Matter in Our Surroundings, class 9, chapter 1

 

Matter in Our Surroundings

Early Indian philosopher classified matter into five basic elements. The” panch tatva”- air, earth, fire, sky and water. According to them, livings as well as non living thing are made up of these elements.

Matter: anything which occupy space, have mass and can be felt by any of our senses is called matter.

• Everything that we can touch, see, hear, taste and also smell is matter.

Physical Nature of Matter

• A physical property is that aspect of the matter that can be observed or measured without changing its nature or composition.
• It is independent of the amount of matter present.
• Physical properties include appearance, colour, odour, density, texture, melting point, boiling point, solubility, etc.

 

1. Particles of matter have spaces between them

• This characteristic is one of the concepts behind the solubility of a substance in other substances. For example, on dissolving sugar in water, there is no rise in water level because the particles of sugar get into the interparticle spaces between the water particles.

Ex- Dissolving a solid in a liquid:

·         When a crystal of potassium permanganate is placed in a beaker of water, the water slowly turns purple on its own, even without stirring.

·         Both potassium permanganate crystal and water are made up of tiny particles.

·         When the potassium permanganate crystal is put in water, the purple colored particles of potassium permanganate spread throughout water making the whole water look purple.

·         Actually, on dissolving, the particles of potassium permanganate get into the spaces between the particles of water.

·         This shows that the particles have spaces between them and are continuously moving on their own.

2. Particles of matter are always in motion

• Particles of the matter show continuous random movements due to the kinetic energy they possess.
• A rise in temperature increases the kinetic energy of the particles, making them move more vigorously.

3. Particles of matter attract each other
In every substance, there is an interparticle force of attraction acting between the particles. To break a substance we need to overcome this force. The strength of the force differs from one substance to another.

Diffusion

When the particles of matter intermix on their own with each other, the phenomenon is called diffusion. For example, spreading of ink in water.

• During diffusion, the particles occupy the interparticle spaces.
• The rate of diffusion increases with increase in the temperature, due to increase in kinetic energy of the particles.
• Solids diffuse at a very slow rate. But, if the temperature of the solid is increased, then the rate of diffusion of the solid particles into air increases. This is due to an increase in the kinetic energy of solid particles. Hence, the smell of hot sizzling food reaches us even at a distance, but to get the smell from cold food we have to go close.

Mixing of two gases:

·         Fragrance of an incense stick (agarbatti) lightened in one corner of a room, spreads in the whole room quickly.

·         The particles of gases (or vapours) produced by burning the incense stick move rapidly in all directions and mix with the moving particles of air in the room

·         We can get the smell of perfume sitting several metres away because perfume contain volatile solvent and diffuse faster and can reach people sitting several metres away.

·         This also shows that the particles of matter are constantly moving.

Difference between three states of matter:

Property	Solid	Liquid	Gas
Shape and volume	Fixed shape and volume	No fixed shape but has volume	Neither definite shape nor volume
Energy	Lowest	Medium	Highest
Compressibility	Difficult	Nearly difficult	Easy
Arrangement of molecules	Regular and closely arranged	Random and little sparsely arranged	Random and more sparsely arranged
Fluidity	Cannot flow	Flows from higher to lower level	Flows in all directions
Movement	Negligible	Depends on interparticle attraction	Free, constant and random
Interparticle space	Very less	More	Large
Interparticle attraction	Maximum	Medium	Minimum
Density	Maximum	Medium	Minimum
Rate of diffusion	Negligible	It depends on interparticle attraction.	Maximum

Density: The mass per unit volume of a substance is called density.
(Density = mass/volume).

Rigidity: The tendency of a substance to retain/maintain their shape when subjected to outside force.
Compressibility: The matter has intermolecular space. The external force applied on the matter can bring these particles closer. This property is called compressibility. Gases and liquids are compressible.

Fluidity: The tendency of particles to flow is called fluidity. Liquids and gases flow.

Kinetic energy: The energy possessed by particles due to their motion is called kinetic energy. Molecules of gases vibrate randomly as they have maximum kinetic energy.

Effect of Change of Pressure:

 The physical state of matter can also be changed by increasing or decreasing the pressure to it. Gases can be liquefied by applying pressure and lowering temperature. On applying pressure, the particles of matter can be brought close together, it gets compressed, and when in addition to it we lower its temperature, it gets converted into liquid. So, we can also say that gases can be turned into liquids by compression and cooling.

