Problem #14: Pure Substance [Solved]
- As a Chemist, how do you define a pure substance?
- How many categories of pure substances exist? Name them and explain the differences between those categories.
- