CLASS 9 Science
CHAPTER 1
Matter In Our Surroundings (NCERT Notes)
CHAPTER 1
Matter In Our Surroundings (NCERT Notes)
Everything in this universe is made up of material that have both mass , volume and occupied space is called matter. The SI unit of mass is kilogram (kg).
Physical Nature of Matter :-
Matter is made up of small particles.
The particles of matter are very small they are small beyond our imagination.
How small are the particle of matter is made up of.
Characteristics of Particles of Matter :-
Particle of matter have space between them . For example sugar solution. In water solution sugar is completely dissolved in it. Another example When we dissolve salt in water, the particles of salt get into the spaces between particles of water.
Particles of matter are continuously moving or show random motion.
Particles of matter are continuously moving or show random motion. That is, they possess the kinetic energy. As the temperature rises, particles move faster. So, we can say that with increase in temperature the kinetic energy of the particles also increases.
Diffusion is defined as the movement of substance from area of high concentration to area of low concentration. As the temperature increases the diffusion becomes faster.
Particle of matter attract each other.
For example steching of a rubber. The particles of matter have force acting between them. This force keeps the particles together. The strength of this force of attraction varies from one kind of matter to another.
States of Matter
SOLID STATE
Solid have a definite shape, distinct boundaries and fixed volumes, that is, have negligible compressibility. Solids have a tendency to maintain their shape when subjected to outside force. Solids may break under force but it is difficult to change their shape, so they are rigid.
For example • A rubber band changes shape under force and regains the same shape when the force is removed. If excessive force is applied, it breaks.
• The shape of each individual sugar or salt crystal remains fixed, whether we take it in our hand, put it in a plate or in a jar.
• A sponge has minute holes, in which air is trapped, when we press it, the air is expelled out and we are able to compress it.
LIQUID STATE
Liquids does not have fixed shape but have a fixed volume. They take up the shape of the container in which they are kept. Liquids f low and change shape, so they are not rigid but can be called fluid.
Solids and liquids can diffuse into liquids. The gases from the atmosphere diffuse and dissolve in water. These gases, especially oxygen and carbon dioxide, are essential for the survival of aquatic animals and plants.
All living creatures need to breathe for survival. The aquatic animals can breathe under water due to the presence of dissolved oxygen in water. Thus, we may conclude that solids, liquids and gases can
diffuse into liquids. The rate of diffusion of liquids is higher than that of solids. This is due to the fact that in the liquid state, particles move freely and have greater space between each other as compared to
particles in the solid state.
GASEOUS STATE
Gases are highly compressible as compared to solids and liquids. The liquefied petroleum gas (LPG) cylinder that we get in our home for cooking or the oxygen supplied to hospitals in cylinders is compressed gas. Compressed natural gas (CNG) is used as fuel these days in vehicles. Due to its high compressibility, large volumes of a gas can be compressed into a small cylinder and transported easily.
Cooked in the kitchen without even entering there, by the smell that reaches our nostrils or smell reach us .Because the particles of the aroma of food mix with the particles of air spread from the kitchen, reach us and even farther away. The smell of hot cooked food reaches us in seconds; compare this with the rate of diffusion of solids and liquids. Due to high speed of particles and large space between them, gases show the property of diffusing very fast into other gases.
In the gaseous state, the particles move randomly at high speed. Due to this random movement, the particles hit each other and also the walls of the container. The pressure exerted by the gas is because of this force exerted by gas particles per unit area on the walls of the container.
Matter Change its State
Water can exist in three states of matter–
• Solid, as ice,
• Liquid, as the familiar water, and
• Gas, as water vapour.
EFFECT OF CHANGE OF TEMPERATURE
On increasing the temperature of solids, the kinetic energy of t he particles increases. Due to the increase in kinetic energy, the particles s tart vibrating with greater speed. The energy supplied by heat overcomes the forces of attraction between the particles. The particles leave their fixed positions
and start moving more freely. A stage is reached when the solid melts and is converted to a liquid.
The minimum temperature at which a solid melts to become a liquid at t he atmospheric pressure is called its melting point. The melting point of ice is 273.15 K*. The process of melting, that is, change of solid state into liquid state is also known as fusion.
When a solid melts, its temperature remains the same, so where does the heat energy go?
During the melting, that the temperature of the system does not change after the melting point is reached, till all the ice melts. This happens even though we continue to heat the beaker, that is, we continue to supply heat. This heat gets used up in changing the state by overcoming the forces of
attraction between the particles. As this heat energy is absorbed by ice without showing any rise in temperature, it is considered that it gets hidden into the contents of the beaker and is known as the latent heat. The amount of heat energy that is required to change 1 kg of a solid into liquid at atmospheric pressure at its melting point is known as the latent heat of fusion. So, particles in water at 00 C (273 K) have more energy as compared to particles in ice at the same temperature.