For example: CO₂ gas can be solidified called Dry ice by applying pressure and lowering temperature.

The atmospheric gases are taken in a cylinder with piston fitted on it. By cooling and applying pressure on them, the gases can be liquefied.

Effect of change of temperature on state of matter

On increasing temperature, the kinetic energy of the particles of the matter increases and they begin to vibrate with a higher energy. Therefore, the interparticle force of attraction between the particles reduces and particles get detached from their position and begin to move freely.

• As a result, the state of matter begins to change.
• Solids undergo a phase change to form liquids.
• Similarly, liquids also undergo a phase change to form gases.

 Temperature remains constant during the change of state:

During the change of state of any matter heat is supplied to the substance. The molecules of this matter use heat to overcome the force of attraction between the particles, at this period of time, temperature remains constant. This extra heat is acquired by the molecules in the form of hidden heat called latent heat to change from one state of matter to the other state.

Melting point

The melting point of a solid is defined as the temperature at which solid melts to become liquid at the atmospheric pressure.

• At melting point, these two phases, i.e., solid and liquid are in equilibrium, i.e., at this point both solid state and liquid state exist simultaneously.

Boiling point

The boiling point of a liquid is defined as the temperature at which the vapour pressure of the liquid is equal to the atmospheric pressure.

Latent heat of fusion

It is the amount of heat energy that is required to change 1 kg of a solid into liquid at atmospheric pressure at its melting point.

Latent heat of vaporisation

It is the amount of heat energy that is required to change 1 kg of a liquid into gas at atmospheric pressure at its boiling point.

Sublimation

The transition of a substance directly from its solid phase to gaseous phase without changing into the liquid phase (or vice versa) is called sublimation.

Ice at 273 K more effective in cooling than water at the same temperature:

 Ice at 273 K will absorb heat energy or latent heat from the medium to overcome the fusion to become water. Hence the cooling effect of ice is more than the water at same temperature because water does not absorb this extra heat from the medium.

Steam produces more severe bums:

Steam at 100°C will produce more severe bums as extra heat is hidden in it called latent heat whereas the boiling water does not have this hidden heat.

Evaporation

The phenomenon by which molecules in liquid state undergo a spontaneous transition to the gaseous phase at any temperature below its boiling point is called evaporation.

For example, the gradual drying of damp clothes is caused by the evaporation of water to water vapour.

Factors affecting evaporation

• Temperature: The rate of evaporation increases with an increase in temperature.
• Surface area: The rate of evaporation increases with an increase in surface area.
• Humidity: The rate of evaporation decreases with an increase in humidity.
• Wind speed: The rate of evaporation increases with an increase in wind speed.

Cooling due to evaporation

During evaporation, the particles of a liquid absorb energy from the surroundings to overcome the inter-particle forces of attraction and undergo the phase change. The absorption of heat from the surrounding makes the surrounding cool.
For example, sweating cools down our body.

Desert cooler cool better on a hot dry day:

The outer walls of the cooler get sprinkled by water constantly. This water evaporates due to hot dry weather. Evaporation causes cooling of inside air of cooler. This cool air is sent in the room by the fan.

Water kept in an earthen pot (matka) becomes cool during summer:

The earthen pot is porous with lot of pores on it, the water oozes out through these pores and the water gets evaporated at the surface of the pot thereby causing cooling effect. This makes the pot cold and the water inside the pot cools by this process.

Palm feels cold when we put some acetone or petrol or perfume on it:

Acetone, petrol or perfume evaporates when they come into contact with air. The evaporation causes cooling sensation in our hands.

We able to sip hot tea or milk faster from a saucer rather than a cup

Tea in a saucer has larger surface area than in a cup. The rate of evaporation is faster with increased surface area. The cooling of tea in saucer takes place sooner than in a cup. Hence we are able to sip hot tea or milk faster from a saucer rather than a cup.

Clothes should we wear in summer

We should wear light coloured cotton clothes in summer. Light colour because it reflects heat. Cotton clothes because it has pores in it, which absorbs sweat and allows the sweat to evaporate faster thereby giving cooling effect.