A shiny brown coloured element $\text{'}\mathrm{X}\text{'}$ on heating in air becomes black in colour. Can you predict the element $\text{'}\mathrm{X}\text{'}$ and the black coloured substance formed? How do you support your predictions?

Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas?

Why are metals used for making cooking vessels? 

Write a chemical equation for the reaction taking place in the test tube. How does the colour of the solution change? What is the change in colour of iron nails? 

What is your observation in the following case:

Burning of magnesium in air.