Pupil sheet · Acids, bases and indicators
Acid, alkali or neutral? — questions
Name ______________________ Date ____________
Try every question before you use a nudge. Write in full sentences where you are asked to explain.
1. identify
Limewater is a solution of calcium hydroxide, an alkali. A drop of limewater is added to each of three indicators. Which row gives the colours correctly?
| Row | Litmus | Methyl orange | Thymolphthalein |
|---|---|---|---|
| A | red | yellow | blue |
| B | blue | red | blue |
| C | blue | yellow | colourless |
| D | blue | yellow | blue |
Circle one answer.
- A
- B
- C
- D
Nudge (only if you are stuck): Limewater is an alkali. Find the alkali colour of each indicator in turn.
2. state
Dilute sulfuric acid is tested with each indicator, and then methyl orange is added to a solution of potassium hydroxide, an alkali. Match each test to the colour you would see.
Write the number of the matching option.
- colourless
- red
- blue
- yellow
- 1. Litmus in dilute sulfuric acid: ______
- 2. Methyl orange in dilute sulfuric acid: ______
- 3. Thymolphthalein in dilute sulfuric acid: ______
- 4. Methyl orange in potassium hydroxide solution: ______
Nudge (only if you are stuck): Three of the tests are in an acid. The last one is in an alkali.
3. deduce
An unlabelled bottle holds one of three colourless solutions: dilute hydrochloric acid, pH 1; sodium chloride solution, pH 7; or sodium hydroxide solution, pH 13. A few drops of thymolphthalein are added, and the solution stays colourless. What can you deduce?
Circle one answer.
- It is the hydrochloric acid or the sodium chloride solution, not the sodium hydroxide.
- It must be the sodium chloride solution.
- It must be the sodium hydroxide solution.
- It must be the hydrochloric acid.
Nudge (only if you are stuck): Which of the three would change the colour of thymolphthalein?
4. identify
Potassium nitrate solution turns universal indicator green. Which row describes the solution correctly?
| Row | pH | Ions in the solution | Litmus |
|---|---|---|---|
| A | 7 | no ions at all | purple |
| B | below 7 | K+, NO3− and spare H+ | red |
| C | 7 | K+ and NO3−, no spare H+ or OH− | purple |
| D | 7 | K+ and NO3−, no spare H+ or OH− | red |
Circle one answer.
- A
- B
- C
- D
Nudge (only if you are stuck): What does green mean on the chart? What is a salt made of?
5. deduce
Five liquids are tested with universal indicator: rainwater pH 6, stomach juice pH 1, oven cleaner pH 13, black coffee pH 5 and seawater pH 8. Put them in order from the most alkaline to the most acidic.
Write the number of each step in order.
- seawater
- rainwater
- stomach juice
- oven cleaner
- black coffee
- First: ______
- Second: ______
- Third: ______
- Fourth: ______
- Last: ______
Nudge (only if you are stuck): Start at the top of the scale: the highest pH is the most alkaline.
6. compare
Solution X turns universal indicator orange, pH 3. Solution Y turns it yellow, pH 6. Which statement is correct?
Circle one answer.
- Y is alkaline, because its pH is higher.
- Y has the higher concentration of hydrogen ions.
- X and Y have the same concentration of hydrogen ions, because both are acids.
- X has the higher concentration of hydrogen ions.
Nudge (only if you are stuck): Both are below 7. Which way along the scale is more acidic?
7. identify / calculate
A pupil measures the pH of a pond four times with a pH meter: 6.4, 6.5, 7.9 and 6.3. One reading does not fit the others. (a) Identify the anomalous reading. (b) Calculate the mean pH of the pond without it, to 1 decimal place.
(a) Anomalous reading: ______________ unit ________
(b) Mean pH: ______________ unit ________
Nudge (only if you are stuck): Three readings agree closely. Leave the fourth out of the mean.
8. explain / suggest
Turmeric, a yellow spice, can be used as an indicator. It is yellow in acidic and in neutral solutions, and red-brown in alkaline solutions. A pupil tries to tell vinegar from salt water with turmeric. (a) Explain why turmeric cannot tell them apart. (b) Suggest an indicator that can, and give the colour it shows in each liquid.
(a) Why turmeric fails
(b) An indicator that works, and its two colours
Nudge (only if you are stuck): Vinegar is an acid and salt water is neutral. Where does turmeric change colour?