Bases
2 guided lessons · 18 checks · 2. Inorganic Chemistry → Acids, Bases & Salt Preparations
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What makes a base act like a base?
A base is any substance that can cancel out an acid, and the products of that reaction are a salt and water. Some bases dissolve in water, and these soluble bases have their own name: alkalis. Because the two words are related but not identical, all alkalis are bases while many bases, such as copper(II) oxide, never dissolve at all.
What you know already
From Reactions of Acids you already know that acids react with metal oxides and hydroxides to give a salt and water. This note names the substances doing the neutralising, separates soluble bases from insoluble ones, and introduces the ion that every alkaline solution shares.
Key points of understanding
- A base neutralises an acid, forming a salt and water.
- An alkali is a base that dissolves in water, so every alkali is a base but not every base is an alkali.
- Alkalis have pH values above 7.
- In basic conditions red litmus paper turns blue.
- Most bases are oxides, hydroxides or carbonates of metals.
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1.What makes a base act like a base?
A base is any substance that can cancel out an acid, and the products of that reaction are a salt and water. Some bases dissolve in water, and these soluble bases have their own name: alkalis. Because the two words are related but not identical, all alkalis are bases while many bases, such as copper(II) oxide, never dissolve at all.
Key points
- A base neutralises an acid, forming a salt and water.
- An alkali is a base that dissolves in water, so every alkali is a base but not every base is an alkali.
- Alkalis have pH values above 7.
- In basic conditions red litmus paper turns blue.
- Most bases are oxides, hydroxides or carbonates of metals.
First pass
A base cancels out an acid to give a salt and water; a soluble base is called an alkali. All alkalis are bases, but metal oxides such as copper(II) oxide are bases that stay solid in water.
Analogy — Think of squares and rectangles: every square is a rectangle, but not every rectangle is a square, just as every alkali is a base without the reverse being true.
Exam pointer — If a question asks whether a named substance is a base or an alkali, first ask whether it dissolves in water, then answer with both words made explicit.
Going deeper
Bases neutralise acids in a reaction whose products are a salt and water. When the base dissolves, the mixture it forms is alkaline and is named an alkali, which is why alkalis sit above pH 7 while insoluble bases give no solution at all.
Analogy — A library and a lending library differ the same way: every lending library is a library, yet most libraries hold their books without lending any, the way most bases hold their ions without dissolving.
Exam pointer — One-mark versions want the salt-and-water products; two-mark versions usually also want the solubility distinction, so phrase answers as base first, alkali second.
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2.Some common alkalis and the ions they contain
The alkalis met by name at this level are sodium hydroxide, potassium hydroxide and ammonia solution. Each one releases hydroxide ions when it mixes with water, and it is those ions that give the mixture its alkaline behaviour. Ammonia is the odd one out, because the gas itself contains no hydroxide and only produces the ions after reacting with water.
Key points
- The hydroxide ion is what makes an aqueous solution alkaline.
- Sodium hydroxide dissolves in water to give sodium ions and hydroxide ions.
- Potassium hydroxide dissolves in water to give potassium ions and hydroxide ions.
- Aqueous ammonia contains ammonium ions and hydroxide ions, formed when ammonia reacts with water.
- Ammonia is the gas NH3 and ammonium is the ion NH4+, so the two names must not be swapped.
First pass
Sodium hydroxide, potassium hydroxide and ammonia solution are the named alkalis, and each supplies hydroxide ions in water. Ammonia gas only makes those ions after it reacts with water, which is why the gas and the solution behave differently.
Analogy — Ammonia gas joining water to release hydroxide ions is like a dry battery finding its socket: the parts only do their job once they are connected.
Exam pointer — Learn the ion table by heart, and always write ammonia for the gas NH3 and ammonium for the ion NH4+ when naming what you see.
Going deeper
In water, sodium hydroxide gives Na+ and OH-, potassium hydroxide gives K+ and OH-, and aqueous ammonia gives NH4+ and OH-. The shared OH- ion explains their common alkaline behaviour, while the ammonium ion shows that ammonia reacts with water rather than simply dissolving.
Analogy — Three different keys can open the same door when each is cut with the same notch: the shared hydroxide notch is what turns each solution alkaline.
Exam pointer — Equation questions often want the ammonia-water step made explicit, so state that the gas reacts with water to give ammonium and hydroxide ions.
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