Molecules that melt easily — questions

1. identify

The table gives data for four substances. Which are likely to be simple molecular substances? Choose all that apply.

Tick every correct statement.

• X
• W
• Z
• Y

Nudge (only if you are stuck): Look at both the temperatures and the conductivity columns.

2. identify

Which of these changes needs the least energy, for the same amount of substance?

Circle one answer.

• Boiling liquid nitrogen, N₂
• Splitting N₂ molecules into atoms
• Melting solid iron, Fe
• Melting magnesium oxide, MgO

Nudge (only if you are stuck): For each change, ask what has to be pulled apart.

3. identify

Sulfur is a simple molecular substance, and calcium chloride, CaCl₂, is an ionic compound. Which row shows whether each one conducts electricity?

Circle one answer.

• A
• B
• C
• D

Nudge (only if you are stuck): For each one, ask: are there charged particles, and can they move?

4. explain

A pupil dissolves sugar in one beaker of pure water and potassium chloride, KCl, in another. The potassium chloride solution lights a bulb in a circuit, but the sugar solution leaves it dark. Which statement explains the difference?

Circle one answer.

• KCl gives free electrons; sugar holds on to all its electrons
• KCl splits the water into ions; sugar leaves the water whole
• KCl gives ions that can move; sugar gives uncharged molecules
• KCl gives small particles; sugar molecules are too large to move

Nudge (only if you are stuck): Which particles in each beaker carry a charge?

5. identify

Supplement. Match each description to a covalent bond or an intermolecular force in ammonia, NH₃.

Write the number of the matching option.

1. Covalent bond
2. Intermolecular force

• Holds the N and H atoms together inside one NH₃ molecule: ______
• Attracts one NH₃ molecule to the next: ______
• Is overcome when liquid ammonia boils: ______
• Is not overcome when liquid ammonia boils: ______
• Needs far more energy to overcome: ______

Nudge (only if you are stuck): Picture two NH₃ molecules side by side. Which attraction acts inside one, and which across the gap?

6. deduce / explain / state

Supplement. Propane, C₃H₈, melts at −188 °C and boils at −42 °C. Pentane, C₅H₁₂, melts at −130 °C and boils at 36 °C. Both are simple molecular substances. (a) Deduce which one has the stronger intermolecular forces. (b) Explain your answer in terms of energy. (c) State which one is a liquid at 20 °C.

(a) Stronger intermolecular forces

(b) The reason, in terms of energy

(c) The liquid at 20 °C

Nudge (only if you are stuck): What does boiling overcome, and which substance needs the higher temperature?

7. explain

Supplement. Iodine, I₂, turns to a purple gas when warmed gently. A pupil writes: “Iodine turns to a gas easily because the covalent bond between its two atoms is weak.” Which statement correctly explains it?

Circle one answer.

• Warming overcomes weak forces between I₂ molecules; the bonds stay
• Warming breaks the weak covalent bonds, giving iodine atoms
• Covalent bonds are strong, so iodine cannot become a gas easily
• Iodine is ionic, and its ions separate when it is warmed

Nudge (only if you are stuck): What is the purple gas made of?

8. deduce / describe / explain

The table shows three Group VII elements, each made of molecules with two atoms. (a) Deduce the state of each element at 20 °C. (b) Describe, in terms of structure and bonding, why all three have low melting and boiling points compared with potassium iodide, an ionic compound that melts at 681 °C. (c) Supplement: the boiling points rise down the group. Deduce what this shows about the intermolecular forces, and explain your answer.

(a) States at 20 °C

(b) Low melting and boiling points

(c) Supplement: down the group

Nudge (only if you are stuck): For (a), compare 20 °C with each melting point and boiling point.