Chemical reactions
Acidic, basic and neutral substances
Sort everyday substances as acidic, basic or neutral, read what each natural indicator is telling you, and use neutralisation to explain an ant bite and a sour field.
At a school science fair, visitors were handed blank white sheets. A volunteer sprayed a liquid over them and the words 'Welcome to the Wonderful World of Science' appeared. Nothing had been written in ink. The message was there the whole time, written in a substance that stays invisible until a second substance meets it and forces a colour change.
Three natures, and the paper that tells them apart
Litmus is a dye taken from lichens, sold as a solution and as blue and red paper strips. Put a drop of a substance on it and the paper answers. Blue litmus going red means the substance is acidic. Red litmus going blue means it is basic. Neither paper changing means it is neutral — neither acidic nor basic. A substance that shows different colours in acid and in base like this is called an acid-base indicator.
Two clues you already have
Sour foods — lemon, amla, tamarind, vinegar — tend to contain acids. Basic substances feel slippery between the fingers and tend to taste bitter. Be careful with the second clue: bitter gourd tastes bitter and is still not basic, so a taste is a hint to be checked, not a verdict.
Worked example
A drop of lemon juice is diluted in a test tube and blue litmus solution is added. The liquid turns red. Lime water is then added drop by drop until the red turns blue. What happened in the tube?
Read the first colour change. Blue litmus went red, so the tube held an acid.
The indicator has done its one job: it has reported the nature of what it is sitting in.
Try it together
Three unlabelled solutions sit on the bench. Work out the nature of each from one test.
Answer each step with a single word: acidic, basic or neutral.
1.A few drops of red litmus solution are added to solution A, and it turns blue. What is A?
Have a go
Have a go on your own. Litmus is not the one plant that can do this job. Name one other vegetable, fruit or flower the chapter says can be made into an acid-base indicator.
Ready to practise?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.