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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. Pour in milk. Sift in flour. Stir. You have cake batter. But what if you decide not to make a cake? Look at what’s happened. You can no longer find the salt or sugar. The eggs are beaten up. 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. In a physical change, the molecules that make up the egg, for example, have not changed. They have not become molecules of macaroni or anything else. You, in your kitchen, might not be able to bring the ingredients back to their original form, but scientists in a lab can. Apart from putting the egg back into its shell. ▶
Chemical Change
◀ After you bake the batter, you can’t ever 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 a chemical change. You can’t undo a chemical change.
Stalactites and stalagmites are the result of a chemical change. It happens when rainwater seeps through rocks rich in calcium. As the water evaporates, it leaves behind tiny amounts of calcium. These are called deposits. They slowly grow into big shapes. ▶
Sculptor Sasson Soffer made physical changes in stainless steel. That’s how he formed 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
When you see a change in color—whether it’s the brilliant red and orange of leaves in the fall or the yucky brown of a banana left unpeeled and uneaten—you know a chemical change has taken place.
Energy Is Released
The bursts of light. The sprays of color. The popping sounds. They’re all part of a fireworks display, in which chemical changes release energy 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. ▶
