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Matter is changing all around you.
Some changes, like melting and freezing, can be undone, so scientists call these changes reversible. Other changes, such as when a match burns or metal rusts, will last forever.
Physical Change
Mix butter and sugar in a bowl, add eggs and a bit of salt, pour in milk, sift in flour, and stir. You’ve made a bowl of delicious cake batter. But what if you decide not to bake that delicious cake after all? Look what’s happened to all your ingredients. You can’t locate the salt or sugar anymore, the eggs are all beaten up, and the milk appears to have vanished. But the truth is, all the matter has changed in appearance only. The molecules in each kind of matter have not been transformed. Scientists call this 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 laboratory could. Apart from putting the egg back into its shell. ▶
Chemical Change
◀ After you bake the batter in the oven, you can never bring the original ingredients back. The heat of the oven 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. Chemical changes are not reversible.
Stalactites and stalagmites are the result of a chemical change that takes place when rainwater seeps through rocks rich in calcium. As the water evaporates, a deposit of calcium is left behind. 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 not the molecules that make it up. ▼
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
If you fry an egg, grill a steak, or boil a potato, the food absorbs energy as it changes from raw to cooked.
Change in Color
A change in color—whether it’s the brilliant scarlet and orange of leaves in the fall or the yucky brown of a too-ripe banana—indicates that 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, an egg is rotten, or meat is spoiled, all by their smells. The ripening and spoiling of foods is a chemical change signaled by a change in smell.
