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Physical Changes and Chemical Changes
Changes in matter can be physical changes or chemical changes. Physical changes are reversible, and chemical changes are irreversible (cannot be reversed).
A physical change is a change in the size, shape, or state of matter with no new matter being formed. Changes in states of matter are always physical changes because they can be reversed. For example, you can melt an ice cube (a solid), then freeze the water (a liquid) to turn it back into ice. Or, you can boil water to make it evaporate into water vapour (a gas), then cool the water vapour to make it condense into liquid water. In both examples, the substance—water—has not changed into something else. The water changes from one state to another, but it is still always water.
Changes in states of matter are not the only types of physical change. If you cut an apple into two pieces, you have changed the shape of the apple but both pieces are still apple. You can reverse the change by putting the two pieces back together.
In a chemical change, a new substance—or more than one new substance—is formed. The change is not reversible. For example, if you burn a piece of wood, two new substances are formed: smoke and ash. The smoke and ash cannot be turned back into wood. If you cook an egg, you cannot change the cooked egg back into a raw egg.
Signs of a Chemical Change
The following are signs that a chemical change is happening:
• Change in colour: When leaves change colour in autumn, they are going through a chemical change.
• Change in odour: You can tell that an egg is rotten because of the awful odour. A rotting egg is undergoing a chemical change.
• A precipitate forms: A precipitate is a solid that forms when certain liquids are mixed together. If you see a precipitate form, you know that a chemical change has happened.
• A gas is produced: When you mix certain substances together, the mixture will begin to bubble. The bubbles are made of a gas that is produced through a chemical change.
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1. Is baking cookie dough a physical, or chemical change?CorrectIncorrect
2. Is crumpling tin foil a physical, or chemical change?CorrectIncorrect
3. Is cracking open an egg a physical, or chemical change?CorrectIncorrect
4. Is bread becoming mouldy a physical, or chemical change?CorrectIncorrect
5. Is cooking rice a physical, or chemical change?CorrectIncorrect