GCSE · Chemistry · Edexcel · Spec 1CH0

Properties of ionic compounds

Salt is made entirely of charged particles, yet a salt crystal won't conduct electricity. Melt it, and suddenly it will.

Heat a crystal of sodium chloride

Temperature: 20 °C. State: Solid: ions locked in place. static

Temperature20 °C

Below 801 °C, its melting point, sodium chloride is a solid. Every ion sits in a fixed position in a giant lattice, pulled on from all directions by the oppositely charged ions around it. Heat it and the ions jiggle harder, but none of them can move away. So they can't carry charge, and solid sodium chloride does not conduct electricity, even though it is made entirely of charged particles.

Each dot is one ion. Drag the temperature up slowly and keep asking one question: can the ions move?

High melting and boiling points

Reason it through

Why do you have to heat sodium chloride all the way to 801 °C before it melts?

Link 1 of 4

First link · your turn

What actually holds the lattice together?

Locked — reveal the link above first

What carries the charge?

Charged is not the same as conducting

Molten sodium chloride conducts electricity. So does sodium chloride dissolved in water.

Something must be carrying the charge. Which is closest to what you think right now?

Electrons: the ones that were transferred when the sodium and chloride ions formed.Ions: but only once they are free to move, as in a liquid or a solution.Electrons flowing through the lattice from one ion to the next.Ions: and because the solid is packed with ions, solid salt conducts just as well.

How sure are you?

I'm sureI think soI'm guessing

Electrons: the ones that were transferred when the sodium and chloride ions formed.

This is a common misconception

A really natural guess, because electrons were transferred to make the ions. But those electrons are now part of the ions themselves. Melting or dissolving doesn't set any electrons loose. What it frees is the ions, and it's the moving ions that carry the charge.

Ions: but only once they are free to move, as in a liquid or a solution.

This matches the evidence

Exactly right. The ions themselves carry the charge. In the solid they are held in fixed positions, so they can't. Melt the compound or dissolve it and the ions are free to move, so they can carry charge through the liquid or the solution.

Electrons flowing through the lattice from one ion to the next.

This is a common misconception

If electrons could flow through the lattice, solid sodium chloride would conduct, and it doesn't. In an ionic compound the charge carriers are the ions, and in the solid every ion is stuck in a fixed position.

Ions: and because the solid is packed with ions, solid salt conducts just as well.

This is a common misconception

Half right! Ions are the charge carriers. But being charged isn't enough: they have to be able to move. In the solid, every ion is held in a fixed position in the lattice, so solid sodium chloride does not conduct electricity.

Ions carry the charge, not electrons. And charged isn't enough: the ions must be free to move, which happens when the compound is molten or dissolved.

Dissolving an ionic compound

Most ionic compounds dissolve in water. Step through what happens to the lattice.

Water meets the latticeDrop a crystal into water. The water molecules are attracted to the ions in the lattice.
The ionic attractions are overcomeThe ionic forces of attraction holding those ions in the lattice are overcome.
The ions separateThe ions separate from the lattice and spread out among the water molecules.

Solubility

Do all ionic compounds dissolve equally well?

At 20 °C, about 35.7 g of sodium chloride dissolves in 100 g of water. Lithium chloride is another ionic compound. How many grams of it do you think dissolve in 100 g of water at the same temperature?

Your estimate

60g

0g120g

Watch out: These figures are here to show the difference, not to be learned by heart.

Your turn to explain

Put it all together in your own words

Sodium chloride is an ionic compound. Explain why sodium chloride has a high melting point, and why it conducts electricity when molten but not when it is solid. [5 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

Sodium chloride is a giant lattice of oppositely charged (positive and negative) ions. (1 mark)
The ions are held by strong electrostatic forces of attraction (ionic bonds) acting in all directions. (1 mark)
A large amount of energy is needed to overcome these forces, so the melting point is high. (1 mark)
In the solid, the ions are held in fixed positions, so they cannot move and cannot carry charge. (1 mark)
When molten, the ions are free to move, so they can carry charge. (1 mark)

Model answer

Sodium chloride has a giant lattice structure made of positive and negative ions. Strong electrostatic forces of attraction act in all directions between these oppositely charged ions. A large amount of energy has to be transferred by heating to overcome these forces and let the ions break free, so the melting point is high. In the solid, the ions are held in fixed positions in the lattice, so they cannot move and cannot carry charge, which is why the solid does not conduct. When it is molten, the ions are free to move, so they can carry charge and the liquid conducts.

