Question

Study the given table carefully.
\[\begin{matrix}
   Substances & Blue\text{ }litmus\text{ }solution~~~ & Phenolphthalein~ & Methyl\text{ }orange & China\text{ }rose\text{ }indicator \\
   1.\text{ }Wasps\text{ }sting & Red & Colourless & Red & Red \\
   2.\text{ }Lime\text{ }water & No\text{ }Change & Pink & Yellow & Green \\
   3.\text{ }Spinach & No\text{ }Change & Pink & Yellow & Green \\
   4.\text{ }Window\text{ }cleaner & Red & Colourless & Red & Red \\
   5.\text{ }Curd & Red & Colourless & Red & Red \\
\end{matrix}\].
Which of them shows the incorrect change in the colour of indicators?
 (A) 1 and 2
 (B) 3 and 5
 (C) 1, 2 and 3
 (D) 1, 3 and 4

Answer 1

Hint: Among the given substances some are alkaline in nature and some are acidic in nature. Different acid-base indicators are used to determine whether the solution is acidic or basic because of their ability to change the color in different pH. Thus by knowing the nature of given substances we can identify the incorrect change in the colour of indicators.

Complete answer:
  - As we know, the solutions can be generally classified as basic or acidic depending on their hydrogen ion concentration relative to pure water. If the value of pH or the potential hydrogen ion concentration is below seven, then the solution is acidic and if the pH is greater than seven then the solution is said to be basic and for a neutral solution the value of pH would be seven.
  - If we use an acid base indicator such as litmus solutions, phenolphthalein, methyl orange etc. we can easily find whether a solution is acidic, basic or neutral. The blue litmus paper can be used to test identify the acidic nature and will turn the blue shade into a red shade if it comes in contact with a solution of acidic nature.
  - Also, phenolphthalein will be colourless in acidic solution and pink in basic solution. Similarly the other acid base indicators will also show apparent colour changes. Let’s check the nature of the given substances. The wasp stings (venom) are alkaline in nature and hence it will not change the blue shade into red. Hence substance 1 is incorrect.
  - As we know spinaches are nutritional powerhouses and they are incredibly alkaline. Also, the window cleaners are made up of ammonium hydroxide which is basic in nature. Hence the substances 1, 3 and 4 are wrong among the given statements since they are all alkaline in nature and will change the red litmus to blue.

Therefore the incorrect change in the colour of indicators are shown by 1, 3 and 4.

That is option (D) is the answer .

Note:
Keep in mind that, based on whether the phenolphthalein indicator is in acid (colourless) or alkali (basic) solution, it has two different structures also. Even though both of these structures absorb light in the ultraviolet region which is not accessible for the human eye, the pink form or phenolphthalein indicator in alkaline solution absorbs light in the visible region (pink) and thus it appears to be pink in the human eye.