GCSE · Chemistry · Edexcel · Spec 1CH0

Acids, alkalis and the pH scale

A clear liquid, no label. You add phenolphthalein and it stays colourless. Is it an acid? Not necessarily, and by the end you'll know exactly why.

Case file · indicators

What can each colour actually prove?

Detective job. An indicator is a substance that changes colour depending on the pH of the solution it is in. Here is your evidence file. LITMUS: in an acid, blue paper turns red and red paper stays red; in an alkali, red paper turns blue and blue paper stays blue; in a neutral solution neither paper changes, and litmus solution is purple. METHYL ORANGE: red in acid, yellow in alkali and in neutral. PHENOLPHTHALEIN: colourless in acid or neutral, pink in alkali. UNIVERSAL INDICATOR: green at neutral. Now take each observation below and tick EVERY condition it could have come from. Some will fit one condition. Some will fit two.

  • A Acidic
  • B Neutral
  • C Alkaline
  1. Blue litmus paper turns red
  2. Red litmus paper stays red
  3. Red litmus paper turns blue
  4. Blue litmus paper stays blue
  5. Litmus solution is purple
  6. Methyl orange is red
  7. Methyl orange is yellow
  8. Phenolphthalein is colourless
  9. Phenolphthalein is pink
  10. Universal indicator is green

The number behind the labels

Where does 'acidic' stop?

You've been sorting solutions into acidic, neutral and alkaline. The pH scale is the number line behind those labels, running from 0 to 14. A solution with a pH less than 7 is acidic. Drag the two ends to mark the acidic part of the scale, then decide whether each end gets a filled circle (that pH is included) or an open circle (left out). Ask yourself about the end at 7: does pH 7 itself count as acidic?

Check your instinct

Which idea is closest to yours?

Acids release hydrogen ions (H⁺) when they dissolve in water, and the concentration of those ions decides how acidic the solution is. Solution A has a pH of 2. Solution B has a pH of 5. Both are acidic.

Which idea is closest to what you think right now?
How sure are you?

Predict, then check

A base is a substance that can neutralise an acid. A base that dissolves in water is called an alkali, and it releases hydroxide ions (OH⁻) into the water.

A beaker holds an acidic solution. You stir in an alkali. What happens to the pH of the solution?

Two ways to find a pH

Universal indicatorvsDigital pH meter

Universal indicator gives you a colour to judge. A digital pH meter gives you a number.

Focus

What you get

Universal indicator

A colour, compared with a reference chart to estimate the pH

Digital pH meter

A numerical pH value

The insight

One method hands you a colour to interpret. The other hands you the number itself.

How it is judged

Universal indicator

By eye, matching the colour to the chart. This comparison is subjective.

Digital pH meter

Read off as a digital value

Accuracy and precision

Universal indicator

An estimate

Digital pH meter

More accurate and more precise than any indicator

The colours or range it shows

Universal indicator

Red, orange and yellow as a solution becomes more acidic; green at neutral; blue and purple as it becomes more alkaline

Digital pH meter

No colours at all, just the number

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Colours are clues, not verdicts. Learn to read them like a detective, and see what the pH scale is really telling you.

What you need to know

  • Decide whether a solution is acidic, neutral or alkaline from its pH, or from what its indicators show.
  • Say what each indicator colour can prove and what it can't, because some colours fit two conditions.
  • Explain pH in terms of hydrogen ions and hydroxide ions, including why an alkali raises the pH of an acid.
  • Explain why a digital pH meter gives a better value than a colour chart.

The big picture

The pH scale runs from 0 to 14: below 7 is acidic, 7 is neutral and above 7 is alkaline. Hydrogen ions (H⁺) make a solution acidic, and the more of them there are, the lower the pH. Hydroxide ions (OH⁻) from an alkali neutralise hydrogen ions and push the pH up towards 7. Indicators change colour with pH, but one colour is evidence, not a verdict, and a digital pH meter gives a more accurate and precise value than any indicator.

Key points

1An acid releases hydrogen ions (H⁺) when dissolved in water. The higher the concentration of hydrogen ions, the more acidic the solution and the lower its pH.
2A base can neutralise an acid. A base that dissolves in water is an alkali, and it releases hydroxide ions (OH⁻) into the water.
3On the pH scale (0 to 14), a pH below 7 is acidic, a pH of 7 is neutral (equal concentrations of H⁺ and OH⁻) and a pH above 7 is alkaline.
4Hydroxide ions neutralise hydrogen ions, producing water, so adding an alkali to an acidic solution raises its pH towards 7.
5An indicator changes colour depending on the pH of the solution. Pink phenolphthalein and red methyl orange each point to one condition. Colourless phenolphthalein and yellow methyl orange each fit two.
6Universal indicator's colour is compared with a reference chart to estimate the pH. A digital pH meter gives a more accurate and precise numerical value.

Worked example

Problem

A student dips red litmus paper into a mystery solution and it stays red. She dips blue litmus paper into the same solution and it stays blue. What can she say about the solution?

⚠ Watch out

Treating one colour as a verdict: 'phenolphthalein is colourless, so it's an acid' is wrong, because colourless fits acidic AND neutral. A second slip is reading a bigger pH number as more hydrogen ions, when a higher concentration of hydrogen ions means a lower pH.

🧠

Memory hook

Low pH, lots of H⁺: the two run in opposite directions. And for any single indicator colour, ask the detective's question: who else could have made this colour?

✓

Check yourself

Methyl orange turns yellow in a mystery solution. A friend says, 'It must be alkaline.' What's wrong with that, and which second result would help?

Flashcards

(13)
In terms of ions, what is an acid?
A substance that releases hydrogen ions (H⁺) when it dissolves in water.
What is the difference between a base and an alkali?
A base is a substance that can neutralise an acid. An alkali is a base that dissolves in water, and it releases hydroxide ions (OH⁻).
On the 0 to 14 pH scale, what do a pH below 7, exactly 7 and above 7 mean?
Below 7: acidic. Exactly 7: neutral. Above 7: alkaline (basic).
How does the concentration of hydrogen ions relate to acidity and pH?
The higher the concentration, the more acidic the solution and the lower the pH.
Going from pH 7 up to pH 14, which ion's concentration increases?
Hydroxide ions (OH⁻). The solution becomes more alkaline.
What is true of hydrogen ions and hydroxide ions in a neutral solution?
They are present in equal concentrations, and the pH is 7.
Why does adding an alkali to an acidic solution raise its pH?
Hydroxide ions from the alkali neutralise some of the hydrogen ions, producing water. So the pH rises towards 7.
What is an indicator?
A substance that changes colour depending on the pH of the solution it is in. It shows whether a solution is acidic, neutral or alkaline.
What does litmus do in acidic, alkaline and neutral solutions?
Acid: blue paper turns red, red paper stays red. Alkali: red paper turns blue, blue paper stays blue. Neutral: neither paper changes, and litmus solution is purple.
Methyl orange: which conditions make it red, and which make it yellow?
Red in acidic conditions. Yellow in alkaline (and neutral) conditions.
Phenolphthalein: which conditions leave it colourless, and which turn it pink?
Colourless in acidic or neutral solutions. Pink in alkaline solutions.
What is universal indicator, and how is it used to estimate a pH?
A mixture of indicators that gives a full range of colours across the pH scale. Its colour is compared with a reference chart to estimate the pH.
Why does a digital pH meter beat universal indicator?
Matching a colour to a chart is subjective. A digital pH meter gives a more accurate and precise numerical pH value than any indicator.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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