Structure → forces → what the ions can do. Get those three links in order and the explanation writes itself.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Why salt needs 801 °C to melt, and why it only conducts once its ions are free to move

What you need to know

The structure of an ionic compound: a giant lattice of oppositely charged ions held by strong forces acting in all directions
Why ionic compounds have high melting and boiling points
How to work out a compound's state from its melting and boiling points
Why ionic compounds conduct when molten or dissolved, but not when solid
What happens to the ions when an ionic compound dissolves, and what solubility means

The big picture

An ionic compound is a giant lattice of oppositely charged ions, held together by strong electrostatic forces of attraction acting in all directions. Overcoming those forces takes a large amount of energy, so ionic compounds have high melting and boiling points and are solids at room temperature. In the solid the ions are held in fixed positions, so it cannot conduct electricity; when the compound is molten or dissolved in water, the ions are free to move and carry the charge. Most ionic compounds dissolve in water, but how much dissolves at a given temperature, the solubility, differs from one compound to another.

Key points

1Ionic compounds are giant lattices of oppositely charged ions. The strong electrostatic forces of attraction between them act in all directions; these forces are the ionic bonds.

2Melting frees the ions from the lattice so they can move, and boiling separates them into a gas. Both need a large amount of energy to overcome the strong forces, so melting and boiling points are high.

3Below the melting point a compound is a solid, between the melting and boiling points it is a liquid, and above the boiling point it is a gas.

4Solid ionic compounds don't conduct electricity because their ions are held in fixed positions. Molten or dissolved, the ions are free to move and carry the charge. The charge is carried by ions, not electrons.

5Most ionic compounds dissolve in water: water molecules are attracted to the ions, the ionic attractions are overcome, and the ions spread out among the water molecules. Solubility, how well a solute dissolves at a particular temperature, differs between compounds.

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Worked example

Problem

Magnesium oxide is an ionic compound that melts at almost 3000 °C. A sample of it is heated to 2000 °C. What state is it in, and does it conduct electricity at that temperature?

⚠ Watch out

Saying electrons carry the charge when a molten or dissolved ionic compound conducts. It's the ions that move and carry the charge, which is exactly why the compound has to be melted or dissolved first.

🧠

Memory hook

Think of the ions as a crowd with their feet glued to the floor. Everyone is charged up, but nobody can go anywhere. Melt it or dissolve it and the glue lets go, and a crowd that can move can carry charge.

✓

Check yourself

Close the page and sketch two quick pictures: a few ions in the solid lattice, and the same ions dissolved in water. Label the one that conducts, and write one 'because' under each picture.

Flashcards

(13)

Describe the structure of an ionic compound.

A giant lattice (giant ionic structure) of oppositely charged ions.

What are the ionic bonds in an ionic compound?

The strong electrostatic forces of attraction between oppositely charged ions, acting in all directions through the lattice.

Why do ionic compounds have high melting points?

To melt, the ions must break free of the lattice so they can move. A large amount of energy must be transferred by heating to overcome the strong forces between the oppositely charged ions.

Why do ionic compounds have high boiling points?

A lot of energy is needed to overcome the strong attraction between the positive and negative ions in the liquid, so that they can move freely as a gas.

What is a melting point?

The temperature at which a substance changes from the solid state to the liquid state.

What is a boiling point?

The temperature at which a substance changes from the liquid state to the gas state.

How do you use melting and boiling points to find the state at a given temperature?

Below the melting point: solid. Between the melting and boiling points: liquid. Above the boiling point: gas.

What state are ionic compounds in at room temperature?

Solid.

Why does a solid ionic compound not conduct electricity?

Its ions are held in fixed positions in the lattice, so they cannot move and cannot act as charge carriers.

In which two situations does an ionic compound conduct electricity, and why?

When molten and when dissolved in water. In both, the ions are free to move and carry the charge.

When a molten or dissolved ionic compound conducts, what carries the charge?

The moving ions. Not the electrons that were transferred when the ions formed.

What happens to the ions when an ionic compound dissolves in water?

Water molecules are attracted to the ions, the ionic forces of attraction are overcome, and the ions separate and spread out among the water molecules to form a solution.

What is solubility?

How well a solute dissolves in a solvent at a particular temperature. It differs from one ionic compound to another.

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Heat a crystal of sodium chloride

Charged is not the same as conducting

Do all ionic compounds dissolve equally well?

Put it all together in your own words

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