Chemical reactions, Year 10: equations, state symbols and gas tests

CaCO3

HintOne atom of the metal, one of carbon, and the -ate ending tells you there are three of the third element.

The whyA compound of a metal with a non-metal and oxygen is named metal first, then the non-metal with the ending -ate. The name tells you which three elements to write, and the formula tells you how many atoms of each: one calcium, one carbon, three oxygen. Read the other way, CaCO3 has to be named calcium carbonate.

potassium hydroxide; hydrogen

HintK is the metal's symbol, taken from its Latin name; OH is the group found in every alkali; the last product is an element on its own.

The whyTurning symbols into words means naming each formula. KOH contains K with an OH group, so it is a hydroxide of the metal. H2 is two atoms of one element joined, so it is simply named as that element. The numbers in front are for balancing and are not part of any name.

H2SO4

HintEvery acid contains hydrogen; this one has two of them, joined to a sulfate group.

The whyThe three common laboratory acids are worth knowing by heart: hydrochloric acid is HCl, nitric acid is HNO3 and sulfuric acid is H2SO4. The sulfate part, SO4, stays together and turns up again in the salts the acid makes, such as copper sulfate, CuSO4.

Mg + 2HCl → MgCl2 + H2

HintGet the four formulae right first: the salt needs two chlorines, and the gas goes about in pairs.

The whyThe salt is MgCl2 and hydrogen gas is H2. That puts two Cl and two H on the right, so two HCl are needed on the left. Check each element: Mg 1 and 1, H 2 and 2, Cl 2 and 2.

3

HintCount the oxygen atoms in two units of the product, then remember that they arrive in pairs.

The whyTwo Al2O3 contain 2 × 3 = 6 oxygen atoms, and each O2 molecule supplies two, so 6 ÷ 2 = 3 molecules are needed. The aluminium checks as well: 2 × 2 = 4 atoms on each side.

Zn + Cu2+ → Zn2+ + Cu

HintSulfate is there at the start and is still there, unchanged, at the end, so strike it from both sides.

The whyIn solution, copper sulfate is separate copper ions and sulfate ions, and zinc sulfate is zinc ions and sulfate ions. The sulfate ions take no part, so they are left out. What remains shows the real change: zinc atoms become ions and copper ions become atoms. The charges balance too, with 2+ on each side.

Oxygen

HintIt is the gas in air that things need in order to burn.

The whyA splint that is only glowing bursts back into flame because burning is much faster in the pure gas than in air, where it makes up only about a fifth.

Burning with a flame

HintA glow is the right starting point for a different gas test.

The whyThe hydrogen test needs a splint that is alight, held at the mouth of the tube. The glowing splint belongs to the oxygen test, where the point is to see it relight. Mixing up the two is the commonest slip in gas tests.

Burning very rapidly

HintThe flame on the splint sets it off, and it is over in an instant.

The whyThe hydrogen at the mouth of the tube has mixed with air, and it reacts with the oxygen so quickly that it makes a small, sharp sound. Only a test tube of gas is ever tested in this way.

The limewater turns milky (cloudy)

HintThe clear liquid stops being clear.

The whyLimewater starts as a clear, colourless solution. Shaking it with carbon dioxide, or bubbling the gas through it, makes it turn milky. No other common gas does this, so it is the test that identifies carbon dioxide.

Limewater

HintIts name joins a word for a white, chalky substance to the liquid it is dissolved in.

The whyIt is clear and colourless, and turns milky when carbon dioxide is shaken with it or bubbled through it. It has nothing to do with the fruit: 'lime' is an old name for compounds of calcium.

It only rules out oxygen

HintA negative result cannot name a gas; think which single gas the splint was looking for.

The whyA glowing splint goes out in many gases; nitrogen is another. To show that a gas is carbon dioxide you need its own positive test, which is limewater turning milky.

Damp litmus paper is bleached white

HintThink what this gas does to the colour of an indicator.

The whyThe litmus loses its colour altogether, which is what bleaching means. Chlorine is a toxic gas, so in school the test is done on a very small scale or in a fume cupboard.

It must be damp

HintA drop of water is all it takes.

The whyThe test is always described with damp litmus paper: the gas has to dissolve in the water on the paper before it can act on the dye.

Chlorine

HintIt comes from the second half of the dissolved salt's name.

The whyChloride ions from the dissolved salt are attracted to the positive electrode, where the gas forms. Bleaching damp litmus paper is the test that identifies it, which is why this gas test is learned alongside electrolysis.

aq

HintIt is short for a word that comes from the Latin for water.

The whyThe four state symbols are (s) solid, (l) liquid, (g) gas and (aq) aqueous, meaning dissolved in water. They are written straight after each formula in an equation.

NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)

HintThree of the four substances are dissolved; the fourth is a pure substance you could pour.

The whySodium hydroxide solution, dilute hydrochloric acid and sodium chloride solution are all dissolved in water, so each is (aq). The water that is made is a pure liquid, so it is (l).

It is a pure liquid, not a substance dissolved in water

HintAsk what the second symbol would have to mean here: mixed into what?

The why(aq) means that a substance is dissolved in water. Water itself is the solvent, so on its own it is simply a liquid and takes (l).