Physical and Chemical Changes — Reading Comprehension Lesson

Reading Passage

Physical and Chemical Changes

Matter is constantly changing around us. Ice melts, wood burns, iron rusts, and bread bakes. All of these are changes in matter, but not all changes are the same kind. Scientists classify changes in matter into two categories: physical changes and chemical changes. Understanding the difference between these two types of changes is a key concept in science.

A physical change is a change in the form or appearance of a substance that does not change what the substance is made of. The substance keeps its chemical identity. Cutting paper, bending a wire, crushing a can, and dissolving sugar in water are all physical changes. When water freezes into ice or evaporates into water vapor, the substance is still H₂O — only its form has changed. Physical changes are often easy to reverse.

In a physical change, no new substance is created. The molecules that make up the matter are rearranged or separated but are not chemically changed. For example, when you tear a piece of paper, you still have paper — just in smaller pieces. When salt dissolves in water, the salt is still salt, even though you can’t see it. It can be recovered by evaporating the water.

A chemical change is a change in which one or more substances are converted into a different substance with different properties. New molecules are formed during a chemical change. Signs that a chemical change has occurred include: a change in color, the production of a gas, the formation of a precipitate (a solid that forms from a liquid), a change in temperature, or the production of light. Burning, rusting, cooking, and digestion are all chemical changes.

Unlike physical changes, chemical changes are usually very difficult or impossible to reverse. When wood burns, it becomes ash, water vapor, and carbon dioxide — you cannot turn ash back into wood. When an egg is cooked, the proteins change permanently — you cannot uncook an egg. Recognizing physical and chemical changes helps scientists understand how matter behaves and how to use those changes in useful ways.

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