Acids, Bases and Indicators in Action
You'll be able to conduct a practical investigation to test pH using indicators.
In 1909, Danish chemist Søren Sørensen invented the pH scale to save his 10,000-litre brewery fermentation vats from going sour.
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Red cabbage juice turns pink in lemon juice and green in baking soda solution. What do you think this colour-changing ability tells a scientist about those substances? Have you ever seen a natural substance change colour like this?
Before digital pH meters existed, chemists still needed to measure how acidic or basic a substance was. How do you think they did this? What limitations might a natural colour-based indicator have compared to a digital meter?
● Know
- That natural indicators such as red cabbage and turmeric change colour with pH
- How to safely test the pH of common substances
- How to record and present investigation results in a table
● Understand
- The advantages and limitations of natural vs synthetic indicators
- That reliable results require systematic testing and repeated observations
- How to link indicator colour changes to pH values
● Can do
- Conduct a practical investigation to test pH using indicators
- Compare natural and synthetic indicators for accuracy and ease of use
- Record results in a clear table and draw conclusions from data
Before electronic pH meters, chemists relied on indicators - natural or synthetic dyes that change colour depending on the acidity or basicity of the solution. The chemistry behind this is elegant and still relevant today.
Indicators are themselves weak acids or bases. In solution, they exist in equilibrium between two molecular forms: one that predominates in acid (HIn) and one that predominates in base (In-). These two forms have different colours because their electron structures differ, causing them to absorb different wavelengths of light.
When H+ concentration is high (low pH), the equilibrium shifts toward the acid form (HIn). When H+ concentration is low (high pH), the equilibrium shifts toward the base form (In-). The eye sees the colour of whichever form predominates.
Different indicators have different transition pH ranges. Litmus changes at pH 5-8. Phenolphthalein is colourless below pH 8.2 and pink above. Methyl orange is red below pH 3.1 and yellow above pH 4.4. By choosing the right indicator, chemists can detect specific pH ranges.
Text description
- Litmus: red (acid), purple (neutral), blue (base).
- Universal indicator: pH 1 red, 3 orange, 5 yellow, 7 green, 9 blue-green, 11 indigo, 13 violet.
- Phenolphthalein: colourless in acid or neutral, pink in base above pH 8.3.
- Red cabbage (natural): red in acid, purple neutral, blue mildly basic, green basic, yellow strong base.
- Different indicators detect different pH ranges, so use more than one for accuracy.
Phenolphthalein is a dramatic indicator used in many school titrations. In a flask containing hydrochloric acid, it is completely colourless. As sodium hydroxide is added drop by drop, the solution remains colourless until the very last drop that neutralises the final H+ ion. At that moment, the pH jumps from acidic to basic, and the solution suddenly turns bright pink. Because the change is sudden rather than gradual, phenolphthalein is good for spotting the exact moment a solution crosses from acid to base — but it tells you only which side of that line you are on, not the pH.
Classroom example: Red cabbage extract contains anthocyanin pigments whose colour changes with pH. In a classroom investigation, the extract is a natural indicator: it is commonly pink/red in acidic solutions, purple near neutral and green/yellow in basic solutions. The exact shade depends on the concentration and preparation, so compare samples under consistent lighting and use the same method each time.
Indicators change colour because the acid destroys the dye. This is false. Indicators change colour through a reversible equilibrium between two molecular forms. If you add acid to turn an indicator red, then add base to turn it blue, then add acid again, it will turn red again. The indicator molecule is not consumed or destroyed - it cycles between forms depending on pH.
You have a colourless solution and add universal indicator. It turns orange. Predict: is the solution acidic, neutral, or basic? Then explain what is happening at the molecular level.
Orange indicates pH around 4-5, so the solution is weakly acidic. The indicator molecules change structure depending on H+ concentration, and the acidic form absorbs light differently from the basic form.
Use these terms in your explanation: indicator · H+ concentration · pH · molecular structure
Testing a shelf of household substances is only an experiment if you test them the same way. Same volume of substance in each well, the same number of drops of the same indicator, the same white background to read the colour against, and a rinse between substances.
Every one of those has a reason, and the reason is the same each time: the only thing you are allowed to read off the well is the indicator’s colour. Use twice as much liquid in one well and the colour is diluted, so you read a pH closer to neutral than the truth. Read one well against a blue bench and another against white and you will call the same green two different things. Skip the rinse and the last substance is still in the pipette, quietly shifting the next reading toward its own pH.
