Four types of chemicals (GCSE Chemistry)

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:

Ionic chemicals

Salt pans evaporating salt from water in Peru
Salt pans evaporating salt from water in Peru.

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.)

At Washington's National Museum of Natural History, a model of the repeating structure of a crystal lattice.
At Washington’s National Museum of Natural History, a model of the repeating structure of a crystal lattice.

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.

The repeating, square structure of salt crystals.
The repeating, square structure of salt crystals.

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

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.

And here's a statue of a methane molecule because why not, you know? Worship me, I am your god!
And here’s a statue of a methane molecule because why not, you know? Worship me, I am your god! (It’s actually at a natural gas extraction field in the Netherlands.)

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!

Giant Covalent

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.

Anything else?
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 rock salt was taken by Piotr Włodarczyk and released under this license.

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.

Why are the lungs good at their job? (Biology AS-level)

Sometimes when understanding a subject, it helps to know what the examiners want you to understand about it. For the lungs, it’s very clear what AQA and OCR want you to know: what the lungs need to have to do their job, and how they tick all those boxes in flesh and blood. So this revision guide will go through the lungs from that angle: what do the lungs need to be like to do their job, and how do they tick those boxes?

The lungs: good thing or bad thing?

Good. (That’s a joke, not one of those silly arts-subject ‘on the one hand, yes…but on the other hand, no…’ questions.) The lungs give your body oxygen and flush out carbon dioxide. You need that for respiration.

Skip this if you’re fine with this topic, but respiration is getting energy out of food, and the main type of it your body does, aerobic respiration, uses oxygen:
Respiration 1(Energy isn’t on the diagram because it’s not a chemical.)

So the lungs need to be good at getting oxygen in and carbon dioxide out.


For the lungs to work, oxygen needs to diffuse in from them to the blood, and carbon dioxide needs to diffuse out of the blood into the lungs. So what’s diffusion?

  • The movement of a chemical in gas or liquid
  • From high concentration to low concentration     
  • Down a concentration gradient (same thing)
  • A passive process, requiring no energy input

And to make diffusion happen from the blood to the lung air spaces fast, the lungs need to tick all these boxes:

  • The concentration gradient needs to be kept steep:
    the blood needs to be low in oxygen and the air spaces high in oxygen.
  • The barrier between blood and air needs to be thin.
  • There needs to be a low diffusion distance everywhere in the lungs:
    every air space in the lungs has to be close to a blood vessel.
  • The lungs have to have a large surface area:
    the more lining in the lungs, the more places there are where blood can pick up oxygen.

So the lungs needs to have a folded-up shape, with lots of surface area and lots of tiny blood vessels behind it. Now let’s see how they tick those boxes.

The structure of the lungs

The lungs have lots of air spaces, called alveoli. (Singular alveolus: it’s borrowed from Latin.) With a folded up structure, they maximise the surface area of lining in the lungs.


The alveolus wall is called epithelium: single, flattened-out cells. Behind them, there’s lots of tiny blood vessels, called capillaries. These, too, have walls only one cell thick. Capillary walls are called their endothelium. That means nowhere in the lungs is far from a capillary: diffusion distance is low.

Maintaining the concentration gradient

To keep the concentration steep, the lungs need to keep two things at extreme levels: there needs to be high-oxygen air in the alveoli, and low-oxygen blood in the air spaces.

That’s done by breathing and the heartbeat:

  1. breathing, or ventilation: moves old air out and new air in
  2. blood being pumped round the body: takes blood away once it’s picked up oxygen and replaces it with some more

The other thing breathing needs to make it work is for the lungs to be squishy. How breathing works is a separate post, but basically the lungs need to be squashed to push all the old air out and stretched to make space for the new air.

Other parts of the lungs you need to know about

This post is focusing on how the lungs work, but you need to know about a few other bits of the lungs:

  • the bronchi (singular bronchus) and bronchioles, big and then smaller tubes going into the lungs
  • the trachea or windpipe


The diagram of the alveolus was created by Mariana Ruiz Villarreal, alias Lady of Hats. (Twitter, blog.) It’s released into the public domain. All pictures used with thanks.

