Matter in Our Surroundings

Everything around us — the air we breathe, the water we drink, the food we eat, a stone, a chair, even a tiny pin — is called matter. Matter is anything that has mass and occupies space (has volume). Things like sunlight, heat, or a feeling of happiness are not matter because they do not have mass and do not occupy space in this way — they are forms of energy or sensations, not matter.

A simple activity helps us understand that matter is made of very small particles. If we dissolve two or three crystals of potassium permanganate in a beaker of water, the whole beaker of water turns pink or purple. If we keep dividing the potassium permanganate crystals into smaller and smaller amounts and repeat the activity, the water still gets coloured, showing that a single crystal itself contains an extremely large number of very small particles. Similarly, the smell of an incense stick (agarbatti) or perfume placed at one corner of a room spreads and can be sensed even far away, because tiny particles of the scent travel through the air and mix with it.

These particles are so small that they cannot be seen even with a powerful ordinary microscope. Scientists use very advanced instruments to observe and even move individual atoms and particles, confirming that all matter — solid, liquid or gas — is built from these tiny building blocks.

Careful observation of activities like dissolving sugar in water, or the spreading smell of a perfume, tells us three important things about the particles that make up matter.

1. Particles of matter have space between them. When sugar or salt dissolves in water, it seems to disappear, but its particles actually occupy the tiny spaces present between the particles of water. This is why a glass filled to the brim with water can still dissolve a spoonful of sugar without overflowing.

2. Particles of matter are continuously moving. This constant, random movement gives the particles kinetic energy — the energy a particle has because it is in motion — and this kinetic energy increases as temperature rises. Because particles keep moving, particles of one substance can mix on their own with particles of another substance — this natural mixing is called diffusion. For example, the smell of hot food cooking in the kitchen reaches other rooms because particles of the aroma diffuse through the air. As temperature increases, particles move faster, so diffusion also becomes faster — this is why the smell of hot food spreads more quickly than that of cold food.

3. Particles of matter attract each other. A force of attraction exists between particles, which keeps them bound together to some extent. This force is different for different substances and is different in the solid, liquid and gaseous states of the same substance — being strongest in solids and weakest in gases.

Depending on how closely particles are packed and how strongly they attract each other, matter is generally found in three physical states: solid, liquid, and gas.

Solids have particles that are very closely packed, with a strong force of attraction between them and very little space to move. This is why solids have a definite shape and a definite volume, are rigid, and are negligibly compressible — for example, an iron rod or a piece of wood keeps its shape and cannot be squeezed into a smaller volume by ordinary pressure. Some solids like sponge and rubber appear compressible, but this is only because of the tiny pores filled with air trapped inside them, not because the solid particles themselves compress.

Liquids have particles that are less tightly packed than in solids, with a moderate force of attraction, allowing them to move around one another. Liquids therefore have no fixed shape and take the shape of their container, but they do have a definite volume. Liquids can flow and are only slightly compressible. Water taking the shape of whatever glass or bottle it is poured into is a common example.

Gases have particles that are very far apart, moving randomly at high speed, with a negligible force of attraction between them. As a result, gases have no definite shape and no definite volume — they expand to fill whatever container they are placed in, like air filling a balloon completely. Because of the large empty spaces between particles, gases are highly compressible, which is why a large volume of air can be compressed into a small cylinder, such as an LPG cylinder.

The state of matter is not always fixed — it can be changed by changing the temperature or the pressure applied to it.

Effect of change of temperature: When a solid is heated, its particles gain kinetic energy and vibrate more vigorously. At a certain fixed temperature, called the melting point, the particles gain enough energy to overcome the force of attraction holding them in fixed positions, and the solid changes into a liquid. This process is called melting or fusion. The melting point of ice, for example, is 0 °C (273 K). The reverse process, where a liquid changes into a solid on cooling, is called freezing or solidification, and it occurs at the same fixed temperature as the melting point.

On further heating, a liquid reaches its boiling point, the fixed temperature at which it changes rapidly into a gas throughout its bulk. The boiling point of water at normal atmospheric pressure is 100 °C (373 K). This change from liquid to gas is called vaporisation, and the reverse change of gas to liquid on cooling is called condensation.

During melting and boiling, the temperature of the substance does not rise even though heat is continuously being supplied — this heat is used to overcome the force of attraction between particles rather than to raise the temperature. This hidden heat energy is called latent heat. The latent heat of fusion is the heat needed to change 1 kg of solid to liquid at its melting point, and the latent heat of vaporisation is the heat needed to change 1 kg of liquid to gas at its boiling point.

Effect of change of pressure: Increasing pressure on a gas, especially along with lowering its temperature, forces its particles closer together and increases the force of attraction between them, which can change it into a liquid or even directly into a solid. Solid carbon dioxide (dry ice) is prepared by applying high pressure and low temperature to carbon dioxide gas. On bringing dry ice back to normal atmospheric pressure, it turns directly into gas without becoming liquid first — this direct change from solid to gas is called sublimation, and other common examples include camphor, naphthalene balls, and ammonium chloride.

Evaporation is the process by which a liquid changes into vapour at any temperature below its boiling point. Unlike boiling, which happens throughout the liquid at a fixed temperature, evaporation is a surface phenomenon — only the particles at the surface of the liquid, which have gained enough kinetic energy to escape the force of attraction of neighbouring particles, turn into vapour and mix with the surrounding air. This is why water in an open dish slowly dries up over hours or days, even though it never reaches its boiling point.

Several factors affect how quickly a liquid evaporates:

• Surface area: A larger surface area exposes more particles to the air, so evaporation is faster. Spreading wet clothes out instead of bunching them up helps them dry quicker.
• Temperature: At higher temperatures, particles have more kinetic energy, so more particles can escape as vapour, increasing the rate of evaporation. Clothes dry faster on a hot day than on a cold day.
• Humidity: Humidity is the amount of water vapour already present in the surrounding air. When humidity is high, the air can absorb less additional vapour, so the rate of evaporation decreases. This is why clothes take longer to dry on a humid day.
• Wind speed: Faster wind carries away water vapour molecules from around the liquid surface, keeping the air near the surface less saturated, which increases the rate of evaporation. This is why clothes dry faster on a windy day.

Evaporation causes cooling: since particles need energy to escape the liquid and turn into vapour, they absorb this energy (latent heat) from the liquid left behind and its surroundings, lowering the temperature. This is why we feel cool when sweat evaporates from our skin, and why water stored in an earthen pot (matka) stays cooler than the surrounding air, as water seeping through its pores keeps evaporating from the outer surface.