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5.1.1Atomic Structure and the Periodic Table

Alkalis and Bases

5 min read 5 parts Lesson 1 of 10 Unit quiz

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Section 1 of 5Bases and Alkalis

5.1.1 Alkalis and Bases

All alkalis are bases, but only the bases that dissolve in water count as alkalis.

Base
  • A substance that neutralises an acid, which can be a metal oxide, a metal hydroxide or a metal carbonate.
Alkali
  • A base that dissolves in water to form an alkaline solution with a pH above 7.
Soluble examples
  • Sodium hydroxide, NaOH, and calcium hydroxide, Ca(OH)₂, are soluble bases, so they are alkalis.
Insoluble examples
  • Copper(II) oxide, CuO, and zinc hydroxide, Zn(OH)₂, are bases that do not dissolve, so they are not alkalis.
Ammonia
  • Ammonia gas dissolves in water to form an alkaline solution, so it is classed as an alkali.

Section 2 of 5Neutralisation Reactions

Every neutralisation produces a salt and water, and the base used decides what else is formed.

1
An acid reacts with a base, and the reaction is neutralisation, which is always exothermic, releasing heat to the surroundings.
2
With a metal hydroxide, the products are a salt and water: acid + hydroxide → salt + water.
3
With a metal oxide, the products are again a salt and water, because the oxide ion takes up hydrogen ions to form water.
4
With a metal carbonate, the products are a salt, water and carbon dioxide, which is why the mixture fizzes.

Section 3 of 5Word and Symbol Equations

Word and Symbol Equations
Word equation
  • Acid + base → salt + water, where the name of the salt depends on the metal and the acid used.
Symbol equation
  • Example2HCl(aq) + CuO(s) → CuCl₂(aq) + H₂O(l), which must be balanced.
Not a neutralisation
  • A metal reacting with an acid gives a salt but no water, so it is not a neutralisation reaction.
Fizzing clue
  • If an acid and base fizz, the base is a metal carbonate, because carbon dioxide is released.

Section 4 of 5Worked Example

Question Write the word equation and balanced symbol equation for the reaction between sodium hydroxide and hydrochloric acid, including state symbols.

  1. Word equationsodium hydroxide + hydrochloric acid → sodium chloride + water.
  2. Write the formulaeNaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l).
  3. Count the atomsone Na, one Cl, one O and two H on each side, so the equation is already balanced.

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

Section 5 of 5Exam Checklist & Tips

5.1.1 – Key Points for the Exam

  • Base A substance that reacts with an acid to form a salt and water.
  • Alkali A soluble base that gives a solution with a pH above 7.
  • Neutralisation A reaction between an acid and a base that produces a salt and water.
  • Exothermic A reaction that transfers energy to the surroundings, so the temperature rises.
  • Salt An ionic compound formed when the hydrogen of an acid is replaced by a metal or ammonium ion.

Common Mistakes

  • Stating that all bases are alkalis; only the soluble bases are alkalis.
  • Describing the reaction of a metal with an acid as neutralisation, when no water is formed.
  • Forgetting that a carbonate reaction also produces carbon dioxide gas.
  • Writing the formula of calcium hydroxide as CaOH instead of Ca(OH)₂.
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