Making salts from neutralisation and the insoluble base rule

A common GCSE chemistry question is how you can make a soluble salt. You can make it by mixing up a base and an acid, with the acid supplying the negative ion like chloride or sulphate, and the base supplying the positive (metal) ion. (See our guide to working out the formula of ionic chemicals for a list of common ions.

That gives you this kind of reaction:
And here’s an example of that, making regular salt:
Neutralisation exampleMaking sure there’s no reactant in the final mix

The problem with doing all that is that the starting chemicals are all water soluble. When you’ve finished, there might be reactant left in the mix if you don’t mix everything in the right amounts:
Problem with soluble reactants A way to deal with that is to make the salt using an acid (which is soluble), and an insoluble base. This is an example with calcium carbonate, a base which doesn’t dissolve in water. Look at the state symbols in red:
Insoluble base The only chemical left dissolved in the solution at the end will be the product.

To make sure that happens, you use an excess of base: more than you need to finish off the reaction. That doesn’t matter, because it’s powder: you can just fish it out of the water with filter paper, then evaporate all the salt off:
The insoluble base rule finalSeparating the salt out at the end:

At the end, you’ll need to separate the salt out from the water. You’ll need to say that you:

  1. Filter that excess unreacted base out.
  2. Heat the mix gently to remove some of the water.
  3. Leave the water to evaporate in an evaporating basin.

How to answer titration questions

8559902400_7c926b8af8_kHow to answer questions on titration? Well:

Know what the point of it all is

Skip this if you want to get to the calculations, but here’s the point of why titrations exist. If you’ve got a bottle of a chemical whose concentration you don’t know (like an acid, say), one of the easiest ways to find its concentration is to do a reaction where you react it with another chemical whose concentration you do know.  If you know how much of the second chemical it reacts with, you can track that back to how much of the first chemical there was in your test, and what concentration it’s at.

Know what you’re doing

Make sure you know what the plan is. A titration calculation works out three things:

  1. It works out the number of moles of the chemical whose concentration you know.
  2. It finds out the number of moles of the mystery chemical.
  3. It works out the concentration of that chemical.

The first step uses the equation:

Moles = concentration x volume

The second uses the equation of the reaction to work out a ratio: how many moles of chemical A react with how many of chemical B? That ratio might be 1:1, 1:2, 2:1, and so on.

The third uses the the same equation as the first, only it’s rearranged because now we want to find concentration from a known number of moles and concentration:

Concentration = moles ÷ volume

Get the units right

classic mistake is to get the volumes wrong. They’re in dm3, or litres (same thing). Your numbers are probably in ml, or cm3 (same thing). To convert to dm3, divide by 1000. So 10cm3 is 0.01dm3, 15cm3 is 0.015dm3, and so on. Don’t let the examiners trick you like this.

Diagram out your answer

As with all calculations, get it right and then get it fast. Most people’s first exposure to titration questions is a set of scribbled lines copied down from the board: make sure you write a model answer that’s well spaced-out and clear, so you know what’s going on at every stage.

Often, it helps to write out the whole plan for what you do before you start working things out on a calculator.

Let’s take this question, a simple 1:1 titration calculation question:

Titration model answer question

Here’s a plan for doing it, working out what sums to do at each stage. No calculations yet:

Titration model answer dry run

And here’s the final answer:

Titration model answer oneone

Know the practical details

Many exam questions on this topic are’t just about the calculation, they’re about doing it. Make sure that you know that you know that you:

  • Measure out the mystery chemical of unknown concentration with a pipette, always the same volume
  • Measure out the chemical whose concentration you known with a burette, dripping it into the beaker.

For measuring, the burette and pipette need to be held vertical, measuring the bottom of the meniscus.

You need an indicator to show that the reaction’s finished. This needs to have a clear colour change when all the acid’s been neutralised or whatever. Because of that, it can’t be universal indicator: that just slowly shades from one colour to another.


The picture of a titration experiment was taken by the UCL Mathematical and Physical Sciences department, and was released under this license. All pictures used with thanks.

