KS3 · Chemistry

The pH scale and universal indicator

Two beakers of clear liquid: one acid, one alkali. Looking will not tell you which, so chemists use a number and a colour.

Chemistry · The pH scale

Where does "acidic" stop?

Chemists measure how acidic or alkaline a solution is on the pH scale, which runs from 1 to 14. A solution with a pH of 1 is the most acidic of all. Water is neutral: its pH is exactly 7. Now mark the acidic part of the scale. Drag the two ends, then decide whether each end should be a filled circle (included) or an open circle (left out). Think hard about the end at 7.

Chemistry · Reading a pH

Slide each solution to its pH

Three solutions need placing on the scale. The pH 5 solution and the pH 8 cleaning solution give their numbers in their names. For water, remember what you just learned. Drag each tag to its pH, then check where each one lands.

Chemistry · Steps on the pH scale

The pH jump

Gut-feeling time. A cleaning solution has a pH of 8. Another alkali has a pH of 11, which is three pH units higher up the scale. How many times more alkaline is the pH 11 solution than the pH 8 solution?

Your estimate

750 times

0 times1500 times

Chemistry · What do you think?

Which idea is closest to yours?

Here are four things people say about acids, alkalis and pH. Pick the one closest to what you think, say how sure you are, and you will get an answer to that exact idea.

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

Chemistry · Universal indicator

What does the colour tell you?

Universal indicator has a wide range of colours. For each colour, decide what it tells you about the solution you are testing.

Still to sort

Acidic (0)

A pH of less than 7.

Neutral (0)

A pH of exactly 7.

Where the line is: Neutral is exactly 7: a solution that is not acidic could still be alkaline.

Alkaline (0)

A pH of more than 7.

6 of 6 still to sort.

Universal indicator is made from a mixture of dyes, so its colour changes with the pH. It comes as a solution, or as paper soaked in the solution and dried. Match its colour to a pH scale chart to read off the pH.

Measuring the pH of a solution

Tap the + on a step to read the extra detail.

  1. Add the test solutionUse a pipette to put three or four drops of the test solution into one well of a spotting tile.
  3. Match the colourThe indicator turns a colour that depends on the pH. Compare that colour with a pH scale chart to read off the pH.
  4. Record the resultWrite down the colour and the pH of the solution in a results table.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Most acids and alkalis are colourless, so how do chemists tell them apart? With a number and a colour.

What you need to know

  • How to use a pH to say whether a solution is acidic, neutral or alkaline.
  • How to work out how many times more acidic or alkaline one pH is than another.
  • How to measure pH with universal indicator and a pH scale chart.

The big picture

The pH scale runs from 1 to 14 and measures how acidic or alkaline a solution is: less than 7 is acidic, more than 7 is alkaline and exactly 7 is neutral. Each pH unit is a factor of ten, and universal indicator, matched to a pH scale chart, gives you the pH.

Key points

1The pH scale is a numerical scale from 1 (most acidic) to 14 (most alkaline).
2A pH of less than 7 is acidic, more than 7 is alkaline, and exactly 7 is neutral.
3pH stands for potential of hydrogen: the higher the concentration of hydrogen, the lower the pH.
4The pH scale goes up and down in factors of ten, so three pH units is 1000 times.
5Universal indicator changes colour with pH, and a pH scale chart turns the colour into a pH.

Worked example

Problem

Solution A has a pH of 2 and solution B has a pH of 5. Which one is more acidic, and how many times more acidic is it?

⚠ Watch out

Treating pH like a ruler. That leads to two slips: thinking a higher pH means more acidic (it is the other way round, because more hydrogen means a lower pH) and thinking that three pH units is "three times" (each unit is ten times, so it is 1000 times).

🧠

Memory hook

pH is the potential of Hydrogen, so more hydrogen means a lower pH. And every step is a lift button that multiplies by ten: three floors up is 1000 times, not 3. (The lift is only a picture. The real scale is just numbers.)

✓

Check yourself

Solution C has a pH of 9 and solution D has a pH of 6. Which is more alkaline, and by what factor? Answer: C, 1000 times (three pH units).

Flashcards

(13)
What does the pH scale measure?
How acidic or alkaline a solution is, on a numerical scale from 1 to 14.
On the pH scale, which end is the most acidic and which is the most alkaline?
1 is the most acidic and 14 is the most alkaline.
Which pH values are acidic, alkaline and neutral?
Acidic: less than 7. Alkaline: more than 7. Neutral: exactly 7, as in water.
What does pH stand for, and why is it written with a lowercase p and a capital H?
Potential of hydrogen. H is the chemical symbol for hydrogen, and acids contain hydrogen.
What is concentration?
The amount of chemical dissolved in a certain volume of solution.
How does the concentration of hydrogen affect pH?
The higher the concentration of hydrogen, the lower the pH. The lower the concentration of hydrogen, the higher the pH.
How many times more acidic is a pH 2 solution than a pH 3 solution?
Ten times, because the pH scale goes up and down in factors of ten.
What happens to the factor when the change is two or three pH units?
Two units is two factors of 10, which is 100 times. Three units is 1000 times.
Why can we not tell an acid from an alkali just by looking?
Most acids and alkalis are colourless solutions, so indicators are used to identify them as acidic, alkaline or neutral.
What is universal indicator?
A mixture of dyes that changes colour depending on the pH of the solution. It comes as a solution or as paper soaked in the solution and dried.
Which universal indicator colours show acidic, neutral and alkaline solutions?
Red, orange or yellow: acidic. Green: neutral. Blue or purple: alkaline.
How do you turn a universal indicator colour into a pH?
Compare the colour of the indicator solution or paper with a pH scale chart.
Why place a drop of the test solution on universal indicator paper instead of dipping the paper in?
Taking a drop of the solution and placing it on the paper prevents contamination of the solution.

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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