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KS3 · Chemistry

K9 Acids and Alkalis

AQA AQA KS3 Science (Chemistry) · Calculator allowed · about 90 minutes

Name: _______________________________ Date: ____ / ____ / ______

Answer ALL questions. Show all your working.

The pH scale runs from 0 to 14 and is used to classify solutions as acidic, neutral or alkaline.

(a)State the pH number that represents a neutral solution.(1)

(b)State the range of pH values that represents an acidic solution.(1)

(c)State the range of pH values that represents an alkaline solution.(1)

Which of these pH values shows the most strongly alkaline solution?

A) 1
B) 6
C) 8
D) 13

When universal indicator is added to a solution, it turns a colour that shows the approximate pH of that solution.

(a)State the colour universal indicator would turn in a solution of pH 2.(1)

(b)State the colour universal indicator would turn in a solution of pH 7.(1)

(c)State the colour universal indicator would turn in a solution of pH 11.(1)

Name the acid described by each clue below.

(a)The acid found in the human stomach that helps digest food.(1)

(b)The strong, corrosive acid used in car batteries.(1)

(c)The weak acid found in vinegar.(1)

(d)The acid found in citrus fruits such as lemons.(1)

Name the alkali described by each clue below.

(a)The strong, corrosive alkali used to make soap and oven cleaners.(1)

(b)The alkaline solution formed when calcium oxide (quicklime) is added to water, often called limewater.(1)

(c)The alkaline gas with a sharp, pungent smell used in cleaning products and fertilisers.(1)

Nadia adds a short piece of magnesium ribbon to a test tube of dilute hydrochloric acid. Bubbles of gas form immediately and the magnesium disappears over a few minutes.

(a)Complete the word equation for this reaction: magnesium + hydrochloric acid -> ____ + ____(2)

(b)Describe a test Nadia could use to confirm the identity of the gas produced, including the result she would see.(1)

Callum adds dilute hydrochloric acid to a spatula of calcium carbonate powder in a boiling tube. Fizzing occurs straight away.

(a)Complete the word equation for this reaction: calcium carbonate + hydrochloric acid -> ____ + ____ + ____(2)

(b)Describe how Callum could test that the gas given off is carbon dioxide.(1)

Complete each word equation below by naming the salt formed in the neutralisation reaction.

(a)hydrochloric acid + sodium hydroxide -> ____ + water(2)

(b)sulfuric acid + potassium hydroxide -> ____ + water(2)

(c)nitric acid + calcium hydroxide -> ____ + water(2)

REQUIRED PRACTICAL. A KS3 class carries out an investigation to find the pH of five household solutions (lemon juice, milk, tap water, soap solution and oven cleaner) using universal indicator solution and a pH colour chart.

(a)State one piece of PPE (personal protective equipment) that must be worn when testing the oven cleaner.(1)

(b)Put these steps of the method into the correct order by writing 1, 2, 3 and 4: (i) record the colour and compare it to the pH colour chart to find the pH number, (ii) add a few drops of universal indicator to the sample, (iii) place a small sample (about 2 cm3) of the solution into a clean test tube, (iv) swirl gently to mix.(2)

(c)Results: lemon juice pH 2, milk pH 6.5, tap water pH 7, soap solution pH 9, oven cleaner pH 13. Using these results, identify (i) the most strongly acidic solution and (ii) the most strongly alkaline solution.(2)

(d)Suggest why the test tube must be rinsed with distilled water between testing each solution.(1)

In a class experiment, 15 cm3 of dilute hydrochloric acid exactly neutralises 25 cm3 of sodium hydroxide solution.

(a)State how the volume of acid needed changes if the volume of alkali used is increased, but the concentrations stay the same.(1)

(b)Calculate the volume of the same acid needed to exactly neutralise 75 cm3 of the same sodium hydroxide solution.(2)

A group of pupils repeated a neutralisation experiment three times, adding acid to 25 cm3 of alkali until the indicator just changed colour. Their volumes of acid added were: trial 1 = 18.2 cm3, trial 2 = 18.3 cm3, trial 3 = 21.6 cm3.

(a)Identify which of the three results is anomalous.(1)

(b)Calculate the mean volume of acid needed, using only the two concordant (closely agreeing) results.(2)

A technician dissolves 6 g of sodium hydroxide in 3 dm3 of water to make a cleaning solution. Calculate the concentration of the solution in g/dm3.

