Quick answer: Acids have a pH below 7, alkalis have a pH above 7, and a neutral liquid such as pure water has a pH of exactly 7. This grade 8 science lab on pH scale gives the aim, the equipment, the safety points, a step-by-step method, sample results and 7 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
Red cabbage juice changes colour across the whole pH scale, so a kitchen can double as a laboratory. In this lab you test six household liquids and decide which are acids and which are alkalis, using a colour card to estimate pH.
Level: Grade 8 science (Year 9 · Class 8) · Curriculum: MS-PS1-2
Aim
To estimate the pH of six household liquids using red cabbage juice as an indicator.
Equipment
- Red cabbage leaves, hot water, a jug and a sieve
- Six clear cups or test tubes and a marker pen
- Lemon juice, white vinegar, fizzy water, tap water
- Baking soda solution (1 teaspoon in 100 mL water) and washing soda solution
- A syringe or spoon to measure 5 mL, and a white sheet of paper
Safety
Wear goggles. Washing soda solution is irritating, so keep it off skin and eyes and rinse splashes with water. Never taste any sample. Do not use bleach or oven cleaner in this lab.
Method
- Chop two cabbage leaves, pour on 200 mL of hot water and leave for 10 minutes. Strain the purple liquid. This is the indicator.
- Label six cups with the sample names and place them on white paper.
- Measure 5 mL of each sample into its cup.
- Add 5 mL of indicator to each cup and swirl gently.
- Record the colour after 1 minute, then use the reference card to estimate the pH range.
- Repeat with fresh samples to check the colours.
Results
Reference card: red is pH 2 to 3, pink pH 4 to 5, purple pH 6 to 7, blue pH 8 to 9, green pH 10 to 12 and yellow pH 13.
| Liquid | Colour with indicator | Estimated pH | Type |
|---|---|---|---|
| Lemon juice | Red | 2 to 3 | Acidic |
| White vinegar | Red | 2 to 3 | Acidic |
| Fizzy water | Pink | 4 to 5 | Weakly acidic |
| Tap water | Purple | 6 to 7 | About neutral |
| Baking soda solution | Blue | 8 to 9 | Weakly alkaline |
| Washing soda solution | Green | 10 to 12 | Alkaline |
Analysis questions
Question 1 Rank the liquids
Rank the six liquids from most acidic to most alkaline.Question 2 Closest pair
State which two liquids are closest in pH and explain how you know.Question 3 Limit of the indicator
Explain why the cabbage method gives a range of pH values and not an exact value.Question 4 Fair test
List three variables you kept the same to make the test fair.Question 5 Diluting vinegar
Predict the colour of the indicator when 10 mL of vinegar is diluted with 90 mL of water. State whether the pH can reach 7.Question 6 Why not bleach?
Give two reasons why bleach was not included.Question 7 Making colours dependable
Propose two changes that would make the pH estimates more dependable for a different cabbage batch.
