Changes Around Us: Physical and Chemical

In Class 6 you learnt that ice melting into water, and water evaporating into vapour, are all still water, just in a different state. No new substance was ever formed; only the size, shape, or state changed. Changes like this, where the substance itself does not change, are called physical changes. Grinding chalk into powder, cutting paper, and stretching a rubber band are also physical changes: the substance is still chalk, paper, or rubber, just in a different form.

It is tempting to assume physical changes can always be undone, but that is not quite right, only that no new substance forms. Melting ice can be refrozen, so that one is reversible. But chop a carrot into small pieces, and however carefully you try, you cannot fit the pieces back together into one whole carrot again; it is still a physical change (still carrot, no new substance), yet practically irreversible. Popping corn into popcorn is similar: still made of the same starch, but you can never turn popcorn back into a raw kernel. So 'physical vs chemical' and 'reversible vs irreversible' are two separate questions, not always the same answer.

Now light a candle and let it burn for a minute. Wax melts and drips, which is a physical change, since the melted wax is still wax. But the flame itself is burning candle vapour, and that produces soot, carbon dioxide, and water vapour, none of which are wax. New substances have appeared that were not there before, and you can never turn the soot and gases back into the original candle wax. A change in which one or more new substances are formed is called a chemical change.

Add a pinch of baking soda to a spoonful of vinegar in a test tube, and you will hear fizzing and see bubbles form. This alone doesn't prove a new substance appeared, so pass the gas produced into a test tube of freshly prepared lime water: it turns milky. Milky lime water is the standard test for carbon dioxide gas, and its appearance here proves a genuinely new substance, carbon dioxide, has formed. This reaction can be written as vinegar + baking soda leads to carbon dioxide + other substances. Repeating the same activity with baking soda and plain water produces no bubbles at all, showing that vinegar is essential to this particular chemical change.

You cannot always see a new substance directly, so scientists look for clues. A change in colour is one clue: a shiny iron nail slowly turning reddish-brown as it rusts shows that iron has combined with oxygen and water in the air to form a new substance, iron oxide, commonly called rust. A gas being given off, like the carbon dioxide from vinegar and baking soda, is another clue. Heat or light being produced, as in burning, and a new smell appearing, are further clues. If you notice any of these, a chemical change has very likely occurred.

Rusting is one of the most common chemical changes we see every day. When iron is exposed to both oxygen and moisture (water) in the air over time, it reacts to form a reddish-brown, flaky substance called rust, chemically iron oxide. Rust is weaker and more brittle than iron, which is why rusted railings, gates and tools become damaged over time. Because rusting needs both oxygen and moisture, painting iron objects, oiling them, or galvanising them (coating with zinc) all work by keeping air and water away from the iron surface, which is why they protect against rust.

Burning magnesium ribbon produces a dazzling white light and leaves behind a white powder, magnesium oxide, a genuinely new substance, along with heat and light: magnesium + oxygen leads to magnesium oxide + heat + light. A chemical change in which a substance reacts with oxygen to release heat and/or light is specifically called combustion, and substances that undergo it, like wood, paper, cotton and kerosene, are called combustible substances. Combustion is chemically the same family of reaction as rusting, both are reactions with oxygen, just far faster and with a visible flame.

Light two identical candles on separate dishes, and cover one with an upturned glass tumbler while leaving the other open. The uncovered candle keeps burning, but the covered one goes out after a short while, once the limited air trapped inside is used up. Test the air inside the used tumbler with a little lime water, and it turns milky, confirming carbon dioxide has formed there, made from carbon in the wax combining with oxygen from the trapped air. Once that oxygen is used up, the flame cannot continue, proving that oxygen is genuinely required for combustion, not just present as a coincidence.

Paper left sitting in ordinary air never catches fire by itself, even though paper is combustible and oxygen is all around it, so something more must be needed. Bring a lit matchstick near paper, and it catches fire almost instantly. Instead, focus sunlight onto paper through a magnifying glass, holding the brightest, smallest spot steady: after a while the paper begins to smoke, then bursts into flame, without any match at all. Both cases show that a combustible substance must be heated up to a minimum temperature, called its ignition temperature, before it will catch fire; the matchstick was already hotter than paper's ignition temperature, while the focused sunlight slowly heated the paper up to it. Combustion, therefore, always needs three things together: a combustible substance (fuel), oxygen, and heat up to the ignition temperature, often drawn as a 'fire triangle'. Remove any one side of the triangle, and burning stops, which is exactly why wrapping a burning person in a blanket (cutting off oxygen) is used to put out a clothing fire.

Many everyday processes involve both physical and chemical changes at once. Baking a cake starts with mixing batter, a physical change, but baking it in the oven causes chemical changes that turn runny batter into a solid, spongy cake that can never be turned back into batter. Cooking rice softens it through a physical-looking change, but starch is also chemically altered so it becomes digestible. Recognising which parts of a process are physical and which are chemical helps you understand what is really happening at each step.

Physical and chemical changes are not only things we do in a kitchen or a lab; they shape the natural world too. Wind and water physically break big rocks into smaller pebbles, sand and clay, a physical change since the rock material itself does not change. But over a very long time, minerals dissolved in water can also react chemically, cementing loose grains together into a completely new kind of rock. So the same natural setting can show both kinds of change: physical breaking-down and chemical building-up, working together over centuries.