Iron filings are attracted by a magnet. This is a physical property of iron but not of most other materials, including sand. How could you use this difference in physical properties to separate a mixture of iron filings and sand?

Explanation:

Direct a magnet towards the mixture of iron and sand. You can observe that only the iron filling attract magnet and come in contact with it. Therefore, after this process you'll be left with iron filling on the surface of the magnet whereas the sand particles are left on the dish

i believe you are right for all except the 3rd ( look at sqdancefan his answer is much better)

nuclear fission produces elements that are heavier than helium.

the elements that are used in nuclear fission and their products are much heavier than helium.