AQA Chemistry 5.4.2.4 (+ 5.4.2.5 Higher) · Combined and Triple
The pH scale and neutralisation
Below 7 acid, 7 neutral, above 7 alkali. Measure it with universal indicator or a probe, watch it climb as alkali is added to acid, and know that each step is ten times.
Triple Science · Higher tier
Start here
Lemon juice.
Squeeze a lemon onto your tongue. Sharp, sour, makes you wince.
If you had to guess, is lemon juice an acid or an alkali?
The scale
Below 7, 7, above 7.
Acids produce hydrogen ions (H+) in aqueous solution; aqueous solutions of alkalis contain hydroxide ions (OH−). The pH scale, from 0 to 14, measures how acidic or alkaline a solution is. Acids have pH values less than 7; alkalis have pH values greater than 7; pH 7 is neutral. The lower the pH, the more acidic. Universal indicator gives an approximate pH by colour; a pH probe gives a number.
Each solution has been tested with universal indicator. Place it.
Vinegar: universal indicator turns orange.
Place it.
Pure water: universal indicator turns green.
Place it.
Oven cleaner: universal indicator turns purple.
Place it.
Black coffee: a probe reads pH 5.
Place it.
Neutralisation
pH climbs through 7.
Add alkali to an acid and the pH rises: slowly at first, then fast through 7, then levelling off above it. The curve is what a probe records. The reaction between the ions is the same whatever acid and alkali you use.
H+(aq) + OH−(aq) H2O(l)
Water is added to an acid of pH 2 instead of alkali. What happens to the pH?
Pick one.
Triple: in a titration a single-colour-change indicator such as phenolphthalein (colourless in acid, pink in alkali) marks the end point. See Titrations.
Higher · each pH unit is ten times
Ten, a hundred, a thousand.
As the pH decreases by one unit, the hydrogen ion concentration of the solution increases by a factor of 10. So a difference of n whole units is a factor of 10n.
factor = 10(difference in pH)
Worked
pH 1 compared with pH 3. Difference 3 − 1 = 2 units. Factor = 102 = 10 × 10 = 100. The pH 1 solution has 100 times the hydrogen ion concentration.
pH 2 compared with pH 5. How many times more H+?
Pick one.
The full treatment, with worked CFIFA, is in Strong and weak acids.
Think again · nearly 7
“pH 6.5 is close enough to 7 to count as neutral.”
Your call
ChooseA probe reads pH 6.5. What is the solution?
Sort
Acidic, neutral or alkaline?
Place each reading.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Command words in this lesson
- Describe
- How to measure pH with universal indicator or a probe.
- Identify
- Acid, neutral or alkali from a pH or a colour.
- Write
- The ionic equation for neutralisation.
Key fact
Acids: pH below 7, hydrogen ions. Alkalis: pH above 7, hydroxide ions. Neutral: 7. Neutralisation: H+(aq) + OH−(aq) → H2O(l). Higher: one pH unit is a factor of 10 in H+ concentration.
Key note · AQA 5.4.2.4 (+ 5.4.2.5 HT) · Combined and Triple
The pH scale and neutralisation
- Acids produce hydrogen ions (H⁺) in aqueous solutions; aqueous solutions of alkalis contain hydroxide ions (OH⁻).
- The pH scale, from 0 to 14, is a measure of the acidity or alkalinity of a solution. Acids: pH less than 7. Alkalis: pH greater than 7. Neutral: pH 7. The lower the pH, the more acidic.
- Measure pH with universal indicator (or a wide range indicator) for an approximate value by colour, or a pH probe for a number.
- In neutralisation between an acid and an alkali, hydrogen ions react with hydroxide ions to produce water: H⁺(aq) + OH⁻(aq) → H2O(l). Adding alkali to acid raises the pH steeply through 7. Diluting an acid raises the pH towards 7 without reaching it.
- Higher: as the pH decreases by one unit, the hydrogen ion concentration increases by a factor of 10 (pH 1 has 100 times the H⁺ concentration of pH 3). Full treatment in Strong and weak acids.
Photograph this card. Tomorrow, cover it and say each line first.
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Ask MrBadmus
Reading pH 6.5 as neutral?
Everyday pH values are approximate: dilute hydrochloric acid about 1, vinegar about 3, black coffee about 5, baking soda solution about 9, sodium hydroxide solution about 13.