Chemical and Physical Changes
Grade 7 · Science · Study Guide
Learn the difference between changes that alter matter's form and changes that create entirely new substances.
Matter is always changing — but not all changes are the same. Scientists divide changes in matter into two categories: physical changes and chemical changes. The key question to ask is: 'Did a new substance form?' If yes, it's a chemical change. If no, it's a physical change.
A physical change alters the form, size, shape, or state of a substance without changing what it fundamentally is. Cutting paper, melting ice, dissolving sugar in water, and bending a wire are all physical changes. The substance is still the same material — you can often reverse a physical change. Ice melts into water, but you can freeze it right back again. When you dissolve sugar in water, the sugar is still there — evaporate the water and you can recover it.
A chemical change (also called a chemical reaction) produces one or more completely new substances with different properties. Burning wood, rusting iron, baking bread, and digesting food are chemical changes. The original materials — the reactants — are converted into new products. Signs that a chemical change has occurred include: a color change, gas production (bubbling), energy released as light or heat, a new smell, or the formation of a precipitate (a solid that appears in a liquid). Chemical changes are generally much harder — or impossible — to reverse. You cannot un-burn wood or un-bake a cake.
Some changes can be tricky. Dissolving salt in water looks like it disappears, but it is still a physical change — the salt molecules are just spread out in the water. However, dissolving an antacid tablet in water creates bubbles of carbon dioxide gas — that is a chemical change because a new substance (CO₂) was produced. Always look for evidence of new substances to decide which type of change occurred.
Key terms
- Physical change
- A change that alters form, shape, or state without creating a new substance.
- Chemical change
- A change that produces one or more new substances with different properties.
- Reactants
- The starting materials in a chemical reaction.
- Products
- The new substances formed by a chemical reaction.
- Precipitate
- A solid that forms and settles out of a liquid solution during a chemical reaction.
- Law of Conservation of Mass
- Matter is neither created nor destroyed in chemical reactions; total mass before equals total mass after.
Flashcards
- Physical change
- A change in form, shape, or state; no new substance is formed. Example: ice melting.
- Chemical change
- A reaction producing new substances with different properties. Example: iron rusting.
- Reactants
- Starting materials in a chemical reaction.
- Products
- New substances formed by a chemical reaction.
- Evidence of chemical change
- Color change, bubbling, light/heat released, new odor, or precipitate formation.
- Law of Conservation of Mass
- Total mass of reactants equals total mass of products in any chemical reaction.
Practice questions
Which of the following is a chemical change?
- Tearing paper
- Melting butter
- Burning wood
- Cutting an apple
Show answer
Answer: Burning wood
Burning wood is a chemical change — wood reacts with oxygen to produce carbon dioxide, water vapor, and ash. These are completely new substances from the original wood.
Dissolving sugar in water is a physical change.
Show answer
Answer: True
When sugar dissolves in water, it spreads out as molecules but remains sugar (C₁₂H₂₂O₁₁). No new substance is formed. Evaporating the water would leave sugar crystals behind.
The starting materials in a chemical reaction are called ___.
Show answer
Answer: reactants
Reactants are the substances you start with before a chemical reaction. They are transformed into products during the reaction.
Which is NOT a sign that a chemical change has occurred?
- Change in color
- Production of gas
- Change in shape
- Release of heat
Show answer
Answer: Change in shape
A change in shape alone (like bending wire or cutting apple) is a physical change — no new substance is formed. Color change, gas, and heat release all indicate new substances.
Chemical changes are generally easy to reverse.
Show answer
Answer: False
Chemical changes produce new substances, making them very difficult or impossible to reverse. You cannot turn ash back into wood or un-bake bread.
Fun facts
- Burning a match releases energy that took the wood years to store through photosynthesis — in less than a second.
- The Law of Conservation of Mass means that when you burn a log, the atoms in the ash plus the gases released equal the mass of the original log.
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