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

CaCO3

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

为什么A 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

提示K 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.

为什么Turning 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

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

为什么The 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

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

为什么The 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

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

为什么Two 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

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

为什么In 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

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

为什么A 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

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

为什么The 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

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

为什么The 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)

提示The clear liquid stops being clear.

为什么Limewater 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

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

为什么It 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

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

为什么A 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

提示Think what this gas does to the colour of an indicator.

为什么The 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

提示A drop of water is all it takes.

为什么The 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

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

为什么Chloride 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

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

为什么The 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)

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

为什么Sodium 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

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

为什么(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).