Ph of Acids and Bases: Overview, Questions, Preparation

The pH may be referred to as ‘power of hydrogen’, ‘percentage of hydrogen’, or ‘potential of hydrogen’. If the pH scale measures any substance with a value 0, the substance is highly acidic. Similarly, the substance with a pH value of 14 is fundamental. You will find various acidic and basic substances in your daily life. Citrus fruits, coffee, milk, are examples of acidic substances, whereas baking soda and ammonia are basic substances. 

Fig 2. The pH of some common substances

Let us have a look at the chemical properties of acids and bases:

Fig 3. The pH variation of acids and bases on a pH scale

Acids release hydrogen ions (H+) when dissolved in water, while bases release hydroxide ions (OH-). When completely ionised in aqueous solution, acids and bases are called strong acid and strong base respectively. Similarly, when acids and bases are partially dissolved in aqueous solution, they are referred to as weak acid or weak base.            

Whenever acid and base react with each other, the process is referred to as neutralisation. The process results in the formation of salt. Therefore, salt has the acidic and basic nature of the acid and base from which it is formed. Similarly, the salt’s pH also depends on the acid and base’s pH combined to form the salt. Some common examples of salts are sodium chloride (NaCl), Epsom salt (MgSO₄), and baking soda (NaHCO₃).

The topic pH is acid and base is covered in the chapter ‘Acids, Bases and Salts’, of CBSE class Xth curriculum. The chapter is a part of the unit “Chemical Substances Behaviour and Nature” of the curriculum that holds 25 marks out of 100. In addition to the pH, the chapter also covers other aspects of acids and bases. These include the chemical properties of acids and bases, reaction with water or metal, and the importance of pH in our everyday lives. 

1. You have been provided with three test tubes. One of them contains distilled water, and the other two contain an acidic solution and a basic solution, respectively. If you are given only red litmus paper, how will you identify each test tube’s contents?

Solution: Red litmus paper reacts with bases, and the blue litmus reacts with acids. Dip red litmus paper in each of the test tubes. The solution that turns red litmus blue is basic. Now use this blue litmus to detect the acidic solution out of the remaining two test tubes. 

2. Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test the presence of the gas?

Solution: The reaction of acids releases hydrogen gas and a salt. Using a moist litmus paper can test the presence of the gas. For example, when calcium reacts with hydrochloric acid, it produces calcium chloride salt and hydrogen gas.  

Ca(metal) + 2HCL(acid) → CaCl2(salt) + H2(gas) 

3. Why should curd and sour substances not be kept in brass and copper vessels?

Solution: Curd or an acidic substance contains lactic acid and reacts with metal copper and brass. This results in the production of salts in a curd that is harmful for human consumption. For example, copper produces cupric lactate salt (C6H14CuO8) on reaction with lactic acid. 

Q: What is a litmus paper?

Q: What is pH?

Q: How do acids and bases affect our daily lives?

Q: How does the reaction between acid and base result?

Q: What is a universal indicator?