Blue litmus solution is added to two test tubes A and B containing dilute hydrochloric acid and sodium hydroxide solutions, respectively. In which test tube a colour change will be observed?

Test tube A contains dilute hydrochloric acid and test tube B contains dilute sodium hydroxide. On adding blue litmus solution to both the test tubes, the colour of test tube A will change from blue to red while the colour of test tube B will remain the same. This is because hydrochloric acid is acidic and acid turns blue litmus to red. Sodium hydroxide being basic will not change the colour of blue litmus.