# 1. Matter in our surroundings | Class 9 Science | PDF and Web notes

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Everything in the universe is made up of matter.

Matters occupy space and have mass. i.e., they have

·    Mass: SI unit of mass is kilogram (kg).

·    Volume: SI unit is cubic metre (m3).

The common unit of measuring volume is litre (L).

1L = 1 dm3, 1L = 1000 mL, 1 mL = 1 cm3.

Early Indian philosophers classified matter as five basic elements (Panch Tatva) - air, earth, fire, sky & water.

Ancient Greek philosophers had a similar classification.

In modern science, there are 2 types of classification of matter based on physical properties & chemical nature.

# PHYSICAL NATURE OF MATTER

MATTER IS MADE UP OF PARTICLES

There were two schools of thought regarding the nature of matter.

1.   Matter is continuous like a block of wood.

2. Matter is made up of particles like sand (particulate).

Experiment to show the nature of matter:

Fill a 100 ml beaker halfway with water.

Dissolve some salt/ sugar.

The water level does not change.

Salt/ sugar particles break and spread throughout water filling the spaces between water particles.

It shows that matter is made up of particles.

HOW SMALL ARE THESE PARTICLES OF MATTER?

The particles of matter are very small.

It can be proven by following experiment:

Dissolve 2–3 crystals of potassium permanganate (KMnO4) in 100 mL of water.

Transfer 10 mL of this solution into 90 mL of clear water.

Transfer 10 mL of this solution into another 90 mL of clear water.

Keep diluting the solution 5 to 8 times.

The water is still coloured (light colour).

-    It shows that a few crystals of KMnO4 can colour a large volume of water (about 1000 L). i.e., each KMnO4 crystal contains millions of tiny particles. It divides into smaller and smaller particles.

-    This activity can be done using 2 ml of Dettol. The smell can be detected even on repeated dilution.

# CHARACTERISTICS OF PARTICLES OF MATTER

PARTICLES OF MATTER HAVE SPACE BETWEEN THEM

-    Particles of sugar, salt, Dettol, or KMnO4 evenly distribute in water.

-  In tea, coffee or lemonade, particles of one type of matter get into spaces between particles of the other.

-    It shows that there is space between particles of matter.

PARTICLES OF MATTER ARE CONTINUOUSLY MOVING

Experiment 1:

An unlit incense stick can be smelled only if we go very close to it.

When it is lit, the smell can be felt from a distance.

Experiment 2:

Take two glasses/beakers filled with water.

Put a drop of blue or red ink slowly along the sides of the first beaker and honey in the second beaker.

The ink spreads faster evenly throughout the water within minutes. For honey, it takes hours.

Experiment 3:

Drop a crystal of copper sulphate or KMnO4 into a glass of hot water and another containing cold water. Do not stir the solution.

In cold water, the crystals settle at the bottom. In hot water, it slowly spreads showing blue/ purple colour.

As time passes, the hot water turns blue/purple. But in cold water, there is no significant color change.

The above experiments show that:

o  Particles of matter are continuously moving, i.e., they possess kinetic energy. As the temperature rises, kinetic energy increases (particles move faster).

o  Particles can intermix by getting into the spaces between them. Intermixing of particles of two different types of matter is called diffusion. On heating, diffusion becomes faster due to increase in kinetic energy.

PARTICLES OF MATTER ATTRACT EACH OTHER

-    Particles of matter are held together by force of attraction. Strength of force of attraction varies in different types of matter. E.g.,

Breaking an iron nail, a piece of chalk, and a rubber band. Iron nails are difficult to break due to greater force of attraction between iron particles.

Water has a weak attraction force than solids. So it is easy to cut the water using fingers. When fingers are withdrawn it joins again.

# STATES OF MATTER

Matter exists in 3 states: solid, liquid and gas.

1. THE SOLID STATE

-    Solids have a definite shape, distinct boundaries and fixed volumes. Show negligible compressibility.

