Matter in Our Surroundings Question Answers: NCERT Class 9 Science

Chair, air, almonds, water, smell of perfume

Detail: Firstly we know that, what is matter? 

Matter is anything which occupies space and has its own mass, matter requires at least one fundamental subatomic particle. For example, that we can see around us all is matter.) Three most common state matters are present around us that are solid, liquid and gas.

Solid      :    Chair, Almonds
Liquid    :     Water
Gas       :     Air, smell of perfume

Hot food exchanges their energy with surrounding particles that leads to increase in kinetic energy of surrounding particles of matter. Due to kinetic energy they can move randomly in their surrounding space and we can easily smell of hot food from a few meters away.

In case of cold food, dew to low temp. the kinetic energy of surrounding particles is low so they can not move far away that's why we have to go closer to smell cold food.

There is a weak intermolecular force of attraction between water particles due to the presence of some space between water molecules. That implies that the liquid state of matter shows some space between their particles.

The important characteristics of particles of matter is:

(i) Particles of matter attract each other because of the force of attractions.
(ii) Particles of matter have space between them.
(iii) The size of particles of matter is very, very small.
(iv) The motion of particles of matter is constant and random.

density = mass / volumes

Arranging substance in their increasing order of densities

Air < exhaust from chimneys < cotton < water < honey < chalk < iron.

Reason: (The most heavy will be the highest density.) Iron is heavy so there is more density and the air is light so its density is low. 

Density is a property of matter which tells about how tightly particles are packed in a given object.

(a) Solid:

 * Particles are rigid and un-incompressible.
 * Particles possess a definite shape and volume.
 * Particles don't have the ability to flow.
 * Particles have the least kinetic energy.
 * Particles have high density. eg.: salt, iron, gold, chalk.

Liquid:

 * Particles are not rigid but can be compressed to a limited extent.
 * Particles have only a definite volume but acquire the shape of a container in which they are kept.
 * Particles can easily flow from higher to the lower level.
 * Particles have more kinetic energy than solid.
 * Particles have less density than solids. eg.: water, alcohol, petrol etc.

Gas:

 * Particles are not rigid and can be compressed easily.
 * Particles don't have a definite shape of volume.
 * Particles can flow in all the possible directions.
 * Particles have maximum kinetic energy.
 * Particles have the least density. eg.: air, CNG, smoke etc.

(b) 

 * Rigidity: It is the property of matter to maintain its shape even if an external force is applied and the solids show it this property.

 * Compressibility: It is the property of matter to allow decrease in volume under high pressure and gases show this property.

 * Fluidity: It is the property of a substance to easily flow and allow change in its shape under external forces and this property is shown by both liquids and gases.

 * Filling a gas container: Gases can be compressed easily hence they can be filled within a vessel at high pressure. This property of gases allows their convenient filling into a small container or cylinder and that also in a large volume. It also allows their easy transport from one place to other e.g. CNG.

 * Shape: According to the type of matter shape differ depending upon the location of particles like solid have a definite shape while liquid acquire the shape of their container and gases as such don't have any shape.

 * Kinetic energy: It is the kind of energy present in an object when it is under motion as the particles of that object are continuously moving therefore the object has kinetic energy. However, greater is the movement, more will be the kinetic energy and vise-versa.

Kinetic Energy = Gas > Liquid > Solid

Kinetic energy of gas is maximum.

 * Density: Mass per unit volume of a substance is known as its density.

Density = Solid > Liquid > Gas

Solids have the greatest density.

(a) Since the attraction force between particles of a gas in negligible i.e., that is extremely less hence particles freely move in all possible directions as a result gas fills completely the vessel in which it is kept.

(b) Freely moving particles of gas hit the walls of its container continuously and randomly therefore such random and erratic motion of gas particles exerts pressure on the walls of the container.

(c) A wooden table should be a solid because the particles of wooden table are quite rigid, have a fixed location and also possess a definite shape and volume. Due to all these properties we should call a wooden table a solid substance.

(d) Air is a mixture of gases and since particles of gal are far apart so the name is true for air therefore we can easily move our hand in air. But a solid block of wood is hard and rigid that resists any change in location of its particles hence we need a karate expert in case of a solid block of wood.

