1) Is chalk a mixture or a compound?
2)We say that reactive metals react with acids to give salt + hydrogen gas.Is it necessary to write "dilute acids"?
1. Tough question to answer. It all depends upon the kind of chalk used. Natural chalk, according to Wikipedia has minor amounts of silt embedded in it which would make it a mixture. Chalk used for blackboards may be manufactured for that purpose from gypsum so it is not a mixture. Chalk used to mark tennis courts is mostly titanium dioxide or silicon dioxide (natural chalk was used in earlier times) which is not a mixture (however, we never know how much diluent may be added to the final product before it is sold). And the list goes on from there.
2. No, it isn't necessary to specify dilute acids. This begs the question, however, of why you ask, because concentrated acids will do this also. One other caution: Nitric acid USUALLY doesn't give hydrogen gas and a salt. The nitric acid undergoes reduction to either NO2 or NO plus the salt is produced.