The Matter in Our Surrounding CBSE Notes Class 9 DailyHomeStudy

DailyHomeStudy: The Matter in Our Surroundings CBSE Notes Class 9 is given with detail explanation. The question answer are also given with detailed explanation. We have also given the MCQs and Sample Paper for help in studies of Class 9 Science.

Look around! You see so many things —living and non-living. These things differ a lot in their properties, such as, colour, shape, size, smell, etc. How are so many different things produced? Before answering this question, we would like to see if these things have anything in common! When we observe these objects closely, we see that in spite of differences in their properties, all these objects have mass, occupy space, and can be felt through one or more of our sense organs. What makes these things to have these common properties? The Greek and the Indian philosophers thought that everything around us, including our body, was made up of five basic elements, namely, Earth, Fire, Water, Air and Sky. These five elements in Indian philosophy are called panch tatva. Science, however, interprets nature in terms of energy and matter.


What is matter?

 Matter is the material of which the universe is composed. Matter may be defined as something that occupies space, possesses mass, and offers resistance to any stress (force applied on it).

A definite variety of matter, all samples of which have the same properties is commonly called a substance. For example, sugar is a substance, because even the smallest particle of sugar is sweet.

Physical Nature of Matter

What is meant by the particulate nature of matter?

We are surrounded by matter in the form of various objects.

How does matter exist in such a large variety of materials?

What constitutes matter?

These were the questions in the minds of people and philosophers since ancient times. The earlier philosophers believed matter to be continuous.

The Greeks believed that Fire, Water, Air and Earth were the primary building blocks of all forms of matter. The properties of any particular material (or any form of matter) were believed to be due to different ratio of the four primary elements. Ancient Indian philosophers considered all forms of matter to be made of five elements called bhutas — akasa (sky), vayu (air), tejas (fire), ap (water), and kshiti (earth).

The great Indian sage, Maharishi Kanada was perhaps the first to suggest that all forms of matter are composed of very small (or tiny) particles. Each such particle was given the name (71111 (31n1) by him. He further believed that each anu may be made of two or more still smaller particles. He called these particles parmanu.

The Greek philosopher Democritus called such tiny particles as atoms (from the Greek word Atomos, meaning uncut).

What is diffusion?

As we have seen in the activities described above that the particles of one torm of matter can easily get spread into another form of matter.

The intermixing of the particles of two or more substances on their own is called diffusion.

• Diffusion in gases is very fast because the particles in gases travel with high speeds. However, different gases diffuse at different rates. Lighter gases diffuse faster than the heavier ones.

 • Diffusion of liquids in liquids and of solids in liquids are relatively much slower.

 • Diffusion of a solid substance into another solid under normal conditions is extremely slow (almost negligible).

 • Diffusion gets faster when temperature is raised.

 How small are the particles of matter?

All forms of matter consist of small particles. The question which arises in the mind is how small are these particles?

States of Matter

When we look around various forms of matter, we find that matter can elxist in three states. These are

  • Solid state
  • Liquid state
  • Gaseous state

 For example, water can exist as ice (solid state), water (liquid state) and steam (gaseous state).

What are the properties of solids, liquids and gases?

Why should the properties of solids, liquids and gases be different?

 What factors govern the states of matter?

What makes the matter to exist in any one of the three states?

Before we find an answer for these questions, let us recollect that,

 • All matter is made up of very small particles. These particles may be atoms, ions or molecules.

 • The constituent particles in any matter are constantly in motion. Thus, they possess kinetic energy. The kinetic energy of these particles increases with a rise in temperature.

• The particles in any matter attract each other. This force of attraction decreases as the distance between the particles increases.

The force of attraction between the particles in any matter is called intermolecular force or force of cohesion.

The intermolecular force or the force of cohesion between the particles tends to bring them closer to each other, whereas motion of the particles tends to move them away from each other. Thus, the physical state of any matter depends upon the net effect of these two factors. Thus,

 • When the intermolecular force or the force of cohesion between the constituent particles is very strong and the kinetic energy is very small, the matter exists as a solid.

 • When the force of cohesion between the constituent particles is negligible and the kinetic energy is very large, the matter exists as a gas.

• When the force of cohesion between the constituent particles is moderately strong and the kinetic energy is sufficient for particles to move to and fro, the matter exists as a liquid.

 How does the strength of intermolecular forces vary in solids, liquids and gases?

The forces operating between the constituent particles (atoms or molecules), in any form of matter are called intermolecular forces.

