Matter & Its States

Matter is anything that has mass and occupies space. The chair you sit on, the water you drink and the air you breathe are all matter; heat, light and thought are not. Ancient Indian thinkers spoke of five elements (Prithvi, Jal, Agni, Vayu, Akash), while modern science sorts matter by what it is made of and by the physical state it is found in. The question for this topic is simple: why does a stone keep its shape, water take the shape of a glass, and air spread into the whole room?

Everything in this topic comes from one tug-of-war: attraction between particles tries to hold them together, while their kinetic energy (motion) tries to pull them apart. In a solid attraction wins, in a gas motion wins, and a liquid sits in between. Heat the substance and motion increases, which is exactly how a solid melts and a liquid boils.

School books stop at solid, liquid and gas, but matter has two more states at the extremes of temperature. Heat a gas to thousands of degrees and its atoms are battered so hard that electrons are knocked out. The result is a cloud of free electrons and positive ions, called plasma, the fourth state. Cool a gas of certain atoms to almost absolute zero and the atoms stop behaving as individuals and merge into one wave-like blob, called the Bose-Einstein Condensate (BEC), the fifth state. One is the hottest state of matter, the other the coldest.

Think of states in order of rising energy: BEC (coldest, least energy) -> solid -> liquid -> gas -> plasma (hottest, most energy). A gas becomes plasma when its atoms are ionised; a gas becomes BEC when it is chilled until all its atoms share one quantum state. The word 'boson' is a tribute to Bose: particles named after him are the ones that can form a BEC.

Put ice on a stove and watch what happens: it melts, the water warms up, then boils and turns into steam. The substance never changed; only the state did. Heat gives particles more kinetic energy until they overcome the attraction holding them, and cooling does the reverse. Strangely, while a state is changing the thermometer stops rising. The heat supplied is being used to loosen the particles and not to raise the temperature; this hidden heat is called latent heat.

Evaporation causes cooling because the fastest particles escape, so the average energy of those left behind falls. This is why sweating cools the body, why water in an earthen pot (matka) stays cool, and why cotton clothes keep us comfortable in summer: cotton absorbs sweat and exposes a large surface to evaporation.

Look at a glass of sweet lemonade, a piece of gold jewellery and a spoon of salt. Chemists sort all of them with one question: is it made of one kind of matter or of many? A substance made of only one kind of particle is pure; if two or more pure substances are simply mixed, the result is a mixture. Pure substances are of two kinds: elements, which cannot be broken into anything simpler by chemical methods, and compounds, in which two or more elements are chemically joined in a fixed proportion.

In a compound the elements lose their own personality. Sodium is a soft, explosive metal and chlorine is a poisonous green gas, yet together they make harmless table salt. In a mixture nothing like this happens: iron filings and sulphur powder stay iron and sulphur, and a magnet can still pull the iron out. If a magnet can separate it, it was a mixture; if you needed a chemical reaction, it was a compound.

Rice has stones, milk has cream, sea water has salt, ink has several dyes, and crude oil has petrol. Every one of these jobs of separation uses a difference in a physical property: size, weight, solubility, boiling point, magnetism, or the ability to sublime. The golden question is therefore never 'which method do I remember?' but 'how do the components differ?' Once you have found the difference, the method is already decided.

Distillation = boiling + condensing. The liquid with the lower boiling point turns to vapour first, the vapour passes into a condenser (a cooled tube), and turns back into liquid in a separate flask. That is why distilled water is so pure: only the water vapour reaches the receiver while dissolved salts stay behind. Fractional distillation simply adds a fractionating column so that the vapours condense and re-evaporate many times, which sharpens the separation when boiling points are close.

A candle teaches the whole topic. When the wax melts and runs down the side, it is still wax; cool it and it hardens again. That is a physical change. But when the wick burns, the wax vapour combines with oxygen and turns into carbon dioxide, water vapour, heat and light: the wax is gone for ever and cannot be got back by cooling. That is a chemical change. A physical change alters only the form or state; a chemical change creates a new substance with new properties.

The decisive test is always: has a new substance been formed? Other clues (colour change, gas, heat) only hint at this, and some physical changes also involve them (a piece of ice gets wet and cold). In chemical changes the atoms are not created or destroyed; they are only rearranged into new substances, which is why the total mass stays the same before and after the change (Law of Conservation of Mass).

Stir sugar into water and the liquid stays clear; stir chalk powder into water and it turns cloudy and later settles; stir a little milk into water and it stays cloudy but never settles. These three behaviours are the three kinds of mixtures made by dispersing one substance in another: a solution, a suspension and a colloid. The only difference between them is the size of the particles of the dispersed substance, and everything else (clear or cloudy, settles or not, passes the filter or not, scatters light or not) follows from that size.

The Tyndall effect is the scattering of a beam of light by particles in its path, so that the path of the beam becomes visible. It is shown by colloids and suspensions, but not by true solutions, because solution particles are too small to scatter visible light. You see it when sunlight streams through the mist of a forest canopy or a hole in the roof of a dusty room, and when a torch beam shines through smoke or fog. It is named after the Irish physicist John Tyndall.