Physical or chemical?
What is the key difference between a physical change and a chemical reaction?
Chemical: new substances form.
Physical: no new substance forms.
Two solutions react and produce bubbles. What could the bubbles show?
Gas may be forming, which can indicate a chemical reaction.
What do we call the starting substances and the substances formed?
Starting substances: reactants. Substances formed: products.
During a chemical reaction, are atoms created, destroyed, or rearranged?
Atoms rearrange. None are created or destroyed.
Which type of reaction transfers thermal energy TO its surroundings?
Exothermic.
Ice melts into liquid water. Physical or chemical? Explain.
Physical. It is still H₂O; only the state changes.
A yellow solid forms when two clear solutions react. What is this solid called?
A precipitate: an insoluble solid formed in solution.
Magnesium burns in oxygen to form magnesium oxide. Write the word equation.
magnesium + oxygen → magnesium oxide
Count each type of atom in MgCl₂. How many atoms are there in total?
Mg = 1, Cl = 2.
Total = 3 atoms.
A chemical hand warmer gets warm. Exothermic or endothermic? Explain the energy transfer.
Exothermic. Thermal energy transfers to the surroundings.
Sugar dissolves in water. Physical or chemical? Explain.
Physical. No new substance forms.
Name FOUR signs that may indicate a chemical reaction.
Colour change, gas, precipitate, and energy change (heat or light).
Hydrogen peroxide breaks down into water and oxygen. Write the word equation.
hydrogen peroxide → water + oxygen
Mg + 2HCl → MgCl₂ + H₂. Count Mg, H and Cl atoms on EACH side.
Each side: Mg = 1, H = 2, Cl = 2.
Total = 5 atoms per side.
An instant cold pack becomes cold. Is the process exothermic or endothermic? Where does thermal energy move?
Endothermic. Thermal energy transfers from the surroundings into the pack.
A candle burns. Identify one physical change and one chemical reaction.
Physical: wax melts.
Chemical: wax burns, forming carbon dioxide and water.
Boiling water produces bubbles. Does this prove a chemical reaction? Explain.
No. The bubbles are water vapour. No new substance forms.
Magnesium reacts with hydrochloric acid to form magnesium chloride and hydrogen. Write the word equation.
magnesium + hydrochloric acid → magnesium chloride + hydrogen
A reaction takes place in a sealed container. Its mass is 150 g before the reaction. What is its mass afterwards? Explain.
150 g. No matter escapes or enters, so mass is conserved.
A reaction mixture cools from 22°C to 15°C without external cooling. Exothermic or endothermic? Explain.
Endothermic. The reaction absorbs thermal energy from the solution, so its temperature falls.
A student says: Every change that is hard to reverse is chemical. Is this a reliable rule? Explain using torn paper.
No. Torn paper is a physical change: no new substance forms.
Two solutions react. A solid forms and the temperature rises. Identify TWO signs and name the energy change.
Precipitate and temperature rise.
Exothermic: thermal energy transfers to the surroundings.
Methane burns in oxygen to produce carbon dioxide and water. Write the word equation.
methane + oxygen → carbon dioxide + water
CH₄ + 2O₂ → CO₂ + 2H₂O. Count C, H and O on each side. How does this show conservation of mass?
Each side: C = 1, H = 4, O = 4.
The same atoms rearrange, so mass is conserved.
Compare the direction of thermal energy transfer in a burning candle and an instant cold pack.
Candle: exothermic, energy to surroundings.
Cold pack: endothermic, energy from surroundings.