(2)

Neutralisation reactions are useful in everyday life.

(a)State why excess stomach acid can cause discomfort, and name the type of substance used in indigestion tablets to treat this.(2)

(b)Explain why adding calcium hydroxide (lime) to acidic soil helps crops grow better.(2)

Aisha mixes 25 cm3 of dilute hydrochloric acid with 25 cm3 of sodium hydroxide solution in a polystyrene cup and measures the temperature. The initial temperature of both solutions was 19.5 degC. The highest temperature reached during the reaction was 26.0 degC.

(a)Calculate the temperature rise during the reaction.(2)

(b)Explain, in terms of energy, why the temperature increased during this reaction.(2)

REQUIRED PRACTICAL. Freya wants to find the exact volume of dilute nitric acid needed to neutralise 20 cm3 of potassium hydroxide solution using phenolphthalein indicator.

(a)State one safety precaution Freya should take during this experiment.(1)

(b)Describe how Freya would know when the alkali has been exactly neutralised, using phenolphthalein indicator.(2)

(c)Freya's teacher suggests using a pH probe and data logger instead of phenolphthalein indicator. Suggest two advantages of using a pH probe and data logger in this investigation.(2)

(d)Suggest one reason why a numerical pH reading is more useful than a colour observation when comparing results between different groups in the class.(1)

Explain how you could carry out an investigation to find the volume of dilute hydrochloric acid needed to exactly neutralise 25 cm3 of sodium hydroxide solution. In your answer, include: the equipment and method you would use, how you would know when the solution is neutral, and how you would make your results more reliable.

(6)

A group's neutralisation investigation gave volume readings that varied a lot between repeats (a range of 4 cm3 between the highest and lowest reading). Suggest one improvement to the method that would make the results more precise, and explain how it would help.

(3)

Limewater is used to test for carbon dioxide gas.

(a)Give the word equation for the reaction between calcium hydroxide (limewater) and carbon dioxide.(1)

(b)Explain why limewater turns cloudy when carbon dioxide is bubbled through it.(2)

A school technician knows that 20 cm3 of dilute sulfuric acid exactly neutralises 30 cm3 of a potassium hydroxide solution. For the next lesson, 8 groups each need 45 cm3 of the potassium hydroxide solution neutralised.

(a)Calculate the total volume of dilute sulfuric acid the technician must prepare for all 8 groups.(3)

(b)Suggest one reason why the technician should prepare slightly more than this calculated volume of acid.(1)

Jamal says: 'Alkalis are always safe to touch because they are not acids.' Evaluate this statement using your knowledge of pH and hazard symbols.

(4)

Mark scheme · K9 Acids and Alkalis

Question 1

(a) B1 7 cao
(a) Answer: 7
(b) B1 0 to 6 oe (accept 'less than 7')
(b) Answer: 0 to 6 (less than 7)
(c) B1 8 to 14 oe (accept 'more than 7')
(c) Answer: 8 to 14 (more than 7)

Question 2

B1 D cao
Answer: D) 13

Question 3

(a) B1 red oe
(a) Answer: red
(b) B1 green cao
(b) Answer: green
(c) B1 purple oe (accept violet)
(c) Answer: purple (violet)

Question 4

(a) B1 hydrochloric acid cao
(a) Answer: hydrochloric acid
(b) B1 sulfuric acid cao
(b) Answer: sulfuric acid
(c) B1 ethanoic acid oe (accept acetic acid)
(c) Answer: ethanoic acid
(d) B1 citric acid cao
(d) Answer: citric acid

Question 5

(a) B1 sodium hydroxide cao
(a) Answer: sodium hydroxide
(b) B1 calcium hydroxide cao
(b) Answer: calcium hydroxide
(c) B1 ammonia oe (accept ammonia solution)
(c) Answer: ammonia

Question 6

(a) B1 magnesium chloride
(a) B1 hydrogen
(a) Answer: magnesium + hydrochloric acid -> magnesium chloride + hydrogen
(b) B1 hold a lit/burning splint at the mouth of the test tube; gas burns with a squeaky pop, confirming hydrogen
(b) Answer: Hold a lit splint at the mouth of the test tube. If the gas is hydrogen it burns rapidly with a squeaky pop.