When we supply heat energy to water, particles start moving even faster. At a certain temperature, a point is reached w hen the particles have enough energy to break free from the forces of at traction of each other. At this temperature the liquid starts changing into gas. The temperature at which a liquid starts boiling at the atmospheric pressure is known as its boiling point. Boiling is a bulk phenomenon. Particles from the bulk of the liquid gain enough energy to change into the vapour state.
For water this temperature is 373 K (100 0C = 273 + 1 00 = 373 K).
Particles in steam, that is, water vapour at 373 K (1000 C) have more energy than water at the same temperature. This is because particles in steam have absorbed extra energy in the form of latent heat of
vaporisation.
Substances around us change state from solid to liquid and from liquid to gas on application of heat. But there are some that change directly from solid state to gaseous state and vice versa without changing into the liquid state.
A change of state directly from solid to gas without changing into liquid state is called sublimation and the direct change of gas to solid without changing into liquid is called deposition.
EFFECT OF CHANGE OF PRESSURE
Applying pressure and reducing temperature can liquefy gases.
Solid carbon dioxide (CO2 ) is stored under high pressure. Solid CO2 gets converted directly to gaseous state on decrease of pressure to 1 atmosphere* without coming into liquid state. This is the reason that solid carbon dioxide is also known as dry ice. Thus, we can say that pressure and temperature determine the state of a substance, whether it will be solid, liquid or gas.
* atmosphere (atm) is a unit of measuring pressure exerted by a gas. The unit of pressure is Pascal (Pa): 1 atmosphere = 1.01 × 105 Pa. The pressure of air in atmosphere is called atmospheric pressure. The atmospheric pressure at sea level is 1 atmosphere, and is taken as the normal atmospheric pressure.
Evaporation
The phenomenon of change of a liquid into vapours at any temperature below its boiling
point is called evaporation.
When water is left uncovered, slowly it changes into vapour. We know that particles of matter are
always moving and are never at rest. At a given temperature in any gas, liquid or solid, there are particles with different amounts of kinetic energy. In the case of liquids, a small fraction of particles at the surface, having higher kinetic energy, is able to break away from the forces of attraction of other particles and gets converted into vapour.
FACTORS AFFECTING EVAPORATION•
• On 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 get enough kinetic energy to go into the vapour state.
• A decrease in humidity:
Humidity is the amount of water vapour present in 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 air is already high,
the rate of evaporation decreases.
• An increase in wind speed:
It is a common observation that clothes dry faster on a windy day. With the increase in wind speed, the particles of water vapour move away with the wind, decreasing the amount of water vapour in the surrounding.
HOW DOES EVAPORATION CAUSE COOLING?
In an open vessel, the liquid keeps on evaporating. The particles of liquid absorb energy from the surrounding to regain the energy lost during evaporation. This absorption of energy from the surroundings make the surroundings cold.
When you pour some acetone (nail polish remover) on your palm .It feel cooler because the particles gain energy from your palm or surroundings and evaporate causing the palm to feel cool.
After a hot sunny day, people sprinkle water on the roof or open ground because the large latent heat of vaporisation of water helps to cool the hot surface.
Why should we wear cotton clothes in summer?
During summer, we perspire more because of the mechanism of our body which keeps us cool. We know that during evaporation, the particles at the surface of the liquid gain energy from the surroundings or body surface and change into vapour. The heat energy equal to the latent heat of
vaporisation is absorbed from the body leaving the body cool. Cotton, being a good absorber of water helps in absorbing the sweat and exposing it to the atmosphere for easy evaporation.
Why do we see water droplets on the outer surface of a glass containing ice-cold water?
When we take some ice-cold water in a tumbler. Soon we will see water droplets on the outer surface of the tumbler. The water vapour present in air, on coming in contact with the cold glass of water, loses energy and gets converted to liquid state, which we see as water droplets.
Lets recall It :
•Matter is made up of small particles.
•The matter around us exists in three states— solid, liquid and gas.
•The forces of attraction between the particles are maximum in solids, intermediate in liquids and minimum in gases.
•The spaces in between the constituent particles and kinetic energy of the particles are minimum in the case of solids, intermediate in liquids and maximum in gases.
•The arrangement of particles is most ordered in the case of solids, in the case of liquids layers of particles can slip and slide over each other while for gases, there is no order, particles just move about randomly.
•The states of matter are inter-convertible. The state of matter can be changed by changing temperature or pressure.
•Sublimation is the change of solid state directly to gaseous state without going through liquid state.
•Deposition is the change of gaseous state directly to solid state without going through liquid state.
•Boiling is a bulk phenomenon. Particles from the bulk (whole) of the liquid change into vapour state.
•Evaporation is a surface phenomenon. Particles from the surface gain enough energy to overcome the forces of attraction present in the liquid and change into the vapour state.
•The rate of evaporation depends upon the surface area exposed to the atmosphere, the temperature, the humidity and the wind speed.
•Evaporation causes cooling.
•Latent heat of vaporisation is the heat energy required to change 1 kg of a liquid to gas at atmospheric pressure at its boiling point.
•Latent heat of fusion is the amount of heat energy required to change 1 kg of solid into liquid at its melting point.
(From NCERT Book)