Possible Clues:

• It will not pass the acid test
• Lit M with us till all is blue
• Substance such as lime or caustic soda
• Lye, e.g.
• Substance that neutralises acid
• Certain soluble salts
• Basic substance
• Chemically, I'm over 7, so it comes in useful to talk a little
• Chemical compound
• Extract of chalk, a limestone, would be this