Question 7

(a) B1 calcium chloride
(a) B1 water and carbon dioxide (both needed)
(a) Answer: calcium carbonate + hydrochloric acid -> calcium chloride + water + carbon dioxide
(b) B1 bubble the gas through limewater; limewater turns cloudy/milky if carbon dioxide is present
(b) Answer: Bubble the gas through limewater. If carbon dioxide is present, the limewater turns cloudy (milky).

Question 8

(a) B1 sodium chloride cao
(a) B1 equation correctly completed with + water
(a) Answer: sodium chloride (+ water)
(b) B1 potassium sulfate cao
(b) B1 equation correctly completed with + water
(b) Answer: potassium sulfate (+ water)
(c) B1 calcium nitrate cao
(c) B1 equation correctly completed with + water
(c) Answer: calcium nitrate (+ water)

Question 9

(a) B1 safety goggles / eye protection
(a) Answer: safety goggles (eye protection)
(b) M1 iii then ii (sample placed in tube, then indicator added)
(b) M1 iv then i (swirl to mix, then read colour against chart)
(b) Answer: iii, ii, iv, i
(c) B1 lemon juice (i)
(c) B1 oven cleaner (ii)
(c) Answer: (i) lemon juice, (ii) oven cleaner
(d) B1 to prevent contamination/carry-over from the previous solution, which would give an inaccurate pH reading for the next solution
(d) Answer: So the previous solution does not contaminate the next sample and cause an inaccurate reading.

Question 10

(a) B1 the volume of acid needed increases in direct proportion to the volume of alkali (oe)
(a) Answer: The acid volume increases in direct proportion to the alkali volume.
(b) M1 scale factor = 75 / 25 = 3 (or equivalent ratio method) ft from (a)
(b) A1 45 cm3 cao
(b) Answer: 45 cm3

Question 11

(a) B1 21.6 cm3 (it does not agree with the other two close results)
(a) Answer: 21.6 cm3
(b) M1 18.2 + 18.3 = 36.5 ft excludes candidate's identified anomaly from (a)
(b) A1 18.25 cm3 (awrt 18.3 cm3) cao
(b) Answer: 18.25 cm3 (awrt 18.3 cm3)

Question 12

M1 concentration = mass / volume = 6 / 3
A1 2 g/dm3 cao
Answer: 2 g/dm3

Question 13

(a) B1 excess (hydrochloric) acid in the stomach causes irritation/pain (indigestion or heartburn)
(a) B1 indigestion tablets contain a base/alkali, e.g. magnesium hydroxide or calcium carbonate
(a) Answer: Too much stomach acid causes pain/indigestion; tablets contain a base such as magnesium hydroxide to neutralise the excess acid.
(b) M1 calcium hydroxide is a base/alkali which reacts with and neutralises the acid in the soil
(b) A1 this raises the soil pH towards neutral, giving the correct conditions for the crop to grow / for nutrients to be available
(b) Answer: Calcium hydroxide neutralises the excess acid in the soil, raising the pH towards neutral so crops grow better.

Question 14

(a) M1 26.0 - 19.5
(a) A1 6.5 degC cao
(a) Answer: 6.5 degC
(b) B1 neutralisation is an exothermic reaction ft from (a) if temperature rise correctly identified
(b) B1 energy is transferred from the reacting chemicals to the surroundings (the solution), increasing its temperature
(b) Answer: Neutralisation is exothermic; energy is transferred to the solution/surroundings, raising its temperature.

Question 15

(a) B1 wear eye protection (goggles), as acids and alkalis can irritate/damage eyes and skin
(a) Answer: Wear eye protection, since acids and alkalis can irritate or damage eyes and skin.
(b) M1 phenolphthalein is pink/magenta in the alkali
(b) A1 acid is added drop by drop, swirling, until the pink colour just disappears (turns colourless), showing the solution is neutral
(b) Answer: Phenolphthalein is pink in the alkali; acid is added drop by drop until the pink colour just disappears, showing the solution is neutral.
(c) B1 gives an exact numerical pH value rather than a subjective colour judgement, reducing human error in identifying the end point
(c) B1 can record continuous/rapid readings and produce a graph showing how pH changes, allowing more precise determination of the neutral point
(c) Answer: It gives an exact numerical pH value (less subjective than judging colour) and can record continuous readings to plot how pH changes.
(d) B1 numerical data can be compared precisely/averaged between groups, whereas colour judgements are subjective and vary between people
(d) Answer: Numbers can be compared and averaged precisely, whereas judging colour is subjective and varies between people.

