A sample of soil is mixed with water and allowed to settle. The clear supernatant solution turns the pH paper yellowish-orange. Which of the following would change the colour of this pH paper to greenish-blue?
Lemon juice
Vinegar
Common salt
An antacid

Correct Answer: (d) An antacid

Explanation: As pH paper turns greenish blue for weakly basic compound and antacids contain weak base like Mg(OH)2. So, an antacid would change the colour of this pH paper to greenish-blue.