LITo classify a substance as acidic or basic using pH and indicators.
SC: I can:
Try these 3questions before today's new content. Click an answer for instant feedback — your teacher will walk through them with you.
The video showed how universal indicator changes colour across the pH scale. Predict: a few drops of universal indicator are added to each of three colourless liquids — vinegar, soapy water, and distilled water. What colour would each turn, and what does that tell you about each liquid's pH?
Releases H⁺ ions (hydrogen ions) in water.
Corrosive — can burn skin and eyes; sour taste in food.
Common examples
Releases OH⁻ ions (hydroxide ions) when dissolved in water (then called an alkali).
Caustic — burns just as badly as acid.
Three types you'll meet
Memory hook: Acid → H⁺ · Base → OH⁻. Every alkali is a base, but not every base is an alkali (only the dissolved ones).
Two of the most commonly occurring groups of compounds are acids and bases. They are found in many everyday items and places, such as cleaning products, swimming pools and kitchens. Many of the foods we eat are also either acidic or basic, and these properties help with digestion. Acids can be dangerous because they are corrosive, and they can react with some metals to produce hydrogen gas and metallic salts. However, when an acid and a base react together, they neutralise each other, forming products that are usually much safer.
Figure 4.14 — Weak acids and bases occur naturally in many of the foods we eat every day.
An acid is a corrosive chemical substance that produces hydrogen ions (H⁺) when mixed with water. The hydrogen ions can react with other substances to produce water and hydrogen gas, as well as ionic salts, which are made up of cations and anions.
The higher the concentration of hydrogen ions produced by an acid, the higher its acidity. Strong acids are very dangerous, especially when they contact skin and eyes. That is why you must always wear protective clothing and eyewear when working with acids in the laboratory.
Weak acids are important in our diet. Citrus fruits contain a weak acid, called citric acid, which contributes to their sour flavour. Soft drinks, coffee and vinegar also contain weak acids.
A base is a substance that reacts with an acid to neutralise it. Bases that dissolve in water are called alkalis. When mixed with water, bases produce hydroxide ions (OH⁻) — an atom of oxygen bonded to an atom of hydrogen, with an overall negative charge.
Strong bases are described as caustic, meaning they cause burns by chemical action; therefore, they can be just as dangerous as acids. Bases such as sodium hydroxide and ammonia react with oils and fats and so are used in many household cleaners. Weak bases are found in toothpaste, conditioners, antacid tablets and baking powder.
Three common types of substances act as bases:
The acidity of a solution is measured on a scale called the pH scale (Figure 4.15). The higher the proportion of H⁺ ions, the lower the pH. The lower the proportion of H⁺ ions in a solution compared to hydroxide ions (OH⁻), the higher the pH. Acids have a low pH, while bases have a high pH. A solution with a pH of 7 is neutral, a solution with a pH of less than 7 is acidic, and a solution with a pH greater than 7 is basic. Pure water is neutral because it has a pH of 7 at 25 °C.
You can measure pH by using a digital pH meter, or by adding an indicator to a solution and matching its colour to a chart. Indicators change colour depending on whether they are mixed with an acid or a base. A common indicator is litmus, which turns red when mixed with acids and blue when mixed with bases.
Figure 4.15 — the pH scale (with everyday examples):
| pH | Class | Everyday example |
|---|---|---|
| 0 | Strongly acidic | Battery acid |
| 1 | Acidic | Stomach acid |
| 2 | Acidic | Lemons |
| 3 | Acidic | Vinegar |
| 4 | Acidic | Acid rain |
| 5 | Weakly acidic | Clean rain |
| 6 | Weakly acidic | Milk |
| 7 | Neutral | Pure water |
| 8 | Weakly basic | Eggs |
| 9 | Basic | Bicarbonate of soda |
| 10 | Basic | Milk of magnesia |
| 11 | Basic | Ammonia-based cleaning products |
| 12 | Strongly basic | Household bleach |
| 13 | Strongly basic | Oven cleaner |
| 14 | Strongly basic | Drain cleaner |
Different indicators show a wide range of colour changes depending on the pH of a solution. For example, a universal indicator can show multiple colours to indicate the pH from 1 to 14, from red in strong acids to violet in strong bases (Figure 4.16):
Figure 4.16 — Universal indicator colour change across pH 0–14: red (strong acid) → orange → yellow → green (neutral) → blue → violet (strong base).
Some indicators display only two or four colours throughout the pH range. This can be useful when we are only concerned about whether a substance is above or below a certain pH. Figure 4.17 shows a variety of different indicators and their colour changes at specific pH values. For example, bromocresol green changes from yellow to blue at pH 4–5, so it would not be able to identify with certainty if a solution is basic (alkaline), though it could identify if its pH is above or below 4–5.
Watch the indicators flip in real time, then use the chart below as your reference whenever you need to recall what each one looks like at a given pH.
Indicator
Figure 4.17 — Colour transitions for eight common pH indicators across the 0–14 scale. The first seven flip across a narrow pH window — useful when you only need to know if a solution is above or below a specific pH. Universal indicator is the only one that gives a distinct colour for every step.
We do · whole class · 3 prompts
A student measures a sample with a pH meter and reads pH 9. Predict what each indicator will show.
Prompt 1 — where on the 0–14 scale, and what does that mean?
Prompt 2 — universal indicator colour?
Prompt 3 — what does litmus paper do?
Wrong — pH and acid strength point in OPPOSITE directions.
The pH scale measures how many H⁺ ions are in solution: more H⁺ means a lower pH and a stronger acid. Lemon juice at pH 2 is much more acidic than milk at pH 6, and battery acid at pH 0 is the strongest of the three.
Rule: low pH = strong acid; high pH = strong base; pH 7 is the middle (neutral).
Wrong — universal indicator gives a colour BAND, not a number.
Universal indicator runs through six colours: red (pH ≈ 1–2) → orange (pH 3–4) → yellow (pH 5–6) → green (pH 7, neutral) → blue (pH 8–11) → purple (pH 12–14). Each colour covers a range of pH values.
If you need a precise reading like "pH 4.7", you must use a digital pH meter — not universal indicator.
Rule: universal indicator narrows the range; a pH meter gives the exact number.
Wrong — litmus is a two-colour indicator: red in acid, blue in alkali. That is the whole signal.
Lemon juice (pH 2), vinegar (pH 3) and concentrated HCl (pH 0) all turn litmus the same red. Litmus cannot distinguish weak from strong acids — it only tells you which side of pH 7 you are on.
Rule: litmus = acid vs alkali; universal indicator = rough pH; pH meter = exact pH.
Starting from the most acidic end and moving up the pH scale, universal indicator runs from red to violet. Fill in the missing colours.
One thing I now understand about pH or indicators that I didn't understand at the start of the lesson:
Next class (Fri 8 May, P2): neutralisation — what happens when an acid meets a base.