  •  In solids, the intermolecular forces are very strong, and the constituent particles are closely packed. That is why, solids are incompressible and have high density.
  • In liquids, the intermolecular forces are strong enough to keep the ‘.,111111 particles tied up to each other but not strong enough to keep them in fixed positions. This is why, liquids can flow and do not have a definite shape.
  • In gases, the intermolecular forces are negligible (extremely weak), and the constituent particles are free to move. As a result, gases can occupy any space available to them.

 The Solid State

 What are the properties of solids?

A solid show the following characteristics:

  • Solids are rigid and incompressible. Because in solids, the constituent particles are closely packed and there is virtually no space between the particles. So, solids cannot be compressed.
  • A sponge can be compressed easily because it has small pores in it. Air is trapped in these pores. When a sponge is pressed, air is expelled out and it gets compressed.
  • A solid has a definite shape and volume. Because in solids, the constituent particles are held together by a strong cohesive force. For example, sugar and salt are solids because the shape of each individual crystal remains unchanged.
  • Solids in general have higher density. Solids have higher density than liquids and much higher than that of gases. This is because the particles in solids are packed tightly due to strong intermolecular forces. Density is described as mass per unit volume.
  •  In solids, intermolecular forces are strong. In solids, the con particles are held closely in fixed positions due to a strong force of attraction. These particles, however, can rotate and/or vibrate about their mean positions. Thus, the particles in solids possess very little kinetic energy.
  •  Diffusion of a solid into another solid is extremely slow. The particles in solids do not move much from their fixed positions. Therefore, diffusion of one solid into another is extremely slow.
  • Solids have high melting points. Due to strong cohesive or intermolecular forces, solids have high melting points.

The Liquid State

What are the properties of liquids?

Liquids show the following general properties

 • Liquids have no fixed (or definite) shape. Liquid, the container they are poured into, because in liquids, thy’ particles can move, over short distances, slip and slide past each and readjust their positions.

• A liquid has a definite volume. In liquids, the cohesive forces are strong enough to keep the constituent particles together.

• Liquids have lower density than solids. In liquids, particles are packed loosely. So, liquids are less dense. There are a few exceptions. The most common example is that of water. Solid water (ice) floats on water. This is because ice is lighter than water.

 • Intermolecular forces (or cohesive forces) in liquids are weaker than those in solids. It is due to this that intermolecular space in liquids is greater than that in solids.

• Liquids are slightly more compressible than solids. Due to greater intermolecular space, liquids are more compressible than solids.

• Liquids diffuse more easily than solids. The particles in liquids move faster and more freely than those in solids. So, the particles in liquids have more kinetic energy than those in solids. As a result, diffusion in liquids is faster than in solids. The diffusion in liquids depends upon the thickness (called viscosity) of the liquids.

 • Liquids flow from higher level to lower level. In liquids, the particles are slightly loosely packed. As a result, the particles can move, slip and slide past each other.

The Gaseous State

 What are the properties of gases?

 In gases, the constituent particles are far away from each other. The cohesive force in gases is negligible. As a result, the constituent particles in gases are free to move in any direction with high speeds.

• Gases have no definite shape and fixed volume. Gases fill the entire space available to them. This is because in gases, the cohesive force between the particles is negligible and these are free to move in any direction.

• Gases are highly compressible. In gases, the particles are far apart and there is enough space available for compression. Due to high compressibility, a large volume of any gas can be compressed into a cylinder of small volume, and transported easily.

 An LPG cylinder contains the petroleum gas under high pressure. The CNG we use in automobiles is highly compressed natural gas. The natural gas is mainly methane.

• Gases have very low densities. As compared to solids and liquids, gases have very low densities (or mass-to-volume ratio). This is because there is a lot of empty space in gases, that is, intermolecular space in gases is very large.

 • Gases diffuse into each other rapidly. This is because the particles in gases can freely move in all directions with high speeds, and there is a lot of intermolecular space in gases. The speed of particles in any form of matter increases with a rise in temperature. That is why, diffusion becomes faster at higher temperatures. You can smell the flavour of the food being cooked in your neighbourhood due to diffusion.

• Gases can easily flow from high pressure to low pressure side.

• Gases exert pressure on the walls of the container. The particles in gases move in all directions at very high speeds. The particles in gases collide with each other and with the walls of the container. When these particles strike the wall, they exert force on it. The force exerted per unit area of the wall is called pressure exerted by the gas. Gases exert equal pressure in all directions.

