Acids and Alkalis
Acids and alkalis are two groups of chemicals with opposite properties, placed at either end of the pH scale, which runs from 0 (strongly acidic) to 14 (strongly alkaline), with 7 as neutral. Acids react with alkalis in neutralisation reactions to form a salt plus water. This KS3 topic covers the pH scale, indicators, and the reactions of acids with metals, carbonates and alkalis.
Before you start
Make sure you're comfortable with these topics first:
Method
- Use the pH scale (0 to 14) to classify a solution: below 7 is acidic, exactly 7 is neutral, above 7 is alkaline.
- Test the pH using universal indicator solution or paper, and match the colour produced against a pH colour chart.
- Recall that acids react with metals to form a salt plus hydrogen gas, and with metal carbonates to form a salt, water and carbon dioxide.
- Write neutralisation word equations as acid + alkali -> salt + water, naming the salt from the acid used (hydrochloric gives chloride, sulfuric gives sulfate, nitric gives nitrate) and the metal in the alkali.
- Test for hydrogen with a lit splint (a squeaky pop confirms hydrogen) and for carbon dioxide by bubbling the gas through limewater, which turns cloudy.
- In titration-style practicals, add acid to alkali gradually while using an indicator (or a pH probe) to detect the exact point of neutralisation.
Worked example
35 cm3 of dilute hydrochloric acid is needed to exactly neutralise 20 cm3 of sodium hydroxide solution. Calculate the volume of the same acid needed to exactly neutralise 60 cm3 of the same sodium hydroxide solution.
- The volume of acid needed is directly proportional to the volume of alkali, since the concentrations stay the same.
- Find the scale factor between the new alkali volume and the original: 60 / 20 = 3.
- Multiply the original acid volume by this scale factor: 35 x 3 = 105.
- Final answer: 105 cm3 of hydrochloric acid is needed.
Practice questions
Try each question, then tap to reveal the answer.
Exam-style questions
Written in the style of a KS3 Science exam paper, with a full mark scheme.
Priya adds a spatula of calcium carbonate powder to a test tube of dilute hydrochloric acid and fizzing occurs immediately. (a) Complete the word equation: calcium carbonate + hydrochloric acid -> ____ + ____ + ____ (2) (b) Describe a test Priya could use to show the gas produced is carbon dioxide, including the result she would see. (1)
A group of students carries out a titration to find the volume of dilute sulfuric acid needed to exactly neutralise 25 cm3 of potassium hydroxide solution, using phenolphthalein indicator. Their results were: trial 1 = 22.4 cm3, trial 2 = 22.5 cm3, trial 3 = 26.1 cm3. (a) Identify which result is anomalous. (1) (b) Calculate the mean volume of acid needed, using only the two concordant results. (2) (c) State the colour change phenolphthalein shows at the neutralisation point. (1)
A school science club wants to know the exact volume of dilute sulfuric acid needed to neutralise 20 cm3 of sodium hydroxide solution, so they can prepare a standard salt solution later in the term. Plan a method the club could follow to find this volume, describing the apparatus and steps involved, how they would recognise the point of neutralisation, and what they could do to make their final result more reliable.
Free printable worksheet
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