Changes Around Us: Physical and Chemical

Fold a sheet of paper into a paper boat, and unfold it again — you still have the exact same sheet of paper, just creased. Inflate a balloon and let the air hiss back out, and you're left with the same rubber you started with, just uninflated again. Crush a stick of chalk into a fine powder, and chemically, it is still exactly the same chalk, just broken into smaller pieces.

In each of these cases, something about the object's appearance changed — its shape, its size, or how it's arranged — but no new substance was ever created. Changes like these, where only physical properties such as shape, size, or state change while the substance itself stays the same, are called physical changes. Water turning to ice, or ice melting back into water, is another classic example: whether it's solid, liquid, or gas, it's still the same substance, water, just in a different state.

Blow your exhaled breath through a straw into a glass of plain water, and nothing much happens besides a few bubbles. Blow the same breath into a glass of lime water instead, and the clear liquid turns milky white right in front of you. Something fundamentally different has occurred in the second glass: the carbon dioxide gas you breathe out has reacted with the lime water (calcium hydroxide) to form a brand-new, insoluble white substance called calcium carbonate, along with a little water. Because an entirely new substance with new properties has appeared, this is called a chemical change, and the underlying process is a chemical reaction.

You can see the same idea in your own kitchen. Add a pinch of baking soda to a spoonful of vinegar or lemon juice, and the mixture fizzes energetically, releasing bubbles of gas. Pass that gas into a test tube of lime water, and it turns milky too — confirming the gas is carbon dioxide, a new substance formed from the reaction between the baking soda and the acid. Chemists often write such reactions in a short, symbolic form called a chemical equation; in plain words, this one reads: Vinegar + Baking soda → Carbon dioxide + Other substances. If you repeated this with baking soda and plain water instead of vinegar, you'd see no fizzing at all — a reminder that a chemical change needs the right combination of substances, not just any liquid.

The rusting of iron, which forms a new reddish-brown substance called iron oxide on the metal's surface, is a chemical change, since iron and rust are simply not the same substance. Burning is a chemical change too, and a particularly dramatic one: when magnesium ribbon is set alight, it blazes with a brilliant white light and leaves behind a completely new white powder, magnesium oxide, along with the heat and light released during the reaction. A chemical reaction in which a substance combines with oxygen and gives out heat, light, or both is called combustion, and substances capable of undergoing it — wood, paper, cotton, kerosene, and magnesium among them — are called combustible substances.

Combustion needs oxygen to continue. Light two identical candles side by side, then trap one under an upturned glass tumbler while leaving the other open to the room: the uncovered candle keeps burning steadily, but the trapped one flickers and dies out within a minute or two, because it has used up the limited oxygen sealed inside the tumbler with it and nothing is flowing in to replace it.

Oxygen and a combustible substance alone aren't quite enough, though. A sheet of paper can sit in open air for years without catching fire on its own, yet it bursts into flame almost instantly when a lit matchstick touches it — or even without any flame at all, if you focus bright sunlight through a magnifying glass onto one tiny spot on the paper and hold it steady for a while. Both cases show that a substance must first be heated to a specific minimum temperature, called its ignition temperature, before it will catch fire; the matchstick's flame simply supplies that heat instantly, while the focused sunlight supplies it gradually. Combustion therefore needs three things together: a combustible substance (fuel), oxygen, and enough heat to reach the fuel's ignition temperature.

Watch a lit candle closely and you'll actually see two completely different kinds of change going on together. Look at any candle that has burned for a while and you'll spot hardened dribbles running down its side; each one started out as liquid wax that cooled and hardened the moment it moved far enough away from the flame's heat. Closer to the wick itself, the opposite is happening: solid wax near the flame turns liquid, that liquid rises up through the threads of the wick, and the heat right at the tip turns it into an invisible vapour. None of this — turning liquid, rising through the wick, turning to vapour, or hardening back into a dribble — changes what the wax actually is; every one of these steps is a physical change.

The vapour rising from the wick, however, is what actually burns in the flame, reacting with oxygen from the air to release the heat and light you see, along with soot, water vapour, and carbon dioxide as new substances. That part of the process, the vapour actually combusting, is a chemical change. A burning candle is therefore a neat demonstration of physical and chemical changes occurring together in a single, everyday object — which is part of why the nineteenth-century scientist Michael Faraday famously used a burning candle as the centrepiece of an entire public lecture series to introduce people to the fundamentals of chemistry.

Not every change is equally permanent. Melted ice can be refrozen back into ice, and water vapour can be condensed back into liquid water — these changes can be reversed. Chopped vegetables, on the other hand, can never be reassembled into their original whole shape, and popped popcorn can never be squeezed back into a hard, unpopped kernel — these changes are irreversible. Reversibility is a separate question from whether a change is physical or chemical: dissolving salt in water is a physical change that happens to be reversible (evaporate the water and the salt reappears), while curdling milk into curd is a chemical change — bacteria convert milk sugar into lactic acid, which makes the milk protein, casein, clump together into solid curd — that cannot be reversed back into milk at all.

Changes also differ in whether we want them. Milk turning to curd, fruit ripening, and food cooking are all generally desirable changes. Rusting of iron tools and food spoiling in storage are generally undesirable. Interestingly, the very same kind of change can be desirable in one setting and undesirable in another: food waste decomposing in your kitchen bin is unwelcome, but that same decomposition, encouraged deliberately in a compost pit, turns waste into useful manure.

Some of the grandest changes of all happen far too slowly for us to notice day to day. Large rocks exposed to the weather crack into smaller fragments over years, as temperature swings, growing tree roots, and water freezing inside tiny cracks physically pry them apart — and at the same time, water and the gases dissolved in it can chemically react with minerals inside the rock; an iron-bearing black rock called basalt, for instance, can develop a reddish outer layer over time as the iron within it reacts with water and air to form iron oxide. Together, these physical and chemical processes are called weathering, and they are what slowly turn solid rock into soil. The broken-down fragments don't just stay put, either: flowing water and wind carry sediment, pebbles, and sand from one place to another in a physical process called erosion, smoothing river pebbles along the way and eventually depositing the material elsewhere, where it can harden into new rock after thousands of years — a timescale far too long for any of us to reverse.