Answers · Physical and chemical changes

Physical and chemical changes — return questions key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. PR1 · Return

Model answer. B: glucose dissolves in water, but the black solid does not, so the solid has a property that the glucose did not have: a new substance. A: the iodine turned to a vapour and back into the same crystals, a change of state. C: the colour came back when the powder cooled; it is still titanium dioxide. D: the solder melted and set into a new shape, and it still conducts: the same substance.

A full answer includes

I chose row B.
I did not choose row C: a colour that comes back on cooling is only a clue.

Also accept

Watch for

“In row A the crystals on the upper glass are a new substance.” — They are the same purple-black iodine crystals. The iodine turned to a vapour and back: a change of state.
“In row C the colour change proves a new substance.” — The powder was yellow only while it was hot and white again when cold. A colour that comes back is only a clue; it is still titanium dioxide.
“In row D the new shape means a new substance.” — The solder still conducts electricity, as before. A new shape is a physical change.

2. PR2 · Return

Model answer. (a) When the wire is bent, the same copper atoms only move into a new shape: it is still copper, with no new kind of particle, so no new substance, a physical change. (b) When hydrogen burns in chlorine, the H₂ and Cl₂ molecules break up and their atoms join in new ways into HCl molecules: a new kind of particle, a new substance, a chemical change.

A full answer includes

(a) I wrote that the copper atoms stay the same and are only arranged in a new shape.
(a) I wrote that bending the wire makes no new substance: a physical change.
(a) I did not write that the new shape is a new substance.
(b) I wrote that hydrogen and chlorine atoms join up in new ways, into hydrogen chloride molecules.
(b) I wrote that burning hydrogen in chlorine makes a new substance: a chemical change.

Also accept

(a) The same copper atoms, only in a new shape: physical. (b) The atoms join in new ways into HCl, a new substance: chemical.

Watch for

“A new shape means a new substance.” — The spring is still copper: it is shiny and it conducts, like the wire. Only the arrangement of the same copper atoms has changed.
“When hydrogen burns in chlorine, the atoms turn into new atoms.” — The atoms stay hydrogen and chlorine atoms. They are only joined in new ways, into HCl molecules.

3. PR3 · Return

Model answer. The bubbles are ethanol vapour: the same ethanol molecules, far apart. Boiling is a change of state, so no new substance is made. Bubbles show only that a gas formed, not that it is a new substance. The ethanol staying colourless is not the reason: a chemical change can give colourless products too.

A full answer includes

I chose: the bubbles are ethanol vapour, so it is a physical change.
I did not choose "the ethanol stays colourless": an unchanged colour does not decide the kind of change.

Also accept

The bubbles are ethanol vapour: physical

Watch for

“Bubbles show that a new substance was made.” — These bubbles are ethanol vapour: ethanol as a gas, still ethanol. Nothing new was made.
“Ethanol contains carbon, so the bubbles are carbon dioxide.” — Carbon dioxide forms only if ethanol burns. Boiling makes ethanol vapour, the same molecules.
“It is physical because the ethanol stays colourless.” — It is physical, but not for that reason: a chemical change can give colourless products too. It is physical because the bubbles are ethanol, so no new substance is made.
“Boiling breaks the molecules into atoms.” — Boiling only moves the molecules apart. Each ethanol molecule stays whole.