Question

what happen when mixture of iron filling and sulphur is heated

Answer 1

they form ferrous sulphide(FeS)

Answer 2

When FE SO^4 which is green in colour is heated it decomposes into -
Fe^2 O ^3 which is reddish brown in colour.
It also emmits 2 gases
SO^2 and SO^3 , which are saffocating gases not to be inhaled.
This is decomposition reaction.
For more reference see page no. 8 (NCERT )