Water is a neutral substance. What colour will you get when you add a few drops of universal indicator to a test-tube containing water?

20 (a) What is a universal indicator? For what purpose is it used? 20 How does a universal indicator work? 21 Which chemical is injected into the skin of a person:

(a) during an ant's sting?

(b) duting the nettle leaf hair sting?

How can the effect of these stings be neutralized?

22 Explain the pH change as the cause of tooth decay. How can tooth decaycaused by pH change be prevented?