Question

Classify each as the following:element, compound, or mixture:
1. helium
2. water
3.sodium
4. a sedimentary rock
5.salt
6.air

Answer 1

1. element 
2. compound
3. element
4. compound
5. compound
6. compound

basically, if its on the periodic table, its an element. for example, rock can be made of many things in the periodic table, therefore its a compound. salt, is NaCl, which are two elements put together, therefore its a compound.