Class 9 NCERT Solutions Chemistry Chapter 1 - Matter in Our Surroundings

Chair - It is matter
Air -  It is matter
Love - It is not matter
Smell - It is not matter
Hate - It is not matter
Almonds - It is matter
Thought - It is not matter
Cold - It is not matter
Cold drink - It is matter
Smell of perfume - It is matter

Concept insight: Anyhting that occupies space and has mass is called matter. Chair, air, almonds, cold drink and smell of perfume are matter since they have mass and occupy space. You might get confused with smell of matter. Is it matter or not? The sense of smell is not matter. However, the smell or odour of a substance is classified as matter. The smell of any substance (say, perfume) is the gaseous form of that substance which our olfactory system can detect (even at very low concentrations). Hence, smell of perfume is matter.

Particles of hot sizzling food possess large kinetic energy and diffuse in air rapidly and reach several metres away whereas particles of cold food possess less kinetic energy and are not able to reach several metres away.

 

Concept insight: 

Particles of matter possess kinetic energy and keep moving constantly. At lower temperature, particles have low kinetic energy and thus move slowly. Particles in cold food have low kinetic energy due to low temperature. At higher temperature, particles have high kinetic energy and move faster hence, the particles of hot vapors from hot food move faster.

This shows that the particles of matter have spaces between them and the interparticle forces of attraction between liquid particles are not very strong hence the force applied by diver is enough to overcome these forces of attraction.

Concept insight: Key to this answer is that a diver will be able to pass through water if there is availability of space between the particles and the forces of attraction between the liquid particles are not very strong. The diver will not be able to cut through solid ice because of less spaces between the solid particles and strong forces of attraction.

The characteristics of particles of matter are:
(i)    All matter is composed of very small particles which can exist independently.
(ii)    Particles of matter have spaces between them.
(iii)    Particles of matter are continuously moving.
(iv)    Particles of mater attract each other.

Concept insight: Mention all the charateristics of particles of matter.

The given substances in the increasing order of their densities can be represented as:
Exhaust from chimney < Air < Cotton < Water < Honey < Chalk < Iron

Concept insight:  Remember that gases have lowest densities, liquids have higher densities than gases but lower than that of solids and solids have highest densities.

(a)    A gas completely fills the vessel in which it is kept because there is negligible force of attraction between the particles of gas. Thus, the gas particles move freely in all directions.

 

(b)    A gas exerts pressure on the walls of the container because the particles of gas move randomly in all directions at high speed. As a result, the particles hit each other and also hit the walls of the container with a force. These collisions of the gas particles with the walls of the container exert pressure on the walls of the container.

(c)    A wooden table should be called a solid because it has a definite shape and volume. It is very rigid and cannot be compressed i.e., it has the characteristics of a solid.

(d)    We can easily move our hand in air, but to do the same through a solid block of wood, we need a karate expert because the particles of air have large intermolecular spaces and least force of attraction between them. Thus, one can easily move the hand in air and push the particles of air apart. However, the particles of solid have minimum amount of intermolecular space and maximum force of attraction between them. Thus, a greater amount of force is required to move the particles of solid apart. Thus, a much greater force is required to move hand through a solid block of wood.

 

Concept insight: For answering reasoning questions, the complete reason should be mentioned. For answering this question, you need to remember all the properties of solids, liquids and gases.

The mass per unit volume of a substance is called density (density = mass/volume). As the volume of a substance increases, its density decreases.

Though ice is a solid, but it has a cage like structure hence there are large number of empty spaces between its particles. These spaces are larger as compared to the spaces present between the particles of water. Thus for a given mass of water, volume of ice is greater than that of water. Hence, the density of ice is less than that of water. A substance with lower density than water can float on water. Therefore, ice floats on water.

Concept insight: For answering this question, link density with floating and sinking. A substance with lower density than water will float on water. A substance with higher density than water will sink in water.

(a) 300 K = (300 - 273)
               = 27 C

(b) 573 K = (573 - 273)
               = 300 C
 
Concept insight: Remember the relationship:
Temperature in Kelvin = Temperature in C + 273
Substitute the given value in the equation and then calculate the other unknown value.

(a) Water at 250 C exists in gaseous state.
(b) At 100 C, water exists in liquid state. At this stage, if we keep on supplying heat energy, water can change to gaseous state.

Concept insight: For answering this question you should recall the melting and boiling points of water.

During a change of state, the temperature remains constant. This is because all the heat supplied to increase the temperature is utilised in changing the state by overcoming the forces of attraction between the particles. This heat is called the latent heat. Latent heat does not contribute in increasing the temperature of the substance.

 

Concept insight:  The key to answer this question is the forces of attraction between the particles and the heat required to overcome these.

By increasing pressure and reducing the temperature, atmospheric gases can be liquefied.

Concept insight:  To answer this question, you should think in which way you can change a gas to liquid. For this conversion, you need to decrease the intermolecular space between the particles and increase the intermolecular forces of attraction between them.

When a liquid evaporates, the particles of the liquid absorb energy from the surroundings to compensate the loss of energy during evaporation. Since evaporation causes cooling, this makes the surroundings cool.

 

In a desert cooler, the water inside it is made to evaporate. This leads to absorption of energy from the surroundings, thereby cooling the surroundings. Evaporation depends on the amount of water vapour present in air (humidity). If the amount of water vapour present in air is less, then evaporation is more. On a hot dry day, the amount of water vapour present in air is less. Thus, water present inside the desert cooler evaporates more, thereby cooling the surroundings more. That is why a desert cooler cools better on a hot dry day.
 
