Acids & Alkalis
Acids & Alkalis
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Acids & Alkalis
Open with familiar examples such as lemon juice, soap, vinegar, or bicarbonate solution, but keep the lesson focused on identifying acidity and alkalinity using indicators and the pH scale.
Learning goal
I can test safe substances with an indicator and arrange them on the pH scale.
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What words do you link with acids, alkalis and pH?
Use this to surface prior ideas such as sour, soap, neutral, colour change, indicator, numbers, safe testing. Pick up misconceptions without correcting all of them yet.
Acidic and alkaline substances
Some substances are acidic.
Some substances are alkaline.
Others are neutral.
We can identify them by testing safely with an indicator.
Keep examples to safe classroom substances only. Avoid introducing strong laboratory acids or alkalis in detail, since the lesson objective centres on safe testing and classification.
What acids and alkalis are like
Acidic substances have a pH below 7.
Neutral substances have a pH of 7.
Alkaline substances have a pH above 7.
The further from 7, the stronger the acidity or alkalinity.
This is the key classification slide. Emphasise that pH is a scale, not just three boxes.
Everyday examples
Lemon juice and vinegar are often acidic.
Water is neutral.
Soap solutions are often alkaline.
Testing shows where each substance fits on the pH scale.
Use examples that are likely to match the safe solutions available in class. If the actual set differs, speak to those instead.
Why we test, not guess
Appearance does not tell us whether a substance is acidic, neutral, or alkaline. Two clear liquids may have different pH values, so chemists use indicators to give evidence.
This slide addresses the common misconception that colour or smell alone reveals acidity or alkalinity.
Indicators
An indicator changes colour depending on pH.
This colour change helps us classify a substance.
In this lesson, the main tools are universal indicator and pH paper.
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Which pH value is neutral?
11
7
3
0
Checks the core classification idea from slides 2–5. Distractors are other values seen or implied on the pH scale.
Acids, alkalis and indicators
A substance with a pH below 7 is acidic. A substance with a pH above 7 is alkaline. We should not guess from appearance, so we use an indicator to test safely.
Although 7 appears twice, each blank is fixed by the sentence meaning because both statements define the same boundary value taught on the slides.
Universal indicator
Universal indicator shows different colours across the pH scale.
The colour is matched to a pH chart.
This gives an estimate of how acidic or alkaline a substance is.
If students already know litmus, briefly note that universal indicator gives more detail because it can suggest a pH value rather than just acid or alkali.
pH paper
pH paper is dipped into a solution.
It changes colour according to the pH.
The colour is compared with a reference scale.
This helps place the substance at the correct point on the pH scale.
Keep the distinction practical: both tools use colour change, but pH paper is the strip itself while universal indicator is the indicator solution.
Reading indicator colours
Red to orange usually shows an acidic pH.
Green usually shows a neutral pH.
Blue to purple usually shows an alkaline pH.
The chart must be used to estimate the pH number.
State that exact shades can vary slightly depending on the indicator chart used in school, so students should always use the chart provided.
The pH scale
The pH scale usually runs from 0 to 14.
7 is neutral.
Numbers below 7 are acidic.
Numbers above 7 are alkaline.
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Placing substances on the scale
Indicator colour → compare with pH chart → estimate pH number → place the substance on the pH scale
This slide gives the lesson’s core chemistry process. Read it as a sequence from evidence to classification.
What does universal indicator help you estimate?
pH value
volume
temperature
mass
Targets slide 7: universal indicator gives an estimate of how acidic or alkaline a substance is by linking colour to pH.
Using indicator results
Universal indicator shows different colours across the pH scale. pH paper is dipped into a solution and the result is compared with a reference scale. This helps place the substance at the correct point on the pH scale.
Echoes the wording of slides 7–11 and checks the practical sequence for reading indicator results.
How do you go from colour change to pH result?
Expected elements: indicator colour is evidence; compare with the pH chart or reference scale; estimate the pH number; then classify as acidic, neutral or alkaline and place it on the pH scale.
Comparing pH values
A substance at pH 3 is acidic.
A substance at pH 7 is neutral.
A substance at pH 11 is alkaline.
Their pH values show where they belong on the same scale.
Use these as model placements only. If your class results differ, connect their tested solutions to the same pattern.
Using safe solutions
Only safe solutions should be tested in this lesson.
Small samples are enough.
Use clean equipment for each substance.
Record the colour and the pH carefully.
Although this is not an activity slide, it anchors good laboratory practice linked directly to the teacher’s stated materials: universal indicator, pH paper, and safe solutions.
From test result to classification
Test the substance with universal indicator or pH paper.
Match the colour to the pH chart.
Decide whether it is acidic, neutral, or alkaline.
Place it in order on the pH scale.
This slide resolves the lesson objective by tying together the equipment, the indicator result, and the final placement on the pH scale.
A substance at pH 11 is...
neutral
acidic
alkaline
unknown
Checks application of the scale from slides 12–14, including the model placements.
From test to classification
Only safe solutions should be tested in this lesson. Match the colour to the pH chart and then decide whether the substance is acidic, neutral, or alkaline. Finally, place it in order on the pH scale.
Reinforces both safe practice and the final classification process from slides 13–14.
What is one thing you will remember about pH?
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What helped you understand indicators best?
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Glossary
acidic - having a pH below 7
alkaline - having a pH above 7
neutral - having a pH of 7
indicator - a substance that changes colour depending on pH
universal indicator - an indicator that shows a range of colours across the pH scale
pH paper - paper that changes colour to show the pH of a solution
pH scale - the scale used to show how acidic or alkaline a substance is
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