Grade 7 · Physical and Chemical Changes · Chemical Changes
Physical vs Chemical Changes
Deciding from evidence whether a new substance has formed.
Learning objectives
- Distinguish physical from chemical changes.
- Identify evidence that a chemical reaction has occurred.
- Interpret data on properties before and after a change.
NGSS alignment
MS-PS1-5
Develop and use a model to describe how the total number of atoms does not change in a chemical reaction and thus mass is conserved.
MS-PS1-3
Gather and make sense of information to describe that synthetic materials come from natural resources and impact society.
MS-PS1-2
Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.
MYP criteria
- Criterion A — Knowing and Understanding
- Criterion C — Processing and Evaluating
ENGAGE
Rusting or wet?
Think about it
Water on iron dries away with no lasting change. Water plus air over weeks leaves flaky orange rust. Which one made a new substance?
EXPLAIN
Evidence for a chemical change
- Colour change that cannot be reversed
- Gas produced (bubbles without boiling)
- Temperature change without heating or cooling
- New solid (precipitate) formed
- Different properties, such as melting point or density, before and after
Careful
Bubbles alone are not proof: boiling water bubbles too, and that is a physical change.
Practice questions
0/2 correct
Level 1
Which observation is the strongest evidence that a chemical reaction has occurred?
Level 2
Melting candle wax is a physical change because...
Reflect & track
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