• The fourth state of matter is called plasma. T plasma state than in the other three states — solid, liquid and gases taken together, in the universe.

• Plasma can occur when matter is heated to very high te At such high temperatures, one or more electrons are freed from the attraction of the nucleus. The matter in plasma state is a collection of free highly energetic and highly excited, electrically charged particles: the negatively charged electrons and the positively charged ions. Such an ionised gas with equal number of positive and negative charges is called plasma.

 • Plasma state exists in stars and the sun due to very high temperature there.

 ■ Fluorescent tubes and neon tubes consist of plasma. The fluorescent tube is filled with argon and mercury vapour. Neon tubes are filled with gas mixtures containing neon. The gas inside the tubes gets ionised when electricity flows through it, and starts glowing.

Change of State

What is meant by a change of state?

Matter can exist in three physical states, viz., solid, liquid and vapour (or gaseous) states.

For example, water can exist in three different physical states: solid (ice), liquid (water), and gas (water vapour).

Change of a substance from one physical state to another is called change of state.

For example, conversion opt ice into liquid water is a change of state (from solid to liquid).

The physical state of any substance at room temperature is considered common form. For exami.7,1e, the common form of water is liquid.

 How can matter change its state?

 The physical state of any matter can be changed by any of the following methods:

• By changing temperature.

 • By changing pressure.

 • By changing pressure and temperature both.

We describe these in the following sections.

Changing the physical state from solid to liquid by changing temperature

 It is our common experience that on heating ice gets converted to liquid water and the liquid water on further heating gets converted into steam.

On the other hand, water vapour when cooled gives water (liquid), which on further cooling gives ice (solid).

The transformation of various physical states of water may be described as follows:

Let us perform an experiment and observe the changes in any form of the matter, e.g., solid on heating.

How can the solid à liquid à gas, transformation be explained

 In solids, the intermolecular forces are very strong, and the constituent particles are closely packed. On heating, the kinetic energy of the particles increases. At a stage, it becomes sufficient to overcome the binding forces in the solids. At this stage, solid starts melting and the solid gets converted into liquid. On further heating, the particles in liquid gain more kinetic energy. At a stage, the kinetic energy of the particles becomes so large that the particles are able to free themselves, and the liquid starts changing into vapour (the gaseous state). In gases, the particles are far away from each other, and the particles do not exert any attractive /repulsive force on each other.


Changing the physical state from solid to vapour by raising temperature

 Certain substances like naphthalene, indigo, iodine and camphor on heating get converted to vapour without passing through the liquid state. The vapour on cooling give crystals of the pure solid compound.

• The process by which a solid on heating gets converted to vapour state without passing through the liquid state is called sublimation.

• The process (or step) by which the vapour of a substance gets converted into solid without passing through the liquid state is called desublimation or vapour deposition.

 • So obtained deposited crystalline material is called sublimate. Sublimation can be described by the equation.

 The technique of sublimation is employed for separating (or purifying} solid substances which sublime on heating from the non-volatile substances.

Changing the physical state by changing pressure

 The physical state can also be changed by changing pressure. For example, a liquid can be vaporised by lowering pressure on it. The LPG we get in the gas cylinder is actually in the liquid form under high pressure. When the regulator/valve is opened, pressure gets lowered and the LPG comes out in the gaseous form.

• Almost all gases can be liquefied by lowering temperature and increasing pressure simultaneously.

Air is liquefied by lowering temperature and increasing pressure simultaneously.

What are melting and melting point?

 • The process in which a solid, on heating, changes into a liquid is called mating or fusion.

Conversion of ice into water is the melting (or fusion) of ice.

• The temperature at which a solid substance gets converted into a liquid is called the Melting point. It is also known as the melting temperature.

The process in which a liquid substance on cooling changes into solid is called freezing or solidification. For example, conversion of water to ice is called freezing or solidification of water.

The temperature at which a liquid gets converted into a solid is called freezing temperature (or freezing point). The melting_ points of some common substances are given in following table.

SubstanceMelting Point (K)

What are boiling and boiling point?

Rapid formation and breaking of bubbles in the bulk of a liquid being heated is called boiling. For example, water on boiling gives steam

Boiling is a bulk phenomenon. During boiling particles from the bulk of the liquid gain enough energy to get converted to vapour.

Boiling of a liquid occurs at a fixed temperature depending on the nature of the liquid. Me fixed temperature at which a liquid boil is called its boiling temperature or boiling point.

