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Matter is changing all around you.
Some changes, like melting and freezing, can be undone. Scientists call these changes reversible. Other changes, such as when a match burns or metal rusts, are forever.
Physical Change
Mix butter and sugar in a bowl, add eggs and a bit of salt, and pour in milk. Sift in flour and stir, and you have cake batter. But what if you decide not to make a cake? Look at what’s happened. You can’t find the salt or sugar anymore, the eggs are all beaten up, and the milk appears to have vanished. But the truth is, the matter has changed only in appearance. The molecules in each kind of matter have not changed. Scientists call this kind of change a physical change because the molecules that make up the egg, for example, have not changed. They haven’t become molecules of macaroni or anything else—yet. You, in your kitchen, might not be able to restore all those ingredients to their original forms, but scientists in a lab can. Apart from putting the egg back into its shell. ▶
Chemical Change
◀ After you bake the batter, you can never bring back the original ingredients. The oven’s heat causes the molecules of butter, sugar, milk, and all the other matter in the batter to break up and re-form. Scientists call this kind of change a chemical change. You can’t undo a chemical change.
Stalactites and stalagmites are the result of a chemical change. It takes place when rainwater seeps through rocks rich in calcium. As the water evaporates, it leaves behind a deposit of calcium. This process is extremely slow. ▶
Sculptor Sasson Soffer made physical changes in stainless steel to form this sculpture. By shaping and smoothing the steel, he changed its physical properties. But he didn’t change the molecules in the steel. ▼
Sensing Change
If the physical properties of something change—for example, its shape, size, texture, volume, mass, weight, or density—scientists say a physical change has taken place. These changes are pretty easy to identify. Here are some signs of a chemical change.
Energy Is Absorbed
Fry an egg. Grill a steak. Boil a potato. In each case, the food absorbs energy as it changes from raw to cooked.
Change in Color
A change in color—whether it’s the brilliant red and orange of leaves in the fall or the yucky brown of a too-ripe banana—means a chemical change has taken place.
Energy Is Released
Bursts of light, sprays of color, and popping sounds are all part of a fireworks display, where chemical changes release energy that we can see and hear.
Change in Smell
You can tell if a peach is ripe when you smell it. You can tell if an egg is rotten or meat is spoiled by its smell. The ripening and spoiling of foods is a chemical change signaled by a change in smell.
