Physical and chemical changes — final check key

1. PK1 · Final check

Model answer. (a) The limewater turns milky, so the gas is carbon dioxide. There was no carbon dioxide before the lump was heated: it was made from the calcium carbonate, so it is a new substance. A gas coming off would on its own be only a clue; the limewater test identifies it. (b) Calcium carbonate does nothing with water, but the solid left reacts with it and gets so hot that it steams. It has a property that the starting solid did not have, so it is a new substance, calcium oxide, even though it is white too.

A full answer includes

• (a) I identified the gas as carbon dioxide.
• (a) I wrote that the carbon dioxide was not there before heating, so it was made in the change.
• (a) I did not write that any gas coming off is proof: the limewater test shows which gas it is.
• (b) I wrote that the solid left behaves differently with water from calcium carbonate.
• (b) I wrote that it is a new substance, so a chemical change happened.
• (b) I did not write that the lump is unchanged because it is still white.

Also accept

• (a) Carbon dioxide; it was not there before heating, so it is a new substance. (b) The solid left has a new property, so it is a new substance.

Watch for

• “The lump still looks white, so it is the same substance.” — A new substance can look like the old one. The test with water shows that the solid left has different properties.
• “Any gas given off proves that a new substance was made.” — A gas comes off in physical changes too, such as steam from boiling water. The limewater test shows that this gas is carbon dioxide, which was not there at the start.

2. PK2 · Final check

Model answer. B: silver is shiny, but the black layer rubs off as a dull powder, so the layer has a property that silver does not have. It is a new substance, made when silver reacts with gases in the air: a chemical change, decided by a new property. A: the dull white surface is the chocolate's own fat and sugar setting as crystals, so it is still chocolate: a physical change, and looking different is only a clue. C: the grains are sugar coming out of the honey as crystals. The sugar was in the honey all along, so no new substance is made: a physical change. D: melting gold is a physical change, so the kind is right, but the reason is not: some chemical changes can go back too. The ring is physical because it is still gold, with the same properties.

A full answer includes

• I chose row B.
• I did not choose row D: the kind is right, but melting it again is not the evidence that decides it.
• I did not choose row C: a solid appearing is only a clue.

Also accept

• B

Watch for

• “In row A the dull white surface shows a new substance.” — Looking different is only a clue. The white surface is the chocolate's own fat and sugar setting as crystals, so it is still chocolate: a physical change.
• “In row C the grains that appear are a new substance.” — A solid appearing is only a clue. The sugar was dissolved in the honey all along and has come out as crystals: no new substance, a physical change.
• “In row D the gold is physical because it can be melted again.” — The kind is right, but the reason is not: some chemical changes go back too. The ring is physical because it is still gold, with the same properties.

3. PK3 · Final check

Model answer. (a) Cutting changes only the shape. Each piece is still shiny and still conducts, so it is still magnesium: a physical change. (b) The white powder does not conduct, but magnesium does, so the powder has different properties: a new substance, a chemical change. The bright light is a clue that energy is given out, not proof. (c) The magnesium atoms join with oxygen atoms from the air into a new substance, magnesium oxide. No atom is made or destroyed. (d) Any test that gives different results for the two will do. For example, add dilute hydrochloric acid: magnesium fizzes and gives off hydrogen, but the white powder dissolves without fizzing. Or hold it in a flame: magnesium burns, but the powder does not burn again. Colour alone would not decide it: it is only a clue.

A full answer includes

• (a) I wrote: a physical change.
• (a) I wrote that the pieces have the same properties: they are shiny and they conduct.
• (b) I wrote: a chemical change.
• (b) I gave the evidence: the powder does not conduct, but magnesium does.
• (b) I did not give the bright light as the proof: it is only a clue.
• (c) I wrote that magnesium atoms join with oxygen atoms from the air.
• (c) I wrote that they make a new substance, magnesium oxide.
• (d) I suggested a test that gives different results for magnesium and the powder, such as adding dilute hydrochloric acid.
• (d) I gave the expected results: magnesium fizzes, but the powder does not.
• (d) I did not suggest comparing colours: a colour is only a clue.

