Where did the salt go? — answer key

1. Q01 · Practice

Model answer. Water is the solvent because the salt dissolves in it. The whole salt-water mixture is the solution.

A full answer includes

• I named water as the solvent.
• I explained that salt dissolves in the water.

Also accept

• Water, because it is the substance in which the salt has dissolved.

Watch for

• “Salt is the solvent.” — Salt is the substance that became dissolved. The solvent is the medium it dissolved in: water.
• “Water is the solvent because every solvent is water.” — Water is the solvent in this case because salt dissolves in it. Some solutions use a different solvent.

2. Q02 · Practice

Model answer. Salt is the solute. It has spread through the water in dissolved form. Losing visible crystals doesn't mean the salt was destroyed.

A full answer includes

• I identified salt as the solute.
• I stated that it is dispersed/dissolved through the water and has not vanished or turned into water.

Also accept

• Salt remains present in dissolved form, even though its crystals are not visible.

Watch for

• “The salt has become water.” — Dissolving changes how the salt is distributed. Water and the dissolved salt are still different components.
• “Invisible means absent.” — Our eyes don’t detect the dispersed salt particles individually. That limits what we can conclude from appearance.

3. Q03 · Practice

Model answer. Ethanol is the solvent because iodine dissolves in it. Iodine is the solute because it is the substance that becomes dissolved. The solution is the resulting homogeneous mixture of ethanol and dissolved iodine, not either substance alone.

A full answer includes

• Ethanol is the solvent because iodine dissolves in it.
• Iodine is the solute because it is the substance that becomes dissolved.
• Solution: the homogeneous mixture containing both ethanol and dissolved iodine.

Also accept

• The whole iodine-in-ethanol liquid mixture is the solution.

Watch for

• “There is no solvent because there is no water.” — The question tells you that iodine dissolves in ethanol. That gives ethanol the solvent role.
• “Iodine alone is the solution.” — Iodine is one component. A solution includes both the solvent and the dissolved solute.

4. Q04 · Practice

Model answer. The salt became dispersed through the water and remained present. Obtaining salt again after removing the water supports that explanation and contradicts the claim that it was destroyed.

A full answer includes

• I stated that salt remained present in dissolved form when crystals were no longer visible.
• I used recovery of salt after water removal as evidence against destruction.

Also accept

• The salt dissolved; it did not disappear from the sample. Its recovery provides evidence.

Watch for

• “Water made new salt when it was removed.” — No source of new salt is provided. The original salt remained in the solution and became visible again as water was removed.
• “It melted because crystals vanished.” — The observations describe salt dissolving in water. Losing visible crystals alone doesn’t show a solid becoming a pure liquid.

5. Q05 · Practice

Options as numbered on the pupil sheet: 1 — Pure substance; 2 — Homogeneous mixture (a solution in these cases); 3 — Heterogeneous mixture.

Model answer. B is a sugar solution, a homogeneous mixture: dissolved sugar is distributed uniformly throughout the water. C is a heterogeneous mixture: sand remains a separate undissolved solid. Pure water A contains one substance, so it belongs to neither category of mixture.

A full answer includes

• B is a solution/homogeneous mixture because sugar is dissolved throughout water.
• C is a heterogeneous mixture because sand remains a separate undissolved solid; the whole sample is not a homogeneous solution.

Also accept

• B has dissolved sugar spread through water. C still has separate, undissolved sand; the whole sample isn't a solution.

Watch for

• “Every liquid mixture is a solution.” — A solution has dissolved material mixed throughout. C contains a separate undissolved solid.
• “B is pure because the sugar is invisible.” — B still contains sugar and water. Invisible dissolved material is still a component.

6. Q06 · Practice

Model answer. The supported statements are ‘Appearance alone can’t establish whether X is pure’ and ‘Y is a solution even though it is blue’. X could be pure or could contain dissolved material. Y contains water and dissolved copper(II) sulfate. ‘X must be pure because it is clear’ and ‘A blue liquid can’t be a solution’ are both unsupported.

A full answer includes

• Appearance alone cannot establish whether X is pure or a solution.
• Y is a solution because copper(II) sulfate is dissolved in water.
• I explained that a clear liquid can contain dissolved solute and that a solution may be coloured.

Also accept

• Clear does not establish purity, and coloured does not rule out a solution.

Watch for

• “X is definitely a solution.” — It might be, but no composition is supplied. The correct conclusion acknowledges the missing evidence.
• “Blue means solid particles are suspended.” — Y is explicitly described as homogeneous, with the solid fully dissolved. Colour doesn’t by itself show suspended grains.

7. Q23 · Apply

Model answer. Water is the solvent and dissolved oxygen is the solute. Both are included in the solution. Oxygen begins as a gas in this case; a solute doesn’t have to be a visible solid.

A full answer includes

• Water is the solvent.
• Dissolved oxygen is the solute.
• The solution includes water and dissolved oxygen.
• I explained that a dissolved solute can be a gas and need not form visible grains/bubbles.

Also accept

• A homogeneous water-oxygen mixture is the stated solution; no solid is required.

Watch for

• “Only solids can be solutes.” — The role describes a substance being dissolved. In this given case oxygen gas is dissolved in water.
• “Bubbles are needed for a gas to count as dissolved.” — Separate visible bubbles are a gas phase. The question describes oxygen dispersed in the liquid without bubbles.

8. Q24 · Apply

Options as numbered on the pupil sheet: 1 — Pure substance; 2 — Homogeneous mixture (a solution in these cases); 3 — Heterogeneous mixture.

Model answer. A is the pure substance, B is the solution, and C is a heterogeneous mixture instead of a solution as a whole. The distinction comes from component identity and whether the second component is dissolved, not from the shapes themselves.

A full answer includes

• A is pure water because only the water component is present.
• B is a homogeneous solution because dissolved sugar is mixed uniformly throughout water.
• C is a heterogeneous mixture because sand remains undissolved and separate.

Also accept

• A: one substance. B: solvent and dissolved solute. C: water and separate solid.

Watch for

• “The circles are empty space.” — Read the key: circles represent the water component. The drawing is a labelled composition map.
• “B has two layers because it has two shapes.” — Both symbols are distributed throughout B. Different symbols distinguish components, not layers.