Atoms, Elements, Compounds and Mixtures, Grade 8
Element, compound or mixture. The real question is how you would prove it.
Grade 8 learners meet atoms, elements, compounds and mixtures in Term 2, and most of them leave able to recite three definitions and sort a page of pictures. That tests memory. This page is about evidence: what is inside an atom, what separates the three kinds of substance, and one practical that turns a chemical formula into something you can measure with a ruler.
What is inside an atom
| Particle | Where it is | Charge | Relative mass |
|---|---|---|---|
| Proton | In the nucleus | +1 | 1 |
| Neutron | In the nucleus | 0 | 1 |
| Electron | Around the nucleus | −1 | about 1/1 840 |
- Atomic number = the number of protons. It decides which element the atom is, and nothing else does
- In a neutral atom, electrons = protons, so the charges cancel
- Mass number = protons + neutrons, so neutrons = mass number − atomic number
Protons and neutrons are not usually equal
You will often see it written that an atom "usually" has the same number of protons and neutrons. That is true for a few light elements and false after that.
| Element | Protons | Neutrons, commonest atom |
|---|---|---|
| Carbon | 6 | 6 |
| Oxygen | 8 | 8 |
| Sodium | 11 | 12 |
| Iron | 26 | 30 |
| Gold | 79 | 118 |
Why chlorine gives you 18,5 neutrons
Most school periodic tables print the relative atomic mass, which is an average, not the mass number of one atom. For most of the first 20 elements it is close to a whole number and the subtraction works. Chlorine is printed as 35,5, and 35,5 − 17 = 18,5.
No atom has half a neutron. Chlorine is a mix of atoms with 18 neutrons and atoms with 20, and 35,5 is their average. Round to the nearest whole number for Grade 8 work, and know why you are rounding.
How small is small
An atom is about 0,000 000 000 1 m across. One gram of gold holds roughly 3 × 1021 atoms: three thousand billion billion. Not millions, and not billions.
No light microscope, and no ordinary electron microscope, shows you a single atom. A scanning tunnelling microscope can map individual atoms on a surface. The colourful "atoms" in most school pictures are models.
Where this fits in the curriculum
| Subject | Natural Sciences |
|---|---|
| Grade | 8 |
| Term | 2 |
| Strand | Matter and materials |
| Topic | Atoms: elements, compounds and mixtures |
| Status | Informal activities |
| Marks | 50 on our worksheet. None prescribed |
Element, compound or mixture
| Element | Compound | Mixture | |
|---|---|---|---|
| Made of | One kind of atom | Two or more elements, chemically bonded | Two or more substances, not bonded |
| Fixed ratio? | n/a | Always the same | Any ratio |
| How to separate it | You cannot split it chemically | Only by a chemical reaction | By a physical method |
| Properties | Its own | New ones. Water is nothing like hydrogen or oxygen | Each part keeps its own |
| Examples | Oxygen, copper, carbon | Water, carbon dioxide, table salt | Air, sea water, tap water, steel |
A compound can only be split by a chemical change, and the parts always come out in the same ratio. A mixture comes apart physically, in whatever ratio you made it. That is the test, and it is the one a class can actually carry out.
That fixed-ratio rule is Proust's law of definite proportions, from the 1790s. It is often credited to Lavoisier, whose law is the conservation of mass.
Three "pure substances" that are not
Tap water, the air you breathe out, and a natural gold nugget are all mixtures, and all three are regularly shown as pure substances.
| Example | Why it is a mixture |
|---|---|
| Tap water | Dissolved minerals, plus a trace of chlorine from treatment |
| Breathed-out air | About 78 % nitrogen, 16 % oxygen and only about 4 % carbon dioxide |
| A gold nugget | Natural gold is nearly always alloyed with some silver and copper |
A molecule is not the same as a compound
Hydrogen gas is made of H2 molecules and oxygen of O2 molecules. Both are elements, because each molecule contains only one kind of atom.
And table salt has no molecules at all. Sodium chloride is a lattice of sodium and chloride ions, packed one to one, with no small separate unit you could call a molecule. NaCl is a ratio, not a molecule.
Practical: split water with electricity
This is the Grade 8 practical that shows a compound being taken apart, and it measures the formula of water.
Use bicarbonate of soda, not table salt
Pure water barely conducts, so something has to be dissolved in it. The widely used "easy" version dissolves table salt. With carbon electrodes, a salt solution gives off a good deal of chlorine at the positive electrode, which is a toxic gas, and the gas is usually labelled oxygen. Bicarbonate of soda conducts well enough and gives only hydrogen and oxygen. This method is ours, for that reason.
You need
- Two carbon electrode rods, 100 mm × 5 mm
- Four AA cells in four battery holders, wired in series for 6 V
- Two crocodile leads
- Two test tubes, 25 mm × 150 mm
- A clear disposable plastic cup, about 300 ml, and a little Prestik
- Two books to stand the cup on
- 5 g of bicarbonate of soda, about a level teaspoon
- Wooden splints, matches, goggles
Method
- Make two holes in the base of the cup, 3 cm apart, with a sharp pencil point, each a little smaller than a rod
- Push a carbon rod up through each hole from underneath until about 5 cm stands inside the cup, and seal round both rods with Prestik, inside and out
- Stand the cup on two books with the rod ends hanging in the gap, and clip a lead to each rod underneath, where it stays dry
- Dissolve 5 g of bicarbonate of soda in 200 ml of tap water and pour it into the cup. Check for drips
- Fill both test tubes to the brim with the same solution, cover the end with a thumb, turn each upside down and lower it over one rod tip so it stays full
- Connect the four cells in series. Bubbles appear on both rods within seconds
- Leave it for 20 to 30 minutes, marking the gas level in each tube every 5 minutes
- Disconnect first. Then lift the tube from the negative rod, still upside down with a thumb over it, and hold a lit splint at its mouth
- Lift the other tube and push a glowing splint into it
Why through the base? The usual drawing shows an upside-down tube over a rod clipped at the top, which cannot be built: the top of the rod ends up inside the tube, under the liquid. Coming up through the base keeps the clips dry and puts every bubble inside a tube.
