaporation, and hence causing a cooling effect on the skin.

Exercise

1. Convert the following temperature to Celsius scale.
(a) 293K
(b) 470K

Answer – 0°C = 273K
(a) 293K
293K = (293 – 273)°C
= 20°C

(b) 470K
470K = (470 – 273)°C
= 197°C

2.Convert the following temperatures to the Kelvin scale.
(a) 25°C
(b) 373°C

Answer – 0°C = 273K
(a) 25°C
25°C = (25 + 273)K
= 298K

(b) 373°C
373°C = (373 + 273)K
= 646K

3. Give reason for the following observations:
(a) Naphthalene balls disappear with time without leaving any solid.
Answer – Naphthalene balls disappear with time without leaving any solid because they undergo sublimation i.e., they directly change into vapour without passing through the liquid state.

(b) We can get the smell of perfume while sitting several metres away.
Answer – We can get the smell of perfume sitting several metres away due to diffusion. The perfumes contain solvent which carries pleasant smelling vapour. They diffuse quite fast and can reach a person sitting several metres away.

4. Arrange the following in increasing order of forces of attraction between the particles – water, sugar, oxygen.
Answer – Oxygen (gas) < water (liquid) < sugar (solid)

5. What is the physical state of water at –
(a) 25°C
(b) 0°C
(c) 100°C
Answer –

(a) 25°C – the water will be in liquid form (normal room temperature)

(b) 0°C – the water is at its freezing point, hence both solid and liquid phases are observed.

(c) 100°C – the water is at its boiling point, hence both liquid and gaseous states of water (water vapour) are observed.

6. Give two reasons to justify –
(a) Water at room temperature is a liquid.
Answer – Water at room temperature is a liquid because its freezing point is 0 °C and boiling point is 100 °C.

(b) An iron almirah is a solid at room temperature.
Answer – An iron almirah is a solid at room temperature because melting point of iron is higher than room temperature.

7. Why is ice at 273K more effective in cooling than water at the same temperature?
Answer – Ice at 273 K will absorb heat energy or latent heat from the medium to overcome the fusion to become water. Hence the cooling effect of ice is more than the water at same temperature because water does not absorb this extra heat from the medium.

8. What produces more severe burns, boiling water or steam?
Answer – Steam at 100 °C will produce more severe burns as extra heat is hidden in it called latent heat whereas the boiling water does not have this hidden heat.

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

Answer –  Interconversion of three states of matter: Using temperature or pressure, any state of matter can be turned into another.
(A) Solid to Liquid → Melting (or) fusion (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 → solidification

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