Molecules that melt easily — answer key

1. VQ11 · Practice

Model answer. W (melts at −95 °C, boils at 69 °C) and Y (melts at 44 °C) have low melting and boiling points and don’t conduct: simple molecular. X melts at 734 °C and conducts only when molten, like an ionic compound. Z melts at 1085 °C and conducts as a solid, like a metal.

A full answer includes

• I chose W and Y.
• I ruled out X: it melts at 734 °C and conducts only when molten, like an ionic compound.
• I ruled out Z: it conducts as a solid, like a metal.
• I did not rule out Y for being a solid at room temperature.

Also accept

• W and Y.

Watch for

• “Y melts at 44 °C, which is above room temperature, so it can’t be molecular.” — Many molecular substances are solids, such as wax. Compare with the hundreds of degrees of X and Z.
• “X is molecular, because it does not conduct as a solid.” — X melts at 734 °C and conducts when molten: its ions can move once it melts. That is an ionic compound.
• “Z is molecular, because it is a solid at room temperature.” — Being a solid is not the test. Z melts at 1085 °C and conducts as a solid, like a metal.

2. VQ12 · Practice

Model answer. Boiling liquid nitrogen only moves whole N₂ molecules apart. The molecules attract each other only weakly, so little energy is needed: nitrogen boils at −196 °C. Splitting N₂ molecules into atoms means breaking the strong triple covalent bond inside each molecule, which needs far more energy. Melting magnesium oxide means pulling apart ions that strongly attract each other in a giant lattice (it melts at 2852 °C), and iron melts only at 1538 °C.

A full answer includes

• I chose boiling liquid nitrogen.
• I can say why: boiling only separates whole N₂ molecules.
• I can say why: N₂ molecules attract each other only weakly.
• I did not choose splitting N₂ molecules: that breaks strong covalent bonds.

Also accept

• Boiling liquid nitrogen, N₂.

Watch for

• “Splitting N₂ molecules into atoms needs the least energy, because covalent bonds are weak.” — Covalent bonds are strong: the triple bond in N₂ is one of the strongest. Boiling only separates whole molecules, which needs far less energy.
• “Melting magnesium oxide needs the least energy, because melting is easier than boiling.” — Melting magnesium oxide means pulling apart ions that attract strongly in a giant lattice: it melts at 2852 °C. Nitrogen boils at −196 °C.
• “Melting iron needs the least energy, because melting is easier than boiling.” — Iron melts at 1538 °C, so it needs a lot of energy. Nitrogen molecules separate at −196 °C.

3. VQ13 · Practice

Model answer. Sulfur is made of molecules with no overall charge, and its electrons stay in the atoms’ shells and shared pairs, so it has no charged particles that can move, solid or molten. Calcium chloride is made of ions: in the solid they are held in place, so it does not conduct; when it melts they can move and carry the current. Row A.

A full answer includes

• I chose row A.
• I can say why sulfur never conducts: its molecules carry no overall charge.
• I can say why only molten calcium chloride conducts: its ions can move only after it melts.
• I did not think molten sulfur conducts because it is a liquid.

Also accept

• Row A

Watch for

• “Row B.” — A liquid conducts only if it has charged particles that can move. Molten sulfur is still uncharged molecules.
• “Row C.” — Solid calcium chloride has ions, but they are fixed in the lattice. They carry a current only when they are free to move, after melting.
• “Row D.” — When calcium chloride melts, its ions are free to move, so molten calcium chloride conducts.

4. VQ14 · Practice

Model answer. Sugar is a simple molecular substance: it dissolves as separate sugar molecules, which have no overall charge, so the sugar solution has no charged particles to carry a current. Potassium chloride is ionic: when it dissolves, its K⁺ and Cl⁻ ions separate and can move through the water, so they carry the current and the bulb lights.

A full answer includes

• I chose: KCl gives ions that can move; sugar gives uncharged molecules.
• I can say why the sugar solution is dark: sugar molecules have no overall charge.
• I did not choose the option where the water splits into ions.

Also accept

• KCl gives ions that can move; sugar gives uncharged molecules.

Watch for

• “KCl splits the water into ions; sugar leaves the water whole.” — The water molecules stay whole. The charged particles come from the potassium chloride: its own ions separate and move through the water.
• “KCl gives small particles; sugar molecules are too large to move.” — Size is not the test. Sugar molecules move through the water too, but they have no overall charge, so they carry no current.
• “KCl gives free electrons; sugar holds on to all its electrons.” — Free electrons carry the current in metals. In a potassium chloride solution the moving charges are ions.

5. VQ15 · Practice

Options as numbered on the pupil sheet: 1 — Covalent bond; 2 — Intermolecular force.

Model answer. A covalent bond holds atoms together inside one molecule: in NH₃, the three N–H bonds. Covalent bonds are strong, so they need far more energy to break, and boiling leaves them in place: ammonia gas is still NH₃ molecules. An intermolecular force attracts one whole molecule to the next. It is weak, and boiling overcomes it.

