An aqueous solution turns red litmus solution blue. Excess addition of which of the following solution would reverse the change?

Which of the following is a weak acid?

The marble statues are often slowly corroded when kept in the open for a long time. Give a suitable reason.

Which of the following relations is correct for a basic aqueous solution?

Which of the following is an oxide of metal?

Tooth enamel is the hardest substance in our body. Name the compound of which it is made up. At what $pH$ of the mouth does it get corroded? State the role of bacteria present in the mouth. Suggest a method to prevent tooth decay.