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Unit 8: Rates of reaction

Investigating acids and alkalis — HIA International - Grade 8 Revision Notes & Topical Questions

5 min read 10 parts Lesson 3 of 10 2 quizzes

📚 HIA International - Grade 8 Hub / Topical Revision
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Lesson summary

Investigating acids and alkalis — HIA INTERNATIONAL notes: Use indicators to find the pH of solutions and plan a fair, testable investigation about acids and alkalis.The pH scale runs from 0 to 14 — the lower the pH the stronger the acid and the higher the pH the stronger the alk…

  • Acids have a pH below 7
  • Alkalis have a pH above 7
  • Neutral solutions have a pH of 7
  • A pH meter
  • Red cabbage juice
  • Universal indicator

Part 1 of 8Acids, alkalis and the pH scale

Acids, alkalis and the pH scale
The pH scale runs from 0 to 14
  • The pH scale runs from 0 to 14 the lower the pH the stronger the acid and the higher the pH the stronger the alkali.
Acids have a pH below 7
  • Acids have a pH below 7 for example hydrochloric acid, citric acid in lemons and ethanoic acid in vinegar.
Alkalis have a pH above 7
  • Alkalis have a pH above 7 for example sodium hydroxide, limewater and ammonia solution.
Neutral solutions have a pH of 7
  • Neutral solutions have a pH of 7 for example pure water and sodium chloride solution.
A pH meter
  • A pH meter gives a more precise reading than an indicator colour chart.

Part 2 of 8Indicators

Indicator
Indicator a substance that changes colour depending on whether it is in an acid or an alkali.
Litmus
  • Litmus red in acid, blue in alkali and purple when neutral.
Red cabbage juice
  • Red cabbage juice a natural indicator.
Universal indicator
  • Universal indicator a mixture of indicators giving a range of colours, so it shows the pH number.
Red (pH 0–2), orange or yellow (3–6), green (7), blue (8–11), purple (12–14).

Part 3 of 8Testing the pH of a solution

1
Put on eye protection and pour a small, equal volume of each solution into separate labelled test tubes.
2
Add the same number of drops of universal indicator to each tube.
3
Swirl gently to mix.
4
Compare the colour with the universal indicator chart in good light.
5
Record the pH and whether the solution is acidic, neutral or alkaline in a results table.
6
Repeat, or check with a pH meter, for more precise results.

Part 4 of 8Safety with acids and alkalis

1Wear eye protection
  • Wear eye protection acids and alkalis can be corrosive or irritant.
2
Use dilute solutions and wash spills with plenty of water.
3Corrosive symbol
  • Corrosive symbol shows a substance that destroys skin and materials.
4Irritant symbol (exclamation mark)
  • Irritant symbol (exclamation mark) shows a substance that causes redness or itching.

Part 5 of 8Asking a testable question

Asking a testable question
Scientists write a question so that it can be tested
  • Scientists write a question so that it can be tested a vague question such as which indigestion remedy is best does not say what best means.
A testable version names what is measured
  • A testable version names what is measured for example which indigestion powder neutralises the acid using the least amount of powder.
Possible questions
  • Possible questions how much lime neutralises an acid lake, or how much toothpaste neutralises mouth acid.

Part 6 of 8Planning a fair test

1Variables
Variables the things that can change in an investigation.
2Independent variable
  • Independent variable the one variable you change, such as the type of indigestion powder.
3Dependent variable
  • Dependent variable the one you measure, such as the number of spatulas needed to neutralise the acid.
4Control variables
  • Control variables the ones kept the same, such as the volume and pH of acid and the number of drops of indicator.
5A model is used instead of the real thing
  • A model is used instead of the real thing a beaker of acid stands in for stomach acid.
6The end point
  • The end point the indicator turns green when the acid is neutralised.

Part 7 of 8Repeating and trusting results

Repeating and trusting results
Repeat the investigation
  • Repeat the investigation repeating makes results more reliable.
Calculate the mean
  • Calculate the mean add the results and divide by the number of repeats.
Spot an anomaly
  • Spot an anomaly a result that looks wrong compared with the others, which may come from a measuring slip.
Leave the anomaly out of the mean
  • Leave the anomaly out of the mean and repeat that test if possible.
When judging someone else's evidence
  • When judging someone else's evidence ask whether the test was fair and whether it was repeated.

Part 8 of 8Recording and displaying results

Draw a results table
  • Draw a results table with the independent variable in the first column and the repeats and mean beside it.
Use a bar chart
  • Use a bar chart when the independent variable is in categories, such as different powders.
Write a conclusion
  • Write a conclusion from the actual results, not from what you expected to happen.

Key terms

TermMeaning
IndicatorA substance that changes colour in acids and alkalis.
Universal indicatorA mixture of indicators that shows the pH as a range of colours.
pH scaleA scale from 0 to 14 that measures how acidic or alkaline a solution is.
Independent variableThe variable you change in an investigation.
Dependent variableThe variable you measure.
Control variableA variable kept the same to make a fair test.
Anomalous resultA result that does not fit the pattern of the others.
MeanThe average of repeated results.

Avoid these mistakes

1
Changing more than one variable — a fair test changes only one variable at a time.
2
Saying a low pH means a strong alkali — a low pH means a strong acid.
3
Including an anomalous result in the mean — identify it and leave it out.
4
Thinking one test is enough — repeating makes the results more reliable.

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