Matter in Our Surroundings

Everything around us — the air we breathe, the water in a steel glass, the dal in a kadhai — is matter. Matter occupies space and has mass.

Matter is made of extremely tiny particles. These particles are so small that they are invisible even under most microscopes. Yet their behaviour explains every property of matter that we can observe.

Ancient Indian thinkers like Kanad proposed the concept of 'anu' (atom) thousands of years ago. Modern science confirms that the particles of matter are atoms, molecules, or ions depending on the substance.

The particles of matter are not touching one another — there are spaces (intermolecular gaps) between them. The size of these gaps is very different in solids, liquids, and gases.

Particles are always in motion. Even in a solid at room temperature, the particles vibrate about fixed positions. In liquids and gases, the motion is more extensive and random.

Particles attract one another with intermolecular forces. In solids, these forces are very strong. In liquids, they are moderate. In gases, they are very weak. This difference in force explains the rigidity of solids and the free flow of gases.

Solids have a definite shape and a definite volume. The particles are packed very close together in regular arrangements and are held in fixed positions by strong intermolecular forces.

Because particles can only vibrate about their fixed positions, solids cannot flow and are very hard to compress. An iron lock, a wooden almirah, a brick wall — all are rigid because their particles are tightly held.

Solids can however be deformed by applying large forces. A lump of clay can be shaped by hand. Metals can be hammered (malleable) or drawn into wires (ductile) because their crystal structure allows some rearrangement under stress.

Liquids have a fixed volume but no fixed shape. Their particles are not as tightly packed as in solids and can slide past one another. This is why liquids flow and take the shape of any container they are poured into.

Water, milk, oil, nimbu pani, and dal — all take the shape of the vessel in which they are kept. Pour water into a round lota, it becomes round; pour it into a rectangular tray, it becomes flat.

Liquids are nearly incompressible because the gaps between particles are small, but their particles can move, which allows diffusion in liquids (though slower than in gases). Liquids also exert pressure in all directions.

Gases have neither a fixed shape nor a fixed volume. Their particles are very far apart with almost no intermolecular forces, so they move randomly at high speeds in all directions.

A gas expands to fill the entire container it is placed in. LPG in a cylinder, CNG in a vehicle's tank, and the air filling a tyre — all expand to occupy every corner of their container.

Gases are highly compressible because of the large spaces between particles. This property is used to store large amounts of gas in small cylinders. It is also why pumping air into a bicycle tyre is possible — you are simply pushing more gas molecules into the same space.

When a substance is heated, the kinetic energy of its particles increases. They vibrate or move faster. If enough heat is supplied, the particles overcome some of the intermolecular forces holding them together, and the substance changes state.

Ice (solid) heated to 0 °C begins to melt and becomes water (liquid). Water heated to 100 °C at sea level begins to boil and becomes steam (vapour/gas). These are changes of state.

Cooling has the reverse effect. Steam cooled below 100 °C condenses back to liquid water. On a cold morning in Delhi in winter, water droplets condense on the outside of a cold steel tumbler or on window glass — these are water vapour from the air condensing on the cold surface.

During melting or boiling, the temperature of a substance does not rise even though heat is being continuously supplied. This is because the heat is used to break the intermolecular forces — not to increase the speed of particles.

The heat absorbed or released during a change of state at constant temperature is called latent heat. The word "latent" means hidden, because this heat does not produce a visible change in temperature.

This is why burns from steam are more severe than burns from boiling water at the same 100 °C temperature. Steam releases extra heat (its latent heat of condensation) as it condenses on skin, causing a much more serious injury.

Temperature is commonly measured in degrees Celsius (°C) in everyday use in India. In science, the SI unit of temperature is the kelvin (K).

Zero kelvin (0 K = −273 °C) is called absolute zero — the lowest possible temperature where particle motion would theoretically stop. At absolute zero, no heat energy remains in a substance.

Converting between the two scales is straightforward: add 273 to Celsius to get Kelvin.

Just as temperature can change the state of matter, so can pressure. Increasing pressure forces particles closer together, which can convert a gas into a liquid or even a solid.

LPG is stored in cylinders in liquid form by applying high pressure. When the valve is opened, pressure decreases and the liquid quickly vaporises to gas, which then burns in the burner.

Decreasing pressure has the opposite effect — it lowers the boiling point of liquids. At high altitudes like Leh, Ladakh, or Shimla, atmospheric pressure is lower than at sea level, so water boils below 100 °C. Food takes longer to cook because the water boils at a lower temperature.

Evaporation is the conversion of a liquid into vapour from its surface, at any temperature below the boiling point. It is a surface phenomenon — only the molecules at or near the surface with enough kinetic energy escape into the vapour phase.

Factors that increase the rate of evaporation: higher temperature, greater surface area, drier air (low humidity), and faster air movement (wind).

In India, wet clothes on a clothesline dry faster on a hot, dry summer day in Rajasthan than during the humid monsoon season in Mumbai, because the drier air and higher temperature both favour evaporation.

Evaporation causes cooling. When the fastest-moving particles escape from the liquid surface, the average kinetic energy of the remaining particles decreases. This cools the liquid. Sweat cools our body for exactly this reason. Water stored in an earthen matka (clay pot) is cooler than tap water because water seeps through the porous clay and evaporates from the surface, cooling the water inside.

Some substances change directly from solid to gas (or gas to solid) without passing through the liquid state. This is called sublimation.

Common examples: camphor (used in puja/prayers in Indian homes) gradually disappears without forming a puddle. Naphthalene (mothballs) placed in cupboards to repel insects also sublimes slowly.

Ammonium chloride sublimes when heated — a fact used in the laboratory separation of ammonium chloride from salt. Dry ice (solid carbon dioxide used to keep ice cream cold) sublimes at room temperature, releasing gaseous CO₂.

Sublimation is used as a separation technique when one component of a mixture can sublime and the other cannot.

Diffusion is the mixing of two substances due to the movement of their particles from a region of higher concentration to a region of lower concentration. It happens in all three states of matter, but is fastest in gases and slowest in solids.

When someone sprinkles attar (ittar) perfume in a corner of a room, the fragrance spreads throughout the room within minutes. The perfume molecules move from the area of high concentration (near the source) toward areas of lower concentration (far corners of the room).

Brownian motion is the random, zig-zag movement of tiny visible particles suspended in a fluid. Robert Brown observed pollen grains dancing erratically in water. This occurs because the much smaller, invisible water molecules hit the pollen from all sides with slightly unequal force at any instant, pushing it in random directions. Brownian motion is direct evidence for the continuous and random motion of particles of matter.

Matter is made of tiny particles with spaces between them, in continuous motion, and with intermolecular forces.

Solids: fixed shape, fixed volume, incompressible. Liquids: no fixed shape, fixed volume. Gases: no fixed shape, no fixed volume, highly compressible.

Change of state: temperature and pressure affect state. Latent heat is absorbed/released at constant temperature during state changes.

Conversion: .

Evaporation: surface process, below boiling point, causes cooling. Factors: temperature, surface area, humidity, wind speed.

Sublimation: solid → gas directly. Examples: camphor, naphthalene.

Diffusion: mixing due to particle movement. Brownian motion: evidence for particle motion.