What you should see
Expect roughly 15 to 25 ml of hydrogen in 25 minutes at 6 V, and about half that of oxygen.
| Negative electrode | Positive electrode | |
|---|---|---|
| Gas | Hydrogen | Oxygen |
| Volume | About twice as much | About half as much |
| Test | Lit splint: a squeaky pop | Glowing splint: it relights |
2H2O → 2H2 + O2
Two water molecules give two hydrogen molecules and one oxygen molecule. Equal numbers of gas molecules take up equal volumes at the same temperature, so the hydrogen tube fills twice as fast. The 2 in H2O has just been measured.
Expect the oxygen tube to come up a little short. Oxygen dissolves in water more readily than hydrogen, and some of it attacks the carbon rod. A ratio of 2,2 or 2,5 to 1 is a real result: clearly about two, not one and not three.
Practical: build the molecules
Modelling clay and toothpicks do this as well as anything. The standard colours are hydrogen white, oxygen red, carbon black and nitrogen blue.
| Build | Atoms | Element or compound? |
|---|---|---|
| H2 | 2 hydrogen | Element |
| O2 | 2 oxygen | Element |
| N2 | 2 nitrogen | Element |
| H2O | 2 hydrogen, 1 oxygen | Compound |
| CO2 | 1 carbon, 2 oxygen | Compound |
| NH3 | 1 nitrogen, 3 hydrogen | Compound |
| CH4 | 1 carbon, 4 hydrogen | Compound |
Then ask the class to build a molecule of table salt, and let them find that there is nothing to build.
If it does not work
| Problem | Cause | Fix |
|---|---|---|
| No bubbles at all | A clip is not touching bare carbon, or the cells are flat | Check the clips under the cup. Try fresh cells |
| The cup drips | A gap round a rod | More Prestik, pressed in on both sides of the base |
| Very few bubbles | Too little bicarbonate, or the rods are far apart | 5 g in 200 ml, holes 3 cm apart |
| No gas in the tubes | The tubes are not over the rods | Lower each tube so its rod goes up inside it |
| The liquid goes grey | Carbon flaking from the positive rod | Normal. Rinse and dry the rods afterwards |
| No pop | Too little hydrogen, or it escaped when lifted | Wait for at least 15 ml. Keep it upside down with a thumb over it |
| The splint does not relight | Too little oxygen | Collect for longer. Blow the splint out just before testing so it glows brightly |
| The ratio is about 1 to 1 | Air was left in the tubes at the start | Refill both to the brim and start again |
Safety
- Goggles. Low voltage and a harmless solution, but the habit is the point
- Never use table salt as the electrolyte. It makes chlorine
- Hydrogen in test-tube amounts only. The pop is the whole point; never collect it in anything larger
- No flames until the circuit is disconnected
- Keep the two clips apart under the cup. If they touch, the cells short and heat up
Disposal: the solution goes down the sink. Rinse and dry the rods; they last for years.
Two demonstrations we would leave out
Electrolysing copper(II) chloride is often suggested next. It exists to make chlorine, and some versions ask learners to describe the smell. The water electrolysis above shows the same idea, decomposition by electricity, with no toxic product.
Heating potassium permanganate is fine as a teacher demonstration, with goggles, a loose plug of mineral wool in the tube and the mouth pointed away from everyone. But be clear about what it makes: potassium manganate, manganese dioxide and oxygen. Two compounds and one element, so it shows a compound splitting into simpler substances, not into its elements.
How the 50 marks are made up
| Section | Marks |
|---|---|
| Inside an atom | 12 |
| Element, compound or mixture | 10 |
| Splitting water | 14 |
| Models | 8 |
| Conclusion | 6 |
No mark allocation is prescribed for this topic. The worksheet and this split are ours.
The most valuable question on the sheet asks how two gas volumes show the formula H2O. A learner who can answer it has understood what a formula is.
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 50 marks, with the particle table, a ten-substance sort, the water electrolysis results and the model-building questions
- Marking memorandum, with the answers, the range of gas ratios to accept, and the five answers that look right and score nothing
Related practicals
- Separating mixtures. The physical methods that take a mixture apart
- Physical and chemical changes. More gases collected and tested
- Reactions of acids, Grade 9. Hydrogen and carbon dioxide again, from acids
What you need to run it
The electrolysis is the part that needs proper equipment, and it is cheap. A pack of ten carbon electrode rods does five groups and lasts for years. Four AA battery holders per group give the 6 V, and a pack of crocodile leads connects them. The cells themselves are best bought fresh by the school; batteries leak in storage.
The rest is two ordinary test tubes, a plastic cup, a pea of Prestik and bicarbonate of soda from the supermarket.
For the atoms half of the topic, a periodic table wall chart on the classroom wall does more than any handout, and it is what the class reads atomic numbers from.
One gap in our range, and we would rather say so. We do not yet stock a chemistry molecular model kit. Modelling clay and toothpicks work well for Grade 8 in the meantime.