When water freezes to form ice, some empty spaces are created. As a result, volume increased for the same mass of water. In some words, mass per unit, volume or density of ice is lower than that of water and hence ice floats on water. (The density of water is maximum at 4^.C.)

K = 273 + C

(a)    K = 300
         K = 273 + C
         300 = 273 + C
         C = 300 - 273
         C = 27^*

(b)     K = 573
         K = 273 + C
         573 = 273 + C
         C = 573 - 273
         C = 300^*

a.  The boiling point of water is 100°c. Therefore at 250°c i.e. at a temperature higher than its boiling point is gaseous.

b.  At 100°c the boiling point of water, water exists both as a liquid as well as a gas.

During the change of state the heat or energy provided to particles of matter is utilized in overcoming the forces of attraction of the particles as a result the temperature of substance or matter remains constant during change of state.

If we decrease temperature and increase pressure we can liquify the atmospheric gases. (The maximum temp. above which a gas can not be liquified known as its critical temp.)

A hot day day means that the temp. of the atmosphere is high and humidity of air is low. Both these factors the rate of evaporation and thus enormous cooling is produced.

During summer the water present on the surface of the earthen pot (matka) evaporates which causes the cooling effect. Besides, earthen pot (matka) bears pores on it hence evaporation occurs continuously and so a large amount of cooling is produced.

Some liquids get quickly evaporated and they are called volatile liquids. Acetone, petrol and perfume are also volatile liquids therefore they get heat from our palm and cause cooling.

Saucer has a bigger surface area as compared to cup since evaporation is a surface phenomenon, by winging a saucer instead of cup. We are increasing the surface area for evaporation to occur; faster evaporation of particles of tea or milk allows cooling and taking a sip becomes easier.

Cotton is a good absorbent of water hence it absorbs sweat quite well and pores in the fabric expose that sweat too early evaporation, hence, we should prefer wearing cotton clothes in summer.

K = 273 + C^*

(a)  K = 300
       K = 273 + C
       300 = 273 + C
       C = 300 - 273
       C = 27^*

(b)  K = 573
       K = 273 + C
       573 = 273 + C
       C = 573 - 273
       C = 300^*

K = 273 + C°

(a)    C = 25
​        K = 273 + C
        K = 273 + 25
        K = 298

(b)   C = 573
​       K = 273 + C
       K = 273 + 573
       K = 646

(a) Some substances possess the property of sublimation like camphor and naphthalene balls. Such substances directly change from solid to gaseous state without changing in to liquid like:

               ice → water → water vapour does.

Therefore, naphthalene balls disappear with time without leaving any solid. 
(Sublimation - Solid to gaseous state directly converted).

(b) Being a volatile substance (gets evaporated easily) perfume changes from liquid to gaseous state very fast. Those particles mix up with air particles and diffuse to reach our nostrils such that we get the smell of perfume sitting several metres away.

(volatile substance → liquid to gaseous state changed immediately).

Sugar > water > oxygen

Because the forces of attraction are solid is more. In solids, molecules are tightly packed. In liquid, forces of attraction are less than solid because molecules move or slide past one another. In gas, weak force of attraction.

(a)  At 25*C water is liquid.

(b)  At 0*C water is solid.

(c)  At 100*C water exists as both liquid and gas.

(a) Water at room temperature is a liquid because at this temperature:

       i). It has fixed volume.
       ii). It can flow.

(b) An iron almirah is solid at room temperature because at this temperature:

       i).  It has a definite shape along with fixed volume.
       ii). It can not flow like water and hence does not possess fluidity.

While melting ice absorbs latent heat of melting from the surroundings and gets changed into water that makes the cooling effect more intense as compared to water at same temperature.

Note: Latent heat: Energy absorbed or released by a substance during a change in its physical state that occurs without changing its temperature.

Steam at 373 K has more heat energy equal to the latent heat of vaporization than boiling water at 373K, therefore steam produces more severe burns than boiling water.

A - fusion (solid to liquid convert)

B - vaporization (liquid to gas convent)

C - cooling - condensation (liquefaction)

D - cooling - freezing (liquid to solid convert)

E - sublimation (solid to gaseous state)

F - sublimation (gaseous to solid state)