Testing acids and bases with universal indicator

Objectives

• Classify solutions as acidic, basic or neutral.
• Relate indicator colour to pH.
• Measure pH with a pH meter.

Theory

The pH scale runs from 0 to 14. Acids have pH below 7, bases above 7 and neutral solutions exactly 7 at 25 °C. Universal indicator is a mixture of dyes that shows a different colour across the scale: red for strong acids, green for neutral and violet for strong alkalis.

Procedure

1. Put on your safety goggles and lab coat.
2. Place three test tubes on the bench.
3. Add about 5 mL of dilute hydrochloric acid to the first tube.
4. Add about 5 mL of sodium hydroxide solution to the second tube.
5. Add about 5 mL of distilled water to the third tube.
6. Add a few drops of universal indicator to each tube and note the colours.
7. Place a pH meter on the bench and measure the pH of all three tubes.

Precautions

• Wear goggles — both solutions are corrosive.
• Use only a few drops of indicator.

Subject

Chemistry

Grade

8–10

Chapter

Acid, Base and Salt (Nepal National Curriculum (CDC), grade 8)

Difficulty

Easy

Duration

15 min

Safety level

Medium

Version

1.0

Apparatus

• Test tube
• pH meter

Chemicals

• Dilute hydrochloric acid HCl Corrosive
• Sodium hydroxide solution NaOH Corrosive
• Distilled water H₂O
• Universal indicator —