Question

$5mL$ of lemon juice was taken in a test tube. Its pH was determined by using a pH paper. Excess of sodium hydroxide solution was then added to the lemon juice in the test tube and the pH of the resulting solution was again determined. What would be the change in colour?

Answer 1

Hint: To answer the given question, we should know that lime juice is acidic in nature while sodium hydroxide is basic in nature. So, try to recall what colour the litmus paper or pH paper shows when it comes in contact with an acid or a base.

Complete step by step solution:
Before going to find the answer, let us first know about some terms.
-Acid: An acid is a substance that releases hydrogen ions when it is dissolved in an aqueous solution. It is any hydrogen-containing substance that is capable of donating a proton i.e. hydrogen ion to another substance. It is usually identified by its sour taste. Acids are known to turn blue litmus paper red.
-Base: A base is a substance that releases hydroxyl ion when dissolved in an aqueous solution. It is any hydroxyl-containing substance which is capable of donating an electron i.e. hydroxyl ion to another substance and it can accept protons from acids. It is usually identified by its bitter taste and soapy feel. Bases are known to turn red litmus paper blue.
So, here in the question, we can see that lime juice is an acid, containing citric acid. Thus, when it is treated with the pH paper, it will turn red from blue. And then, when it is treated with excess sodium hydroxide which is a base, it will release hydroxyl ions and will change the pH paper blue from red.

Hence, the change in colour will be from red to blue.

Note: Litmus paper or pH paper (used to test the pH of liquids) is made by treating the paper with natural dyes. Remember, red litmus turns blue which indicates a basic pH whereas, blue turns red for the acidic pH.