-    E.g., pen, book, needle, wooden stick etc.

-  Solids maintain their shape when subjected to outside force. They may break under force but changing shape is difficult. So they are rigid.

-    A rubber band changes shape under force and regains the same shape when the force is removed. If excessive force is applied, it breaks. So rubber band is a solid.

-    When sugar and salt kept in different jars, they take the shape of the jar. But these are solids because each sugar or salt crystal has fixed shape.

-    Sponges are also solid. But it can be compressed due to the presence of air-filled holes. When it is pressed, the air is expelled out.

2. THE LIQUID STATE

-    Liquids have no fixed shape but have a fixed volume. They take up the shape of the container in which they are kept. Liquids flow and change shape, so they are not rigid but is called fluid.

-    E.g., water, cooking oil, milk, juice, cold drinks etc.

-    Solids, liquids and gases can diffuse into liquids. The atmospheric gases diffuse and dissolve in water.

-    Rate of diffusion in liquids is higher than in solids because liquid particles move more freely and have more space between them compared to solid particles.

3. THE GASEOUS STATE

Gases are highly compressible as compared to solids &

liquids. It can be shown by the following experiment.

Take three 100 mL syringes and close their nozzles by rubber corks.

Remove the pistons of all syringes.

Leave one syringe empty, fill the second with water, and add pieces of chalk to the third.

Apply some Vaseline on the pistons (for smooth movement) and insert into the syringes.

Try to compress the content by pushing the piston in each syringe.

The piston in the syringe with air can be easily pushed because gas is highly compressible.

-    Due to high compressibility, large volumes of a gas can be transported in a small cylinder. E.g.,

·    Liquefied petroleum gas (LPG) cylinder.

·    Oxygen cylinder in hospitals.

·    Compressed natural gas (CNG) is used as fuel in vehicles.

·    Helium gas cylinder is used to fill balloons.

-    The rate of diffusion of gases is higher than that of solids & liquids. It is due to high speed of particles and large space between them. By this, gases diffuse very fast into other gases. E.g., particles of the aroma of food mix with air particles and reach us quickly.

-    In gaseous state, the particles move randomly at high speed. As a result, the particles hit each other and the walls of the container. The pressure of the gas is due to this force exerted by gas particles per unit area on the walls of the container.

Motion of the particles in the three states of matter

# CAN MATTER CHANGE ITS STATE?

-    Water can exist in three states of matter: solid (ice), liquid (water) and gas (water vapour).

-    Temperature and pressure determine the state of a substance.

EFFECT OF CHANGE OF TEMPERATURE

Experiment:

Take about 150 g of ice in a beaker and suspend a thermometer so that its bulb is in contact with the ice.

Heat the beaker on a low flame.

The temperature when the ice starts melting is 0° C.

The temperature when all the ice has converted into water (liquid state) is still 0° C.

Now stir the water with a glass rod till the water starts boiling and vapourising (gaseous state). Temperature increases and reaches 100° C.

Conversion of ice to water

Conversion of water to water vapour

-    On increasing the temperature of solids, kinetic energy of the particles increases. As a result, the particles vibrate faster. The energy supplied by heat overcomes the forces of attraction between the particles allowing them to move freely. Thus the solid state changes into liquid state. It is called melting (fusion).

-    The minimum temperature at which a solid melt to become a liquid at the atmospheric pressure is called its melting point. It indicates the strength of the force of attraction between its particles.

-    The melting point of ice is 273.15 K.

Kelvin: SI unit of temperature. 0° C =273.15 K (≈ 273 K).

Kelvin to Celsius: Subtract 273 from the given temperature.

Celsius to Kelvin: Add 273 to the given temperature.

-    During the melting of a solid (e.g., ice), its temperature does not change after the melting point is reached, till all the solid melts.

-    Ice (solid) absorbs heat energy without increasing its temperature, which is used to change the state by overcoming the forces of attraction between particles. This is called latent heat.