So the method is not fussiness. Each rule closes off one way for something other than the substance to change the colour you end up writing down.
A class tests lemon juice, tap water, oven cleaner, cola and milk with universal indicator, five drops into 2 mL of each. Lemon juice goes red, water green, oven cleaner deep purple. So far so good.
Then cola turns a muddy brown that matches nothing on the chart, and the milk goes cloudy and pale. Neither is a pH reading. Cola is already dark brown, so what you are looking at is cola with a hint of indicator in it; milk is opaque, so the colour never gets out to your eye.
The rule that carries: before you trust a colour, ask whether anything other than pH could have produced it. Run it across the whole tray. Lemon juice is pale and see-through, so it passes. Tap water passes. Oven cleaner is colourless, so it passes. Cola fails on its own colour, milk fails on being opaque — and bleach fails for a third reason, because it destroys the dye outright and leaves you with a clear well that means nothing at all. For any substance that fails, you need a pH meter, which reads the solution rather than looking at it.
Why the method matters outside a classroom: pH is checked constantly in Australian industry — in winery must and finished wine, in dairy processing, in swimming pools, and in river monitoring for water quality. Almost all of it is done with a meter rather than by eye, for exactly the reason the tray above shows: wine is dark, milk is opaque, and river water carries silt. A meter reads the solution instead of relying on someone judging a colour, so two operators on two days get the same number.
Universal indicator tells you the pH. It tells you it to about the nearest whole number, by eye, and that is a different claim. Green is somewhere near 7; you cannot get 6.4 out of it, and two people reading the same well in different light can disagree by a unit. That is why the key terms in this lesson separate qualitative data from quantitative data: the colour is a description, the meter reading is a measurement. Neither is the better one in general — universal indicator is fast, cheap and enough to sort a tray of household substances, and a meter is what you use when the answer has to be a number, or when the sample is too dark or too cloudy to read a colour at all.
- Litmus
- Universal indicator
- pH meter
- A systematic test
- A number you can record, and the only method that works on a dark or cloudy sample
- Acid or base, and nothing finer — one boundary, two colours
- The same volume, drops and background for every substance, so the only thing that varies is the substance
- A colour you read against a chart to estimate pH to about the nearest whole number
Good scientists record their results systematically so that others can understand and trust their findings. A well-organised results table is essential.
Tips for recording results
- Use a ruler to draw neat tables with clear headings.
- Include units in column headings where appropriate (e.g., "pH" not just "reading").
- Record what you actually see, not what you expect to see. If the colour is "pinkish-orange," write that rather than just "red."
- Be consistent describe colours using the same words each time, or better still, compare to a colour chart.
- Note any anomalies unexpected results should be recorded, not ignored.
Drawing conclusions
After collecting your data, look for patterns:
- Which substances were acidic? Which were alkaline? Which were neutral?
- Did all three indicators agree on whether each substance was acidic or basic?
- Which indicator gave the most detailed information about pH?
- Did the pH meter readings match the indicator colours?
Wrong: "Natural indicators are less accurate than synthetic ones, so they are useless." No. Natural indicators can be useful for some purposes. The key is to understand the response range and limitations of each tool.
Right: Natural indicators can be useful for some purposes, though their response ranges and colour readings may be less consistent than manufactured indicators. Knowing the limitations of your tool is good science.
Wrong: "If an indicator does not change colour, the substance must be neutral." Not necessarily, some indicators only change in certain pH ranges. Turmeric does not change in acid, so a yellow result with turmeric could mean acid or neutral. You need multiple indicators to be sure.
Right: No colour change only means the pH is outside that particular indicator's response range. To determine whether a substance is truly neutral, you need to test with multiple indicators or use a pH meter.
Wrong: "You only need to test each substance once." No, repeating measurements and using multiple indicators improves reliability and helps identify mistakes or anomalies.
Right: Repeating tests and using multiple indicators improves reliability. A single result could be an error or anomaly, repeated consistent results give you confidence that your conclusion is correct.
Plant Indicators and Soil Conditions
Plants can also provide clues about local soil conditions. A plant species may grow more successfully in some soils than others, but observing where a plant grows is not the same method as adding a chemical pH indicator to a water sample.