Working out the formula of ionic chemicals (GCSE Chemistry)

This post is mostly for students taking GCSE chemistry, but it may be helpful for AS-level students struggling a bit balancing equations.

One standard challenge is when you’re given a reaction but not the formulae of all the chemicals in it, and told to balance it.

Often, you need to make sure you’ve correctly worked out the formula of ionic chemicals in the reaction. Ionic chemicals include all the compounds made of metals and non-metals at GCSE chemistry, like iron oxide, copper carbonate and calcium sulphate. Here’s how to do that.

1) Get the facts

We’ve written a well-formatted list of the ions that make up these ionic chemicals-you can download it as a one-page pdf with some extra advice here. Ions are particles-often atoms-that have gained or lost electrons.

Ions listRemember what forms what: metals always form positive ions, non-metals form negative ions, with the exception of the hydrogen and ammonium ions, formula H+ and NH4+.

Charges are written in superscript: which means they’re above the line of text. (The numbers in subscript are numbers of atoms or ions.)

2) Remember that all chemicals have a total charge of zero.

You need to balance the charges on chemicals until the charge of the + ions and the – ions are equal.

Then, don’t put any charges in the final formula of your chemical.

Sometimes, this is simple.
Formulae of chemicals 1
For sodium chloride, the charges are just +1 and -1. A one-one ratio gives a neutral chemical.

Sometimes, it’s simple, but the charges aren’t just +1 and -1. Barium oxide is like that:

Formulae of chemicals 2In this case, +2 and -2 add to give 0.

When the two ions have different charges, you need to balance their numbers to get to a total charge of 0.This is calcium chloride, which often comes up as it gets made when you react marble chips with hydrochloric acid:
Formulae of chemicals 5

Some ions contain more than one atom inside them. Carbonate and sulphate ions are common examples of that:
Formulae of chemicals 3Sometimes, it’s hard and you need to plan out the formula. You need to find the first number you can get to on the times tables of the + charge and the – charge: there, the charges will balance and you’ll have a neutral-charge chemical:

Formulae of chemicals 4 Iron (III) oxide, like in rust, has a similar formula, Fe2O3.

Structuring questions on succession

Grass growing on a coal mine spoil tip in Loos-en-Gohelle, northern France.
Grass growing on a coal mine spoil tip in Loos-en-Gohelle, northern France.

This blog already has an article on succession as an abstract topic. This is a post about answering questions on it. It’s targeted for Edexcel A-level biology unit 4 (6BI04/01), and uses question 3 on this paper from June 2011 (mark scheme) as a case study.


How does succession happen over time? It’s an easy question to structure: you work chronologically from the start. You need to say this, with key points in italics.

1) A pioneer community like lichens and mosses arrives in the environment.
2) They are able to grow in little or no soil.
3) They break up the rock and form a thin soil layer.
4) They make the environment less extremeso plants with shallow roots can grow in the thin soil.
5) These other plants can outcompete the pioneer community.
6) They die and…
7) …decompose to form form a topsoil that can…
8) …hold minerals.
9) Bigger plants like trees and shrubs can now colonise the area.

In the case study question, you need to refer back to the list of plants that grow in the environment at different times. The lichens and mosses that are first to colonise the environment (a bing, a coal mine spoil heap) are the pioneer community, and the trees and shrubs are the climax community. However, the thin roots point is not a reference to any facts you’re given: you have to work that one out for yourself. (Use common sense, though. Plants in sand dunes often have very deep roots for their size, so they can burrow down to water. The problem here is that the rock prevents that.)

Climax community

Here’s how you describe the final, climax community:

1) It’s stable unless the environment changes.
2) It has high biodiversity (because there are many species and niches).
3) There’s a lot of interaction between species.
4) And finally, if that term isn’t given, say it’s a climax community!!

If you’re reading this but studying other boards, climax community is a big topic on Edexcel but less on other boards. Focus on the succession question.

The picture of a spoil tip was taken by Troye Owens (Tumblr), and released under this license. All images used with thanks.