-    The heat energy needed to change 1 kg of a solid into liquid at atmospheric pressure at its melting point is called the latent heat of fusion.

-    So, particles in water at 0° C (273 K) have more energy as compared to particles in ice at the same temperature.

-    When heat energy is supplied to water, the particles move faster. At a certain temperature, they gain enough energy to overcome the forces of attraction and the liquid starts changing into gas. The temperature at which a liquid starts boiling at the atmospheric pressure is called its boiling point.

-    Boiling is a bulk phenomenon. Particles from the bulk of the liquid gain enough energy to change into vapour.

-    Boiling point of water is 373 K (100°C).

-    The heat energy needed to change 1 kg of a liquid into gas at atmospheric pressure at its boiling point is called the latent heat of vaporisation.

-    Particles in steam (water vapour) at 373 K have more energy than water at the same temperature because the steam particles have absorbed extra energy as latent heat of vaporization.

Sublimation and Deposition

-  The direct change of solid to gas without changing into liquid state is called sublimation. E.g., camphor.

-    The direct change of gas to solid without changing into liquid is called deposition. E.g., camphor.

Experiment:

Crush some camphor and put it in a China dish.

Put an inverted funnel over the China dish.

Put a cotton plug on the stem of the funnel.

Heat slowly. Camphor changes directly from solid to gas. This gas condenses on the upper part of the funnel.

Sublimation of camphor

EFFECT OF CHANGE OF PRESSURE

-    Difference in states of matter is due to the difference in the distances between the particles.

-    Change in pressure can change the state of matter.

-    When pressure is exerted and compress a gas in a cylinder, the particles come closer. Gases liquefy on applying pressure and reducing temperature.

-  Solid carbon dioxide (CO2) is stored under high pressure. When pressure is decreased to 1 atmosphere, it transforms into gaseous state without coming into liquid state. So it is also known as dry ice.

Atmosphere (atm) is a unit of measuring pressure exerted by a gas.

1 atmosphere = 1.01 × 105 Pascal (Pa - unit of pressure).

The atmospheric pressure (pressure of air in atmosphere) at sea level is 1 atmosphere (normal atmospheric pressure).

Interconversion of the three states of matter

# EVAPORATION

-    It is the change of a liquid into vapour at any temperature below its boiling point (i.e., without reaching the boiling point). E.g., Water, when exposed, slowly changes into vapour, wet clothes dry up etc.

-    Particles of matter are always moving. In liquids, some surface particles with higher kinetic energy can overcome the forces of attraction and escape as vapour.

FACTORS AFFECTING EVAPORATION

Surface area: Evaporation is a surface phenomenon. Increasing the surface area increases the rate of evaporation. E.g.,

o  Water taken in an open China dish evaporates faster than in a test tube.

o  Spreading wet clothes helps to dry up fast.

Temperature: As temperature increases, more particles get enough kinetic energy increasing evaporation.

Humidity: Humidity is the amount of water vapour present in air. The air can only hold a certain amount of water vapor at a given temperature. High humidity decreases the rate of evaporation.

Wind speed (velocity): As the wind speed increases, the particles of water vapour move away with the wind. It decreases the amount of surrounding water vapour. That’s why clothes dry faster on a windy day.

HOW DOES EVAPORATION CAUSE COOLING?

-    In an open vessel, the liquid keeps on evaporating.

-    The liquid particles absorb energy from the surrounding to regain the energy lost during evaporation. This makes the surroundings cold.

-    When acetone (nail polish remover) is poured on palm, the particles gain energy from the palm or surroundings and evaporate. It causes the palm to feel cool.

-    After a hot 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?

-    In summer, we perspire more to keep the body cool.

-   During evaporation, the particles at the surface of the sweat 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 is a good absorber of water. So it absorbs sweat and exposes 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 water vapor in the air contacts a cold glass of water, it loses energy and condenses into liquid droplets.

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