Memory Anchor:

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Key Questions:

1. In what way can universal indicator be used to measure the approximate pH of a solution?
   • Universal indicator is used to measure the approximate pH of a solution as when it is added to a solution strongly acidic solutions appear red, weakly acidic solutions appear orange/yellow, neutral solutions appear green, weakly alkaline solutions appear blue and strongly alkaline solutions appear purple.
2. If a substance has the following pH is it classified as acid, alkali or neutral: 1) pH 3 2) pH 11 3) pH 7
   • 1) pH 3 = acid 2) pH 11 = alkali 3) pH 7 = neutral
3. What ions are produced in alkalis in aqueous solutions?
   • Aqueous solutions of alkalis contain hydroxide ions (OH-).
4. What happens to strong acids in an aqueous solution in terms of ionisation?
   • A strong acid is completely ionised in aqueous solution. Examples of strong acids are hydrochloric, nitric and sulfuric acids.
5. What happens to weak acids in an aqueous solution in terms of ionisation?
   • A weak acid is only partially ionised in aqueous solution. Examples of weak acids are ethanoic, citric and carbonic acids.
6. What ions are produced in acids in aqueous solutions?
   • Acids produce hydrogen ions (H ) in aqueous solutions.
7. When an acid is stronger, what effect does this have on the pH?
   • For a given concentration of aqueous solutions, the stronger an acid, the lower the pH.
8. What happens to the hydrogen ion concentration of a solution when pH decreases by one unit?
   • As the pH decreases by one unit, the hydrogen ion concentration of the solution increases by a factor of 10.
9. What is meant by 'dilute' and 'concentrated' (in terms of amount of substance) and 'weak acid' and 'strong acid' (in terms of the degree of ionisation)?
   • The strength of an acid is a measure of the degree of its ionisation - strong acids are fully ionised but weak acids are only partly ionised. Remember that the opposite of strong is weak. The concentration of an acid is a measure of the number of moles of acid in 1 dm3 of acid solution. For example, 2 mol/dm3 hydrochloric acid is twice as concentrated as 1 mol/dm3 hydrochloric acid or 1 mol/dm3 ethanoic acid. Remember: the opposite of concentrated is dilute.