Question

Classify the following into groups of substances having $pH < 7,pH = 7,pH > 7$
Tamarind juice, lemon juice, soap water, distilled water, washing soda, soda water, tea.

Answer 1

Hint: pH scale is a logarithmic scale used to identify whether the substance is acidic, basic or neutral. Substances which are acidic will have $pH < 7$ , neutral substances have $pH = 7$ and basic substances have $pH > 7$

Complete step by step answer:
> Let us consider the chemical composition of each substance given to identify whether it is acidic, basic or neutral. Consider tamarind juice. As you very well know, tamarind juice tastes sour. This sour taste is one of the characteristic features of an acidic substance. The acid present in tamarind juice is tartaric acid. As it is acidic, its pH will be less than 7.
> Similarly, when we speak of lemon juice, we know its sour taste. This soreness is caused by the presence of citric acid. Again, due to its acidic nature, the pH of lemon juice will be less than 7.
> To know whether soaps are acidic or basic, let us recall their definition. Soaps are sodium or potassium salts of long chain carboxylic acids. That is, soaps are formed when weak carboxylic acids react with a strong alkali like sodium hydroxide or potassium hydroxide. As the reaction is between a weak acid and a strong base, the product which is formed will show basic character. That is, soaps are basic in nature. So their pH value will be greater than 7.
> Now coming to distilled water. Distilled water is purified water which does not contain any ions. Due to this reason, distilled water is neither acidic, nor basic and is characterised by the neutral pH, that is 7.
> Moving on to washing soda, the chemical name of washing soda is sodium carbonate. Its chemical formula is $N{a_2}C{O_3}$ and it is formed by the reaction between carbonic acid $\left( {{H_2}C{O_3}} \right)$ which is a weak acid and sodium hydroxide $\left( {NaOH} \right)$ which is a strong base. The product of this neutralisation therefore is basic in nature and will have its pH to be greater than 7.
> Let us discuss soda water now. It is nothing but carbonated water. Essentially, it contains carbon dioxide which has been dissolved in water under pressure. That is, it contains carbonic acid. As the name of the compound itself suggests, it will be acidic with a pH less than 7.
Lastly, tea contains tannins which are acidic in nature, making tea acidic with a pH below 7.
So to conclude, the substances can be classified as
$pH < 7:$ Tamarind juice, lemon juice, soda water, tea.
$pH = 7:$ Distilled water.
$pH > 7:$ Soap water, washing soda.

Note: Lower the value of pH, stronger the acid and if the value of pH is close to 14 that indicates a very strong base.