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## Chapter: Matter in Our Surroundings

Earlier, matter was said to be composed of atoms. In later stages, phenomena such as sound, light were included in it. This concept of matter may be widespread from atoms to any such objects which possess mass even when the object is at rest. There is no standard universal definition for matter and it is not a fundamental concept in physics now. Matter is also defined as the substance that holds all observable physical objects.

All the objects whichever we see, touch, feel are composed of atoms. This atomic matter consists of interacting sub-atomic particles, which are generally a nucleus of protons and neutrons along with a group of orbiting electrons. Normally, science considers these composite particles as matter because they are said to have both rest mass and volume. But particles such as photons, which have no mass are mass are not termed as matter as they don’t have mass or volume at rest.

## Matter is Made Up of Particles

Matter is made up of particles like sand. Let us perform an activity to decide about the nature of matter – is it continuous or particulate?

When we dissolve salt in water, the particles of salt get into the spaces between the particles of water.

## Characteristics of Particles of Matter

Similar particles of one type of matter move into the gaps between the particles of the other. Hence it proves that there is no space left between the particles of matter. These particles of matter are moving continuously, so we can say that they contain an energy called kinetic energy. In addition, we can notice the particles moving faster when the temperature of these particles increases. So we can conclude that the kinetic energy of the particles increases as the temperature increases. Also, the inter-mixing of particles occurs on its own by moving into the spaces between the particles. This property of inter-mixing is called diffusion. Diffusion occurs between two different types of matter on their own. Diffusion becomes faster when the particles are heated.

## States of Matter

Matter around us exists in three different states– solid, liquid and gas. These states of matter come up due to the distinction in the characteristics of the particles of matter.

### 1. Solid State

The particles are closely packed in this state. The forces between the particles are so strong that the particles can only vibrate and cannot move freely. Because of this, a solid has a definite shape, definite volume and it is termed to be stable. The deformation of solids occur only due to the applying of force or if it is broken or cut.

### 2. Liquid State

A liquid is defined as an incompressible fluid that takes the shape of its container but retains a constant volume which doesn’t depend on pressure. If the pressure and temperature are constant, there is said to be a definite volume.

Liquid is obtained by heating a solid above its melting point and on condition that the pressure is higher than the triple point of substance. The molecules have adequate energy to move relatively close to each other and thus the structure is mobile. It concludes that the liquid does not have any definite shape and it takes the shape of its container.

### 3. Gaseous State

A gas is a compressible fluid. The gas will take the shape of the container and in addition, it will expand to fill the whole container. The molecules of gas have so much kinetic energy that the effect of inter-molecular forces is small. A gas has no definite shape or volume.

## Can Matter Change its State?

We all know from our observation that water can exist in three states of matter–

i- Solid, as ice.

ii- Liquid, as the familiar water.

iii- Gas, as water vapor.

## Effect of Change of Temperature

A gradual increase in the temperature of solids leads to the increase in kinetic energy. Because of this increase in kinetic energy, the particles start vibrating with greater speed and we can see that the particles no more stay in their fixed positions and they start moving freely. This happens as the forces of attraction between the particles get overcome by the temperature rise. After a stage is reached, solid slowly converts into liquid. This temperature at which a solid melts and converts into a liquid is called the melting point. This happens at atmospheric pressure.

Now let us provide heat energy to water and see what happens? We observe that the particles start moving even faster. After some time at a particular temperature, a certain point is reached where the particles have sufficient energy to breakup from the attractive forces of each other. Now the liquid starts changing into gas at this temperature. This is called as boiling and the boiling point is defined as the temperature at which liquid is converted into gas i.e. liquid starts boiling. This also occurs at an atmospheric pressure. Boiling is termed to be a bulk phenomenon. Liquid particles gain adequate energy to convert into vapor state.

## Effect of Change of Pressure

Liquefaction of gases happens due to the applying of pressure and by reducing the temperature. Have you ever heard about what solid carbon dioxide is? It is actually preserved under high pressure. It is directly converted into gas without undergoing the liquid state. This process happens by decreasing pressure to 1 atmosphere. Solid carbon dioxide is also called as dry ice. Pressure and temperature are the two vital parameters to determine any substance and its state (solid, liquid or gas).

## Evaporation

From the above mentioned case of change of pressure, we need to think if we always have to heat or change pressure for changing the state of matter?
Let us quote some examples from everyday life in which liquid directly converts to vapor without reaching the boiling point. Water, when left uncovered, gradually changes into vapor. Wet clothes dry up when they are kept at rest. Have you ever thought what exactly happens to water in the above two examples?

We can see that the matter particles are always moving and are never said to be at rest. There are some particles with different amounts of kinetic energy at a given temperature in any solid, liquid or gas forms. In liquids, some part of particles which have higher kinetic energy are able to break up the attractive forces of the other particles and thus gets converted into vapor. This is called as evaporation. Evaporation is defined as the process of converting liquids into vapors at any temperature below the boiling point.

## Factors Affecting Evaporation

The rate of evaporation increases with-

### •  An increase of surface area:

We know that evaporation is a surface phenomenon. If the surface area is increased, the rate of evaporation increases. For example, while putting clothes for drying up, we spread them out.

### •  An increase of temperature:

With the increase of temperature, more number of particles gets enough kinetic energy to go into the vapour state.

### •  A decrease in humidity:

Humidity is the amount of water vapour present in the air. The air around us cannot hold more than a definite amount of water vapour at a given temperature. If the amount of water in the air is already high, the rate of evaporation decreases.

### •  An increase in wind speed:

You must have seen that clothes dry faster on a windy day. With the increase in the wind speed, the particles of water vapour move away with the wind, decreasing the amount of water vapour in the surrounding.