A full answer includes

• I matched “inside one molecule” to a covalent bond.
• I matched “one molecule to the next” to an intermolecular force.
• I matched “overcome when liquid ammonia boils” to an intermolecular force.
• I matched “not overcome when liquid ammonia boils” to a covalent bond.
• I matched “far more energy to overcome” to a covalent bond.
• I did not match boiling to the covalent bonds.

Also accept

• Bond, force, force, bond, bond.

Watch for

• “Covalent bonds are weak, so molecular substances melt easily.” — Covalent bonds are strong. The weak attractions are between the molecules, and those are what melting and boiling overcome.
• “The covalent bonds break when the substance melts or boils.” — Melting and boiling only separate the molecules. Every covalent bond inside them stays.

6. VQ16 · Practice

Model answer. (a) Pentane. (b) Boiling overcomes the intermolecular forces between the molecules. Pentane needs a higher temperature, so more energy, to separate its molecules: its intermolecular forces are stronger. The covalent bonds inside the molecules are not broken in either case. (c) Pentane: 20 °C is above its melting point, −130 °C, and below its boiling point, 36 °C. Propane is a gas at 20 °C, above its boiling point of −42 °C.

A full answer includes

• (a) I chose pentane.
• (b) I wrote that pentane boils at a higher temperature: 36 °C, not −42 °C.
• (b) I wrote that more energy is needed to separate pentane’s molecules.
• (b) I wrote that more energy means stronger forces between the molecules.
• (b) I did not write that pentane’s covalent bonds are stronger.
• (c) I chose pentane: 20 °C is between its melting and boiling points.

Also accept

• (a) Pentane. (b) A higher boiling point: more energy to overcome the intermolecular forces. (c) Pentane.

Watch for

• “Pentane boils at a higher temperature because its covalent bonds are stronger.” — Boiling does not break covalent bonds. A higher boiling point means stronger forces between molecules.
• “Propane has the stronger forces, because it turns to a gas more easily.” — Turning to a gas easily means weak forces between molecules: little energy separates them.

7. VQ17 · Practice

Model answer. The purple gas is still I₂ molecules, so the covalent bond inside each molecule has not broken. What warming overcomes is the weak intermolecular forces between the molecules, which needs little energy.

A full answer includes

• I chose: warming overcomes weak forces between I₂ molecules; the bonds stay.
• I can say why the pupil is wrong: the purple gas is still I₂ molecules.
• I did not choose the option where the covalent bonds break.

Also accept

• Warming overcomes weak forces between I₂ molecules; the bonds stay.

Watch for

• “Covalent bonds are weak, so molecular substances melt easily.” — Covalent bonds are strong. The weak attractions are between the molecules, and those are what melting and boiling overcome.
• “The covalent bonds break when the substance melts or boils.” — Melting and boiling only separate the molecules. Every covalent bond inside them stays.
• “Iodine is ionic, and its ions separate when it is warmed.” — Iodine is a non-metal element whose atoms are joined in pairs by a covalent bond: I₂ molecules, not ions.
• “Covalent bonds are strong, so iodine cannot become a gas easily.” — The bonds are strong, but turning to a gas does not break them. It only overcomes the weak forces between molecules, which needs little energy.

8. VT2 · Apply

Model answer. (a) At 20 °C fluorine is a gas (above its boiling point, −188 °C), bromine a liquid (between −7 °C and 59 °C) and iodine a solid (below its melting point, 114 °C). (b) The halogens are simple molecular substances: separate F₂, Br₂ and I₂ molecules. The molecules attract each other only weakly, so little energy is needed to separate them. In potassium iodide, every ion is strongly attracted to the oppositely charged ions around it in a giant lattice, which needs much more energy to overcome. (c) A higher boiling point means more energy is needed to separate the molecules, so the intermolecular forces get stronger down the group (the molecules get bigger).

A full answer includes

• (a) I wrote fluorine: gas.
• (a) I wrote bromine: liquid.
• (a) I wrote iodine: solid.
• (b) I wrote that each element is made of small separate molecules.
• (b) I wrote that the molecules attract each other only weakly.
• (b) I wrote that little energy is needed to separate them.
• (b) I wrote that potassium iodide has strong attractions between oppositely charged ions.
• (b) I did not write that covalent bonds are weaker than ionic bonds.
• (c) I wrote that the intermolecular forces get stronger down the group.
• (c) I wrote that a higher boiling point means more energy to separate the molecules.

Also accept

• (a) Gas, liquid, solid. (b) Separate molecules that attract weakly, so little energy separates them; strong attractions between the ions in KI. (c) Stronger intermolecular forces down the group.

Watch for

• “The halogens melt easily because covalent bonds are weaker than ionic bonds.” — The covalent bond inside each halogen molecule is strong. The halogens melt easily because the separate molecules attract each other only weakly; in potassium iodide every ion is strongly attracted to its neighbours.
• “The boiling points rise because the covalent bonds get stronger down the group.” — Boiling does not break the covalent bonds. A higher boiling point shows stronger forces between the molecules.