One of the most important, simple things GCSE chemistry teaches you to do about understanding chemicals is to just split them up into groups. And yet, because it’s in no specific place on many syllabuses, it’s often hard to figure out what’s going on, and why different chemicals are, well, so different. I certainly didn’t really get it until after I finished my chemistry GCSE. But when you know it, it lets you understand a lot about why chemicals work the way they do, and it explains how to answer a lot of questions. So:
In five words: salt and things like salt.
The ionic chemicals you’ll see at GCSE have a lot in common. They’re hard, crystals, powdery, like salt. They’re always solid at room temperature: that means they have high melting points. Not all dissolve in water, but many do. They don’t conduct electricity when solid. They’re brittle: hit them with a hammer and they smash, not bend. Zoom in with a microscope, and you see a lattice structure: a repeating pattern inside the crystal. (Lattice means a repeating pattern, like squared paper.)
Now let’s look at what they’re made of, and why they have those properties.
Ionic chemicals are made, well, of ions: charged particles, often atoms that have given away or picked up electrons. Those ions have + and – charges, which attract one another: ionic chemicals are held together by the electrostatic attraction between + and – ions. That attraction is very strong, so ionic chemicals are solid at room temperature: to break the ionic bonds and melt salt you need to put in lots of heat energy. Salt melts at 801°C, for example.
In an ionic chemical, all the ions are locked in place. Plus ion next to minus ion next to plus ion next to minus ion. That makes them hard but brittle: they can’t bend because the ions are locked in place. And it gives them their regular, lattice structure. Given that, you might just hear ionic chemicals called ionic crystals, because they’re crystalline.
Things can conduct electricity if charged particles inside them can move, mostly electrons. And ionic chemicals don’t have any charged particles that can move. However, they do dissolve in water and melt, and that lets them conduct electricity. The ions aren’t locked in place any more, so they can move and conduct current.
Is it an ionic chemical? At GCSE, yes if it’s a metal compound: a metal reacted with some other chemical. That includes metal ores. All acids are ionic chemicals, and so are any compounds with ammonium in their name. Many chemicals in rocks are ionic chemicals, especially limestone and chalk. (Getting ready for chemistry A-level? Some metal compounds, especially ones with transition metals, in the bottom centre of the periodic table, can have covalent or covalent-ish bonds. Beyond the scope of this article, though.)
How to explain it: Why does an ionic chemical like sodium oxide have such a high melting point? Well…
- It’s an ionic chemical. (You have to say this!)
- The ions in it are held together by strong ionic bonds
- These take a lot of energy to break up.
What to watch out for: never, ever, ever say ionic chemicals are made of molecules or atoms. On some boards, that’s an instant no-marks point: as far as the examiners are concerned, nothing can redeem your answer if you say that! They’re made of ions. We’ll get to molecules in a second, don’t worry. In fact:
Molecular chemicals are a varied bunch, from the gases in the air to jet fuel to tar to the solid chemicals that make up things like leather and wood. So let’s sort of summarise them. It’s probably a molecular chemical if:
- It’s in your body and isn’t a mineral or salt
- It’s not got any metals in its formula
- It’s not a giant covalent chemical. (See below.)
- It’s found in or made from oil.
- It’s in the air and it isn’t a noble gas.
So that includes sugar, starch, proteins, enzymes, plastics, water, oxygen gas, nitrogen gas, chlorine gas, and so on.
Molecular chemicals tick some general boxes. They’re often liquid or gas at room temperature, but the more atoms in the formula the melting and boiling points tend to tick upwards. Even so, they’re certainly nothing like as durable as ionic chemicals or most metals. Salt and sugar look the same, but salt melts at 801°C and sugar at only about 150°C. (Depends on which sugar you mean, though.)
Molecular chemicals don’t conduct electricity, whether liquid, solid, or dissolved in water. And speaking of that, many like oils won’t dissolve in water, but they will in organic solvents. (That’s why many perfumes and aftershaves are bottled with alcohol: the scent chemicals won’t dissolve in water on its own.)
Why do molecules have low melting points? Well, molecules are held together with covalent bonds (where two atoms share electrons-you see them in dot and cross diagrams) which are strong. However, the only things holding one molecule to the one next door are something called intermolecular forces, which are quite weak. It’s not hard to pull one molecule away from the one next door, and it doesn’t take a lot of heat energy. That’s why most molecules melt at quite low temperatures compared to ionic chemicals and many metals.
What to say in exams: Practice explaining why they have low melting and boiling points. Take methane, for example, the gas in a gas cooker. Why does it boil at so low a temperature, -161.5 °C? The answer is:
- It’s a molecular chemical. (Obvious, but you have to say this!)
- The molecules are held to one another with intermolecular forces, which…
- …are quite weak!
- It doesn’t take a lot of heat energy to separate the molecules.
Learn those points!
These are, to be honest, a bit of a topic in themselves. There’s only a few of them, and yet they’re all very different to one another. The main ones are diamond, graphite and silicon dioxide (the main ingredient of sand, glass and many rocks). But their general properties are:
- They’re made of lots of atoms held together by covalent bonds
- They’re solid at room temperature and have very high melting points
- They won’t dissolve in water
You learn about these individually, rather than as a group. However, you do need to know why they don’t melt until you heat them up a lot. It’s because:
- All the atoms are held together with strong covalent bonds
- To melt them, you have to break the covalent bonds
- That takes a lot of heat energy.
For comparison, when you melt chemicals made of molecules like sugar, you have to make the sugar molecules let go of one another, but you don’t have to break up the sugar molecules themselves. That makes it much easier to do and need less energy.
And finally, these. Everyone knows what metals are: they’re grey (normally), hard (normally) and shiny. But let’s focus on some of their other properties:
- They conduct electricity, whether solid or as liquids
- They’re malleable: hit them with a hammer and they normally bend
Why do they have those properties? In a metal, the outer shells of electrons on the atoms become delocalised: released to move around freely. That makes them able to conduct electricity: the minus-charged electrons are free to move.
The atoms become positive ions, attracted to the minus-charged electrons. The electrons act as a kind of glue that sticks together the positive ions. People say that the positive ions are in a ‘sea’ of electrons. And like ice cubes in water, they can move around a bit, making metals far more beat-uppable than ionic crystals are.
You don’t have to worry about explaining the melting point of metals, because they vary a lot. Some melt at incredibly high temperatures, one is liquid at room temperature, another melts in the heat of the palm of your hand.
Some metal compounds, as noted above, do weird things beyond the scope of this article. Multi-atom ions like ammonium and carbonate ions have covalent bonds inside them-you’ll see this at A-level. Otherwise, the main chemicals that don’t fit into this list are probably noble gases, which exist as single atoms since they don’t need to form ionic or covalent bonds with anything else. Self-sufficiency, that’s the stuff.
The picture of a salt pan was taken by Kevin Tao, the picture of a crystal lattice model by Fovea Centralis and the picture of the holy shrine to methane molecules by Detlef Schobert. All three are released under this license. All pictures used with thanks.