Work each one out on paper first, then tap "show answer" to check. They get harder as you go, mixing word equations, calculations and reasoning questions tied to the lab.
A chemical change — a new substance (rust, iron oxide) has formed. It's not reversible by simple physical means, which is the other clue.
A physical change — no new substance forms, and refreezing it reverses the change.
magnesium + oxygen → magnesium oxide
Mass in = mass out: 4 + 3 = 7 g
This demonstrates conservation of mass — atoms are rearranged in a chemical reaction, never created or destroyed.
(a) Thermal decomposition · (b) Combustion · (c) Oxidation (rusting)
Strongly alkaline — on the UI colour chart, purple/violet corresponds to the top of the scale, around pH 13–14.
hydrochloric acid + sodium hydroxide → sodium chloride + water
(a) An acid + metal reaction.
(b) sulfuric acid + magnesium → magnesium sulfate + hydrogen
(c) Bubbles form and the magnesium ribbon gradually dissolves. Test the gas with a lit splint — a squeaky pop confirms hydrogen.
(a) hydrochloric acid + calcium carbonate → calcium chloride + water + carbon dioxide
(b) Bubble the gas through limewater — it turning cloudy confirms carbon dioxide.
Neither tube rusts.
The boiled-water-with-oil tube has water but no oxygen — boiling removed the dissolved air, and the oil layer stops more air getting in.
The drying-agent tube has air (oxygen) but no water — the drying agent absorbs all the moisture before it reaches the nail.
Rusting needs water and oxygen together — each tube is missing exactly one of the two, so neither rusts.
A physical change — still the same substance, just a different state. Cool it back down and it solidifies again.
A physical change — still wood, just smaller pieces. No new substance has formed.
copper carbonate → copper oxide + carbon dioxide
Mass in = mass out: 10 + 30 = 40 g of products in total.
Mass of water = 40 − 22 = 18 g
Same idea as always — the atoms are all still there, just repackaged, so nothing in the mass total can go missing.
(a) Oxidation · (b) Combustion · (c) Acid + carbonate reaction (vinegar is a weak acid, baking soda is a carbonate)
Acidic — orange sits on the acidic half of the UI colour chart, roughly pH 3–4.
nitric acid + potassium hydroxide → potassium nitrate + water
(a) hydrochloric acid + zinc → zinc chloride + hydrogen
(b) The gas is hydrogen. Hold a lit splint at the mouth of the tube — a squeaky pop confirms it.
(a) sulfuric acid + magnesium carbonate → magnesium sulfate + water + carbon dioxide
(b) Bubbling/fizzing as carbon dioxide gas is released, and the solid carbonate gradually dissolves and disappears.
No, it doesn't contradict the rule.
Salt doesn't change whether rusting can happen — you still need both water and oxygen present, or there's no rust at all, exactly as in the lab. What salt changes is the rate: salt water conducts electricity better than plain water, which speeds up the reaction once both required conditions are already met.
Necessary conditions (water + oxygen) and rate factors (like dissolved salt) are two different things — this question is testing whether you can tell them apart.