酸、碱与盐

An acid 酸 is a substance that forms hydrogen ions ($\text{H}^{+}$) when dissolved in water. Acids have three typical reactions:

• with metals 金属: acid + metal → a salt 盐 + hydrogen 氢气
• with bases: acid + base 碱 → a salt + water (this is neutralisation 中和)
• with carbonates 碳酸盐: acid + carbonate → a salt + water + carbon dioxide 二氧化碳

Describing what you see

Naming the products is only half of what the exam wants. "State the observations" asks what you would see with your eyes, and it carries its own marks - so learn the words:

So for magnesium added to hydrochloric acid, a full answer is: effervescence, and the solid dissolves (and the tube gets warm). For copper(II) oxide added to warm sulfuric acid: the black solid dissolves and the solution turns blue.

Two rules save most of the lost marks. Never write "hydrogen is given off" as an observation - you cannot see hydrogen, you can only see the bubbles, so write effervescence. And do not write a name you cannot see: "a salt forms" is a conclusion, not an observation.

Indicators

An indicator 指示剂 is a dye that changes colour to show whether a solution is acidic or alkaline.

Aqueous solutions of acids contain hydrogen ions 离子 ($\text{H}^{+}$). Aqueous solutions of alkalis contain hydroxide ions ($\text{OH}^{-}$).

Bases are oxides 氧化物 or hydroxides 氢氧化物 of metals. An alkali 可溶性碱 is a base that dissolves in water (a soluble base).

Bases have two typical reactions:

• with acids: base + acid → a salt + water (neutralisation again)
• with ammonium salts 铵盐: this releases ammonia gas.

An acid is a proton 质子 donor — it gives away $\text{H}^{+}$ ions (a hydrogen ion is just a proton). A base is a proton acceptor.

How strong an acid is depends on how much of it splits into ions in water:

• A strong acid 强酸 is completely dissociated 电离 (fully split into ions) in water. Hydrochloric acid 盐酸 is strong:

• A weak acid 弱酸 is only partly dissociated. Ethanoic acid 乙酸 is weak, so the equation uses the reversible arrow:

Note: 'strong' and 'weak' are about dissociation, not about being dilute or concentrated.

The pH scale runs from 0 to 14 and tells you how acidic or alkaline a solution is. A higher hydrogen ion concentration means a lower pH.

You can find pH using universal indicator 通用指示剂, which turns different colours:

When an acid and an alkali react, the $\text{H}^{+}$ and $\text{OH}^{-}$ ions join to make water:

Oxides can be sorted by how they behave:

• Acidic oxides are oxides of non-metals, such as $\text{SO}_2$ and $\text{CO}_2$.
• Basic oxides are oxides of metals, such as $\text{CuO}$ and $\text{CaO}$.
• Amphoteric 两性 oxides react with both acids and bases to make a salt and water. $\text{Al}_2\text{O}_3$ and $\text{ZnO}$ are amphoteric.

So metal oxides tend to be basic and non-metal oxides tend to be acidic.

Whether a salt can be made by a certain method depends on whether it is soluble 可溶 (dissolves) or insoluble 不溶 (does not dissolve).

Solubility rules

Making a soluble salt

You react an acid with one of these:

• an alkali, using titration 滴定 (since both are solutions, you must measure the exact volumes);
• an excess 过量 of a metal, an insoluble base, or an insoluble carbonate.

When you use an excess of a solid, you then filter 过滤 to remove the leftover solid. Finally you evaporate 蒸发 some water and let the solution crystallise 结晶 to get the salt.

Worked example. Which method would you use to prepare (a) copper(II) sulfate and (b) barium sulfate? Check the solubility rules first. (a) Copper(II) sulfate is soluble (only barium, calcium and lead sulfates are not), so make it from an insoluble solid and an acid: add excess copper(II) oxide to warm dilute sulfuric acid, filter off the leftover solid, then evaporate some water and crystallise. (b) Barium sulfate is insoluble, so make it by precipitation: mix two solutions that each carry one of the needed ions, then filter, wash and dry the solid. The solubility of the salt you want is what decides the method - consult the rules before you choose.

Making an insoluble salt

An insoluble salt is made by precipitation 沉淀: mix two solutions that each contain one of the needed ions, and the insoluble salt forms as a solid. You then filter, wash and dry it.

Worked example. Describe how to make pure, dry crystals of copper(II) sulfate from copper(II) oxide and dilute sulfuric acid.

1. Warm the dilute sulfuric acid in a beaker. Add copper(II) oxide a little at a time and stir, until some black solid is left over and will not dissolve. The left-over solid shows the oxide is in excess, so all the acid has reacted.
2. Filter to remove the excess copper(II) oxide. The blue filtrate 滤液 is copper(II) sulfate solution.
3. Heat the filtrate to evaporate some of the water, until crystals start to form when a drop is taken out on a glass rod and cooled.
4. Leave the solution to cool, so crystals form. Filter them off and dry them between filter papers.

Why use an excess of the solid? An excess of acid would stay in the solution and end up in the crystals, but an excess of solid can simply be filtered off.

Water in salts

• A hydrated 水合 substance is chemically joined with water.
• An anhydrous 无水 substance contains no water.

The water molecules inside hydrated crystals are called the water of crystallisation 结晶水. For example, $\text{CuSO}_4 \bullet 5\text{H}_2\text{O}$ has five water molecules for each formula unit.

Common mistakes.

• Saying all bases are alkalis. An alkali is only a base that dissolves in water; copper(II) oxide is a base but not an alkali.
• Heating the salt solution until it is dry. This can break up the crystals and drive off the water of crystallisation; heat only until crystals start to form, then let it cool.
• Using an excess of acid to make a soluble salt. The extra acid cannot be filtered off; use an excess of the solid instead.
• Forgetting to wash a precipitate. Rinse it with a little distilled water, or the soluble ions from the solutions stay on it as impurities.
• Choosing precipitation for a soluble salt, or the excess-solid method for an insoluble one. Check the solubility rules first.

• Learn the three acid reactions: acid + metal → salt + hydrogen; acid + base → salt + water (neutralisation); acid + carbonate → salt + water + carbon dioxide.
• Strong and weak describe how much the acid splits into ions (dissociation), not how concentrated it is. A strong acid is fully dissociated; a weak acid only partly.
• A lower pH means a higher hydrogen-ion concentration. pH 7 is neutral, below 7 is acidic, above 7 is alkaline.
• Choose the salt method by solubility: a soluble salt is made from an acid + excess solid (then filter, evaporate, crystallise), or by titration with an alkali; an insoluble salt is made by precipitation (mix two solutions, then filter).
• Learn the solubility rules — all sodium, potassium, ammonium and nitrate salts are soluble.