In agriculture, soil pH can be measured with indicators or a pH meter. If a soil is too acidic for a crop, lime (calcium carbonate) can be used to raise its pH; the appropriate amount depends on the soil test and crop.
✍ Copy Into Your Books
▾Natural Indicators
- Red cabbage: pink (acid), purple (neutral), green/yellow (base)
- Turmeric: yellow (acid/neutral), reddish-brown (base)
- Made from plant pigments; vary in strength
Synthetic Indicators
- Litmus: red (acid), blue (base)
- Universal indicator: red to purple across pH 0–14
- More consistent and reliable than natural indicators
Recording Results
- Use clear tables with headings and units
- Record what you actually observe
- Repeat measurements for reliability
Indicator Comparison
Data Detective
At the start of this lesson, the hook reminded you that red cabbage juice changes through 11 different shades depending on how acidic or alkaline a liquid is, and that early chemists squeezed berries and flower petals to test their chemicals.
Now that you've used and compared different indicators, how would you explain to a friend why red cabbage juice works as a pH indicator? What limitations does it have compared to a digital pH meter, and how did your initial ideas compare with what you discovered?
Q1. 1. Describe the difference between qualitative and quantitative data. Give one example of each from the pH testing investigation. 4 MARKS
Q2. 2. A student wants to test whether the water from their rainwater tank is acidic. They have red cabbage indicator, universal indicator paper, and a pH meter. Design a systematic method they could use, including which tool(s) they should choose and why. 4 MARKS
Q3. 3. In the practical investigation, a student finds that red cabbage indicator turns slightly different colours for the same substance on two different days. Explain TWO factors that could cause this variation, and suggest how the student could improve the reliability of their results. 4 MARKS
Revisit Your Thinking
Go back to your Think First answer. Has your understanding changed?
- Would you now choose different equipment to test your five mystery liquids?
- What have you learned about recording results that you did not know before?
Model answers (click to reveal)
Answers
▾MCQ 1
B Red cabbage extract is prepared from a vegetable. Litmus dye is also natural in origin because it is extracted from lichens, although litmus paper itself is a manufactured test strip; the question asks specifically about a fresh vegetable extract. ACS red cabbage indicator activity; Australian National Botanic Gardens on lichen dyes.
MCQ 2
D Turmeric only changes colour in alkaline conditions. If it stays yellow, the solution could be acidic or neutral, but it is definitely not alkaline.
MCQ 3
A Synthetic indicators are manufactured with controlled chemical composition, giving more consistent and reproducible results. Natural indicators can vary depending on how they are grown and prepared.
MCQ 4
C Pink with red cabbage and orange with universal indicator both indicate an acidic pH. pH 4 is the only acidic option listed.
MCQ 5
B Substance Y shows blue with universal indicator (alkaline) but pH 7.0 (neutral) on the meter. This is inconsistent and should be retested. Substance X (green + pH 7.0) and Substance Z (red + pH 2.5) are both consistent.
Short Answer 1
Model answer: Qualitative data describes qualities or characteristics that cannot be measured with numbers. For example, recording that universal indicator turned "red" is qualitative data. Quantitative data involves numerical measurements. For example, recording that the pH meter read 3.5 is quantitative data. Both types are important: qualitative data describes what happened, while quantitative data allows precise comparison.
Short Answer 2
Model answer: The student should use the pH meter as their primary tool because it gives a precise numerical reading, which is needed to know if the water is acidic (pH < 7) and by how much. They should also use universal indicator paper as a quick check to confirm the meter reading is reasonable. The method should be: (1) collect a sample of rainwater, (2) calibrate the pH meter, (3) rinse the probe and dip it into the sample, (4) record the reading, (5) dip universal indicator paper into the sample and compare to the colour chart, (6) record both results in a table, (7) repeat the measurement twice for reliability.
Short Answer 3
Model answer: Factor 1: The concentration of the red cabbage indicator may have differed between preparations, more concentrated indicator can produce deeper colours. Factor 2: Lighting conditions when observing colours can affect perception, natural light versus artificial light may make colours appear different. To improve reliability, the student should: (1) prepare the indicator using a standardised method each time, (2) observe colours under consistent lighting, (3) use a colour chart for comparison rather than relying on memory, and (4) repeat tests with the same substance multiple times and compare results.