Question 16

Level 1 (1-2): Basic or isolated statements about equipment or a single step in the method are given, with little logical structure and limited use of scientific terms.
Level 2 (3-4): A method with some relevant steps in a logical order is described, including reference to measuring volumes and using an indicator, but with limited detail on how the exact neutralisation point is identified and/or reliability is not addressed.
Level 3 (5-6): A clear, logically ordered method is described: measuring a fixed volume of alkali, adding acid gradually with a suitable indicator to identify the exact neutralisation point, and repeating the experiment to obtain a reliable mean. Relevant safety precautions are included.
Indicative content:
- Measure a fixed volume of sodium hydroxide solution (e.g. 25 cm3) into a conical flask using a pipette or measuring cylinder.
- Add a few drops of a suitable indicator (e.g. phenolphthalein or universal indicator) to the alkali.
- Note the initial colour of the solution in the flask.
- Fill a burette with the dilute hydrochloric acid and record the starting volume.
- Add the acid to the flask a little at a time, swirling the flask continuously to mix.
- Slow down and add the acid drop by drop as the colour starts to change, to avoid overshooting the end point.
- Stop adding acid at the point where the indicator just changes colour, showing the solution is neutral.
- Record the volume of acid used (final burette reading minus starting reading).
- Repeat the experiment at least twice more and calculate a mean volume from concordant results, ignoring any anomalous readings, to improve reliability.
- Wear eye protection throughout, since acids and alkalis can irritate skin and eyes.

Question 17

B1 named improvement, e.g. use a burette instead of a measuring cylinder to add the acid, or add acid drop by drop near the expected end point, or use a pH probe for an objective reading
B1 explanation linking the improvement to greater precision, e.g. a burette measures to 0.1 cm3 rather than the nearest 1 cm3, reducing the uncertainty in identifying the exact neutralisation point
B1 a second distinct valid point, e.g. repeat the experiment and take a mean of concordant results to reduce the effect of random error
Answer: Use a burette (measures to 0.1 cm3) instead of a measuring cylinder, and add acid drop by drop near the end point; this reduces uncertainty and the chance of overshooting, giving more precise, closely agreeing results.

Question 18

(a) B1 calcium hydroxide + carbon dioxide -> calcium carbonate + water
(a) Answer: calcium hydroxide + carbon dioxide -> calcium carbonate + water
(b) M1 calcium carbonate is produced, which is insoluble in water ft from (a)
(b) A1 the insoluble solid forms a suspended (white) precipitate, making the solution look cloudy/milky
(b) Answer: Insoluble calcium carbonate is formed; it stays suspended as a white precipitate, making the limewater look cloudy.

Question 19

(a) M1 scale factor per group = 45 / 30 = 1.5 (or equivalent ratio method)
(a) M1 acid per group = 20 x 1.5 = 30 cm3
(a) A1 total for 8 groups = 30 x 8 = 240 cm3 (accept 0.24 dm3) cao
(a) Answer: 240 cm3 (0.24 dm3)
(b) B1 any sensible reason, e.g. to allow for spillages, measurement errors, or in case a group needs to repeat the experiment
(b) Answer: To allow for spillages, measurement errors, or repeats.

Question 20

B1 the statement is incorrect / not fully correct
B1 alkalis can also be corrosive or irritant, especially when concentrated, e.g. concentrated sodium hydroxide can burn skin and damage eyes, shown by the corrosive hazard symbol
B1 the further the pH is from 7 (towards 0 or towards 14), the more hazardous the substance is generally likely to be, for both acids and alkalis
B1 safety depends on concentration and identity of the substance, not simply on whether it is an acid or an alkali, e.g. dilute vinegar (a weak, dilute acid) is safe to eat, but concentrated sodium hydroxide (an alkali) is highly corrosive
Answer: The statement is incorrect: alkalis can be just as hazardous as acids, especially when concentrated (e.g. sodium hydroxide is corrosive), and hazard depends on concentration/identity, not simply on being an acid or alkali.

Acids and Alkalis - Worksheets, Questions and Revision