Year 9 Science Exam Revision Sem 1, 2025

Identify why chemical equations need to be balanced.

• No atoms are created or destroyed during a chemical reaction

• Atoms are destroyed during a chemical reaction and the balanced equation shows how many

• New atoms are created during a chemical reaction and the balanced equation shows how many

Jeremy writes the following symbol equation: K (s) + H2O (l) → KOH (aq) + H2 (g) Determine why this equation is unbalanced.

• There are 2 hydrogens on the reactant side and 3 hydrogens on the product side

• There are 2 hydrogens on the reactant side and 1 oxygen on the product side

• There is K on the reactant side and KOH on the product side

• There are 2 hydrogens on the reactant side and only 1 hydrogen on the product side

Ammonia reacts with chlorine gas to produce nitrogen gas and hydrogen chloride. The unbalanced chemical equation is: NH₃ + Cl₂ → N₂ + HCl. Determine the balanced chemical equation.

• NH₃ + 3Cl₂ → N₂ + 6HCl

• 2NH₃ + Cl₂ → N₂ + 6HCl

• 2NH₃ + 3Cl₂ → N₂ + 6HCl

Identify the best description of a chemical reaction.

• A change in the state of matter

• The formation of a new solid

• A change that results in new substances

• The conversion of products into reactants

Recall what happens at the atomic level during a chemical reaction.

• Atoms are rearranged as bonds break and form

• Atoms are rearranged but the bonds stay the same

• Some atoms disappear and bonds break

• New atoms are created and make new bonds

Classify each of these examples as either a physical change or a chemical reaction.

Baking a pie in an oven: (a)  

• physical change

• chemical change

Classify each of these examples as either a physical change or a chemical reaction.

Spreading jam on bread: (a)  

• physical change

• chemical change

Identify which of the following are elements.

• Silver chloride (AgCl)

• Copper (Cu)

• Hydrogen (H₂)

• Methane (CH₄)

When magnesium (Mg) is heated with oxygen (O₂), a new solid forms: magnesium oxide (MgO). Determine which of the following statements about this reaction are true.

• Magnesium oxide is a reactant

• Two elements are converted into a compound

• Oxygen is a reactant

• A compound is converted into two elements

Hydrogen (H₂) and oxygen (O₂) gas react to form the product water (H₂O). This happens when the gases are ignited. Determine both the word equation and the symbol equation for this reaction.

• water + oxygen → hydrogen

• 2H₂ + O₂ → 2H₂O

• hydrogen + oxygen → water

• H₂O + O₂ → H₂