★ Must-know
📌 A physical change alters the state or shape of a substance without changing its chemical composition.
📌 A chemical change produces new substances with chemical properties different from those of the original substances.
Further detail
Examples of physical changes include:
Examples of chemical changes include wood combustion, yogurt formation from milk, wax combustion, rusting iron, burning bread, limewater becoming milky with carbon dioxide, a universal indicator changing color near ammonia gas, cooking food, fireworks, soap formation from oil and caustic soda, and food rotting.
Physical changes alter state or shape, whereas chemical changes form new substances.
1. A sample of ice melts into liquid water without producing a different substance. How should this change be classified?
2. Why is the formation of rust on iron classified as a chemical change?
3. What happens at the atomic level during a chemical reaction?
What does a physical change alter in a substance?
Its state or shape without changing chemical composition.
What results from a chemical change in substances?
New substances with different chemical properties.
What is a chemical reaction in terms of atomic bonds?
It is breaking bonds in reactants and forming new bonds in products.
What are the main signs indicating a chemical reaction?
Precipitate formation, gas evolution, color change, consistency change, odour, light and heat emission.
What gas is produced when hydrochloric acid reacts with magnesium?
Hydrogen gas is produced.
How can hydrogen gas from magnesium and hydrochloric acid be tested?
It burns with a pop sound near a burning match.
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