Concept insight: The key to this answer is evaporation. If the particles evaporate more, it will lead to more cooling. This is because evaporation causes cooling.

There are a large number of extremely small pores in an earthen pot through which the water kept inside the pot keeps on evaporating and takes the latent heat required for vaporisation from the earthen pot and remaining water. The earthen pot and water hence loses heat and this makes the water inside the pot cool.

Concept insight: The key to this answer is evaporation and the property of evaporation that it causes cooling.

When we put some acetone or petrol or perfume on our palm, it evaporates. During evaporation, particles of the liquid absorb energy from the surface of the palm to compensate for the loss of energy, making the surroundings cool. Hence, our palm feels cold when we put some acetone or petrol or perfume on it.
   
Concept insight: The key to this answer is evaporation and the property of evaporation that it causes cooling.

A liquid has a larger surface area in a saucer than in a cup. One of the factors affecting evaporation is surface area. Larger is the surface area more is the evaporation. Thus evaporation is faster in a saucer causing more cooling than in a cup. For this reason, we are able to sip hot tea or milk faster from a saucer than a cup.  

 

 Concept insight: The key to this answer is evaporation and the property of evaporation that it causes cooling.  The more the surface area the more is the cooling.

We should wear cotton clothes and light coloured clothes in summers. We sweat a lot in summers. Cotton is a good absorber of water. Thus, it absorbs sweat from our body and exposes the sweat to the atmosphere, making its evaporation faster. During this evaporation, particles on the surface of the sweat gain energy from our body surface, making the body cool. Further light coloured clothes are poorer absorber of heat and would be better for summer.

Concept insight: The key to this answer is evaporation and the property of evaporation that it causes cooling. Link evaporation to the types of clothes.  

 (a) 293 K = (293 - 273)
= 20 C
(b) 470 K = (470 - 273)
= 197 C
 
Concept insight: Remember the relationship:
Temperature in Kelvin = Temperature in C + 273
Substitute the given value in the equation and then calculate the other unknown value.

(a) 25°C = (25 + 273) K
= 298 K
(b) 373°C = (373 + 273) K
= 646 K

Concept insight: Remember the relationship:
Temperature in Kelvin = Temperature in ^oC + 273
Substitute the given value in the equation and then calculate the other unknown value.

(a) Naphthalene undergoes sublimation easily i.e., the change of state of naphthalene from solid to gas without intervention of liquid state. Thus, naphthalene balls keeps on forming naphthalene vapours which disappear into the air with time without leaving any solid.

(b) Gaseous particles possess high speed and move very rapidly in all directions. When perfume is sprayed, its particles diffuse into the particles of air at a very fast rate and reach our nostrils. This enables us to smell the perfume from a distance.

 

 Concept insight: For answering reasoning questions, the complete reason should be mentioned. Remember the properties of all the three states of matter and their state conversion processes.

Sugar is a solid; the forces of attraction between the particles of sugar are strong.

Water is a liquid; the forces of attraction here are weaker than sugar.

Oxygen is a gas; the forces of attraction are the weakest in gases.

 

Thus, the increasing order of forces of attraction between the particles of water, sugar and oxygen is

 

Oxygen < Water < Sugar

Concept insight: The key to this answer is the strength of the forces of attraction in the different states of matter. Oxygen is a gas, water is a liquid and sugar is a solid.

(a)Water at 25 C is present in the liquid state.

 

(b) At 0 C, water can exist as both solid and liquid.
NOTE: At 0 C temperature, after getting the heat equal to the latent heat of fusion, the solid form of water i.e., ice starts changing into its liquid form i.e., water.
(c) At 100 C, water exists in liquid state at 100^oC. At this stage, if we keep on supplying heat energy, water can change to gaseous state.

Concept insight: For answering this question, you should recall the melting and boiling point of water.

(a) At room temperature (25 C), water is a liquid because it has the following characteristics of liquid:
(i) At room temperature, water has no shape but has a fixed volume i.e., it occupies the shape of the container in which it is kept.
(ii) At room temperature, water flows easily.

 

(b) An iron almirah is a solid at room temperature (25 C) because:
(i) It has a definite shape and volume like a solid at room temperature.
(ii) It is rigid as solid at room temperature.
 
Concept insight: For answering reasoning questions, the complete reason should be mentioned. Remember the characteristics of all the three states of matter to answer such questions.

Cooling takes place when heat is removed from a system. In case of ice at 0^oC, it will first take heat (latent heat) from the medium to convert itself into water at 0^oC and then into water at higher temperature. In short, in case of ice at 0^oC, there will be change in phase so more energy will be required, whereas in case of water at 0^oC, there will be no change in phase. Hence, lesser energy will be taken from the surroundings. So, at same temperature of 273 K, large amount of heat will be removed by ice from the surroundings than in case of water. Hence, ice at 273 K is more effective in cooling than water at the same temperature.
 

Concept insight: Remember that when steam will be cooled, it will change into water. Cooling will be more in that case in which there is more removal of heat from the system.

Steam has more energy than boiling water. It possesses the additional latent heat of vaporisation. So, when steam falls on skin and condenses to produce water it gives out 22.5 x 10⁵ J/kg more heat than boiling water at the same temperature. Therefore, burns produced by steam are more severe than those produced by boiling water.
 
Concept insight: The key to this answer is that in comparison to water, steam contains more heat in the form of latent heat of vaporisation.

 

 

 

A. Fusion
B. Vaporisation
C. Condensation
D. Solidification
E. Sublimation
F. Sublimation

Concept insight: The key to this answer is to recall the names of the processess of conversion of one state to another.