1. As a Chemist, how do you define a pure substance?
  2. How many categories of pure substances exist? Name them and explain the differences between those categories.
  3. Do those categories have sub-categories? Explain
  4. Given the following pure substances: He, Na2O, Cl2, Au, NaCl, H2CO3. Classify them as either elemental substances or compounds and justify.

Solution:

A pure substance is made of one kind of molecule. For example, water (H2O), ethanol (C2H5OH), oxygen (O2), gold (Au).
A pure substance has its own and specific properties such as melting point(M.P), boiling point(B.P), density, etc. Water: B.P: 100oC, M.P: 0oC, Density: 1Kg/L (or 1000 Kg/m-3)

  1. There are two categories of pure substances, elemental substances and compounds.

Elemental Substance

Compound

The Molecule is made of atoms of the same chemical element

Ex: H2, O2, S8, Na

The Molecule or Formula unit is made of atoms of different chemical elements

Ex: H2O, HCl, NaCl, CaCO3

It cannot be decomposed into other simpler substances

It can be decomposed into other simpler substances

Ex: CaCO3(s) →  CaO(s)  +  CO2(g)

2H2O(l) electrolysis → 2H2(g)  +  O2(g)

The category of elemental substances has the following sub-categories:

  • Non-metal elemental substances: ex: H2, Cl2, O2
  • Metals: ex: Na, Fe, Au, Cu ...
  • Semi-metals (Metalloids): ex: B, Si, As ...
  • Rare gases: ex: He, Ne, Ar ...

The category of compounds has two sub-categories:

  • Molecular/Covalent compounds; ex: H2O, CH4, C2H5OH, NH3, CO2.
  • Ionic compounds: ex: NaCl, CaO, MgBr2.

Elemental substances

Compounds

He, Cl2, Au, 

They are made of one kind of atom

Na2O, NaCl, H2CO3

They are made of more than one kind of atom

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