Physical vs chemical changes

Year 8 answers

Fluency

(a) P, (b) C, (c) C, (d) P, (e) C, (f) P.
Colour change, gas produced, precipitate formed, temperature change, light/sound emitted.
(a) Any reasonable physical change, e.g. melting butter, dissolving sugar, cutting vegetables. (b) Any reasonable chemical change, e.g. baking a cake, frying an egg, browning toast.
False — mass is conserved in a sealed container.
Each pigment is still the same substance after mixing; you could in principle separate them again. No new substance has formed, so the change is physical.

Fluency

Oxygen ( $\text{O}_2$ ).
Carbon dioxide ( $\text{CO}_2$ ).
A lit splint at the mouth of the tube gives a squeaky pop.
Not hydrogen (no pop). Could be nitrogen, which gives no response to any of those tests.
Calcium hydroxide, $\text{Ca(OH)}_2$ — it reacts with CO $_2$ to form calcium carbonate, a white solid that clouds the water.

Reasoning

In a chemical change, atoms are rearranged into new groupings (new molecules), which have new properties. In a physical change the atoms stay in the same molecules — only their arrangement or state changes.
Rusting adds oxygen atoms from the air to the iron atoms, forming iron oxide. Total mass of iron + oxygen before equals mass of rust after. Mass was not lost — the rust is heavier than the iron was.
Endothermic reactions absorb energy from the surroundings, so the reaction mixture feels cold. Whether heat is taken in or given out, a new substance has formed, so it is still a chemical reaction.
Bubble the gas through limewater. If it turns cloudy/milky, the gas is CO $_2$ .

Problem solving

Evidence: bright white light given off; new white powder formed (was silvery metal). Word equation: magnesium + oxygen $\to$ magnesium oxide.
Less than 200 g. Carbon dioxide escaped as gas, taking some mass with it.
Chemical change. A precipitate (the yellow solid, lead iodide) has formed — a new substance.
Different gases can sometimes come from similar-looking reactions, and some reactions produce a mixture. Testing confirms which gas you actually have and avoids wrong conclusions.

Reasoning

The extra $6.6$ g came from oxygen atoms in the air that combined with the magnesium to form magnesium oxide. Total mass (Mg + O + crucible) is still conserved; it just was not all on the scales at the start.
The gas does not react with limewater (rules out CO $_2$ ) but re-lights a glowing splint — so it is oxygen. Example lab preparation: decomposition of hydrogen peroxide using a catalyst (manganese dioxide): hydrogen peroxide $\to$ water + oxygen.
Combustion is a fuel reacting rapidly with oxygen to give out heat and light. It is exothermic because it releases energy. Everyday example: a gas stove burning natural gas.
carbonic acid $\to$ water + carbon dioxide. The CO $_2$ leaves as a gas, so the liquid volume shrinks — it looks like something has “been lost,” but in a sealed container the total mass is unchanged.