Acids, Bases and SaltsIntroduction
Sunday lunch at Riya's house: rajma-chawal, extra pickle and two helpings of gulab jamun. An hour later Dadaji is rubbing his chest — acidity. On the kitchen shelf Riya finds three bottles: lemon juice, vinegar, and baking soda dissolved in water.
Which one should she offer him?
Baking soda is the odd one out. Lemon juice and vinegar taste sour because they are acids; baking soda is a base, and a base can cancel the effect of an acid. Without being told, you have used the big idea of this chapter: acids and bases nullify each other.
From earlier classes you know two more things. Acids taste sour and turn blue litmus red; bases taste bitter and turn red litmus blue. But nobody tastes chemicals in a laboratory. So how do we tell acids and bases apart safely? Let's start with a stain.
The Curry Stain Mystery
At lunch, a drop of dal lands on Riya's white school shirt. The stain is bright yellow — that colour comes from the haldi, or turmeric. She scrubs it with soap, and the stain turns reddish-brown! Is the shirt ruined?
Be Riya. Use the three buttons to complete all three missions.
Nothing was ruined: turmeric simply changes colour. It is yellow in water and in an acid, but reddish-brown in a base such as soap. A substance that shows by its colour, or its smell, whether something is acidic or basic is called an indicator. Turmeric and litmus are natural indicators; methyl orange and phenolphthalein are synthetic indicators, made in factories.
Note
Litmus solution is a purple dye extracted from a lichen, a plant belonging to the division Thallophyta. In a solution that is neither acidic nor basic, it stays purple. Red cabbage leaves, turmeric, and the coloured petals of flowers such as Hydrangea, Petunia and Geranium also change colour with acids and bases. All of these are called acid–base indicators, or simply indicators.
In this chapter you will see how acids and bases react, how they cancel each other out, how to measure how acidic or basic something is, and where many of the salts in your house come from.