The boiling occurs when the pressure exerted by the vapour of the liquid becomes equal to the external or atmospheric pressure.

The boiling point of a liquid under 1 atmospheric pressure is called normal boiling point. Under 1 atmospheric pressure, water boils at 1000 C.

So, Normal boiling point of water = 1000 C = 373.15K

The boiling points of some common liquids
SubstanceBoiling Point (K)
Petroleum ether307.5
Ethyl alcohol351.5

What are vaporisation and condensation?

 • The process in which a liquid on heating changes into vapour is called vaporisation. For example The conversion of water into its vapour (steam) by heating at its boiling temperature is termed as vaporisation of water.

• The process in which vapour, on cooling, gets converted into a liquid is called liquefaction or condensation. Condensation is the reverse of vaporisation. For example, the conversion of water vapour into liquid water is called the condensation of water vapour.

♦ The fixed temperature at which vapour gets converted into liquid is called condensation temperature.

The boiling point (a vaporisation temperature) and condensation temperature of a pure substance are equal.

Why does temperature remain constant during melting and boiling?

The temperature remains constant during melting of ice and also during boiling (or vaporisation) of water even when heating is continued.

Why does the temperature of a substance remain constant during melting?

and boiling even when heat is being supplied to it continuously?

This is because the heat supplied to the substance is used up (absorbed) in overcoming the intermolecular forces, and therefore, it does not show up as a rise in the temperature.

 Thus, the heat supplied during melting and boiling (or vaporisation) remains hidden from the thermometer and is called latent heat. Depending upon the nature of the process, the kinds of latent heat are:

• Latent heat of fusion (or Latent heat of melting)

■ Latent heat of vaporisation (or Latent heat of boiling) These latent heats are described as follows.

What is the latent heat of fusion?

The latent heat of fusion or the latent heat of melting is defined as follows

The latent heat of fusion of a substance is the quantity of heat required for

Surface. On reaching there, it evaporates. The heat required for evaporation is taken from the earthen pot and from the water in it. As a result, the water m an earthen pot gets cooled down.

Why do we feel relief under a fan when we perspire?

The air from the fan causes rapid evaporation of sweat. During evaporation, the sweat takes away heat from the body. As a result, we feel relief under a fan.

Why do trees acquire more leaves during summer Evaporation of water from the leaves is called transpiration?

Transpiration helps plants/trees to keep cool. During summer, when the temperature is high, a tree must transpire more to keep itself cool. More transpiration requires more leaves. So, trees acquire more leaves during summers.

Why does the air passing through a desert cooler become cooler?

In a desert cooler, hot and dry air passes through wet pads of wood-shavings. Water takes heat from the hot air and evaporates. The evaporation of water cools the pads, and the circulating water too. As a result, the incoming air also gets cooled down.

Water Vapour in the air

Air always contains some water vapour in it. This water vapour present in the air comes from the evaporation of water in rivers, lakes and oceans, and also from many biological activities such as respiration etc. The amount of water vapour in the air, however, varies considerably from time to time and from place to place.

What is meant by humidity and absolute humidity?

The amount of water vapour in the air is expressed in terms of humidity. The humidity of air is defined as the quantity of water vapour present in one unit volume of the air. The humidity of air is generally measured in the units of gram per cubic metre (g/m3).

The maximum limit of the water vapour which could be present in one cubic metre of air at any temperature is called the absolute humidity or saturation value.

Absolute humidity values of air at various temperatures

What is meant by relative humidity?

The actual amount of water vapour present in air is usually lower than the absolute humidity (or saturation value) of the air at the same temperature. The wetness of air is generally expressed in terms of its relative humidity (denoted by RH).

If m is the actual amount of water vapour present in a certain volume of air, and rn, is the amount of water vapour required to saturate the same volume of air at the same temperature, then relative humidity can be expressed as,

Thus, the percentage of the amount of water vapour actually present in a certain volume of the air to the amount of water vapour needed to saturate it, is called its relative humidity.

If the actual amount of water vapour present in a certain volume of the air is equal to the amount of water vapour needed to saturate the same volume of the air at the same temperature, then the relative humidity of air is 100%. Mathematically, when


The 100% relative humidity means that the air is fully saturated with water vapour. Therefore, when the relative humidity is 100%, then no evaporation is possible.

 During the rainy season and in the coastal areas the relative humidity is generally high, while during winter, in deserts and in extremely cold places the humidity is low. Both low and high relative humidity make us uncomfortable.

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