Also accept

• (a) Physical: the same properties. (b) Chemical: the powder does not conduct. (c) Magnesium atoms join with oxygen atoms into magnesium oxide. (d) A test such as dilute acid: magnesium fizzes, the powder does not.

Watch for

• “Cutting the ribbon makes a new substance, because the pieces look different.” — The pieces are only smaller. They are still shiny and still conduct, so they are still magnesium.
• “The bright light proves a chemical change.” — The light is a clue that energy is given out. The proof is the powder: it does not conduct, so it is not magnesium.
• “Comparing colours is enough to tell two substances apart.” — Colour is only a clue: different substances can look alike. A test that gives different results, such as fizzing in acid, decides it.

4. PK4 · Final check

Model answer. Left: 3 carbon and 8 hydrogen atoms in C₃H₈, and 5 × 2 = 10 oxygen atoms in 5O₂. Right: 3 carbon and 3 × 2 = 6 oxygen atoms in 3CO₂, and 4 × 2 = 8 hydrogen and 4 oxygen atoms in 4H₂O, so 6 + 4 = 10 oxygen atoms. Both sides have 3 carbon, 8 hydrogen and 10 oxygen atoms. The C₃H₈ and O₂ molecules break up, and their atoms join into CO₂ and H₂O molecules, which were not there before: new substances, so a chemical change. Each kind of atom has the same number on both sides, so no atom is made or destroyed; the atoms are only joined in new ways.

Checked automatically

• Carbon atoms on the left: 3 (exact value)
• Hydrogen atoms on the left: 8 (exact value)
  • Common wrong answer 3 : That is the carbon. C₃H₈ has 8 hydrogen atoms.
• Oxygen atoms on the left: 10 (exact value)
  • Common wrong answer 5 : Each O₂ molecule has 2 oxygen atoms, so 5O₂ has 5 × 2 = 10.
  • Common wrong answer 2 : The 5 in front means five O₂ molecules: 5 × 2 = 10 oxygen atoms.
• Carbon atoms on the right: 3 (exact value)
  • Common wrong answer 1 : The 3 in front of CO₂ means three molecules, each with one carbon atom.
• Hydrogen atoms on the right: 8 (exact value)
  • Common wrong answer 4 : Each H₂O molecule has 2 hydrogen atoms, so 4H₂O has 4 × 2 = 8.
  • Common wrong answer 2 : The 4 in front means four H₂O molecules: 4 × 2 = 8 hydrogen atoms.
• Oxygen atoms on the right: 10 (exact value)
  • Common wrong answer 6 : That is the oxygen in 3CO₂ only. Add the 4 oxygen atoms in 4H₂O.
  • Common wrong answer 7 : Each CO₂ has 2 oxygen atoms: 3 × 2 = 6, then add 4 from 4H₂O.

A full answer includes

• I found 3 carbon atoms on each side.
• I found 8 hydrogen atoms on each side.
• I worked out the oxygen as 5 × 2 on the left and 3 × 2 + 4 × 1 on the right.
• I found 10 oxygen atoms on each side.
• I did not count 5 oxygen atoms on the left: 5O₂ has 10.
• I wrote that the atoms join up in new ways into new molecules, carbon dioxide and water.
• I wrote that no atoms are made or destroyed, because each kind of atom has the same number on both sides.

Also accept

• 3 C, 8 H and 10 O on each side. Chemical: the atoms join into new molecules, and none are made or destroyed.

Watch for

• “In a chemical change some atoms are destroyed.” — Atoms are never made or destroyed. They join up in new ways, so new substances form.
• “Burning is a change of state, like melting.” — Burning is a reaction with oxygen that makes new substances, such as carbon dioxide and water. Melting makes no new substance.
• “5O₂ has 5 oxygen atoms.” — Each O₂ molecule has 2 oxygen atoms, so 5O₂ has 5 × 2 = 10 oxygen atoms.