Which metal burns with a dazzling white light when heated at a high temperature?

$(a)$ When calcium metal is added to water,the gas evolved does not catch fire,but the same gas evolved on adding potassium metal to water catches fire. Explain why?
$(b)$ Name a metal for each case:
$(i)$ It displaces hydrogen gas from nitric acid.
$(ii)$ It does not react with any physical state of water.
$(iii)$ It does not react with cold as well as hot water but reacts with steam.

Which metal does not react with acid?

How many carats is pure gold?

Which metal reacts with cold water violently?

Which gas is produced when zinc reacts with diluted hydrochloric acid?