1Physical or chemical?
In a chemical change new substances are made. In a physical change, like evaporating, the particles only move apart: no new substance.
chemical changephysical change
Lesson 11.1 notes
Chemical changes and equations
Physical and chemical changes, signs of a reaction, bonds breaking and forming, and balanced equations.
2Signs of a reaction
Fizzing, a colour change, a new solid (a precipitate) or a temperature change are clues that a new substance may have formed.
precipitate
3Bubbles are not always a reaction
Boiling water makes bubbles, but they are steam: water particles as a gas. No new substance is made, so boiling is a physical change.
physical change
4Bonds break and form
In a reaction the bonds between atoms break, the atoms rearrange, and new bonds form. No atoms are made or destroyed.
bond
5Reactants and products
The substances you start with are the reactants. The new substances are the products: methane + oxygen → carbon dioxide + water.
reactantproduct
6Balancing equations
Atoms are not made or destroyed, so a balanced equation has the same number of each type of atom on both sides.
balanced equation
7Only change the big numbers
Balance an equation by changing the big numbers in front of formulae. Changing a small number makes a different substance: H₂O₂ is not water.
formula
Explore: Equation balancer
Pick an equation, then add or take away molecules until every type of atom balances.
Common mistake: Bubbles do not always mean a reaction
Boiling water bubbles, but the steam is still water: a physical change. A chemical reaction only happens if a new substance is made.
Remember
- Chemical change: new substances are made. Physical change: no new substance.
- In a reaction, bonds break, atoms rearrange and new bonds form. No atoms are made or destroyed.
- A balanced equation has the same number of each type of atom on both sides. Only change the big numbers.
Going further at GCSE
GCSE: Formulae and word equations → GCSE: Balancing equations → GCSE: Conservation of mass →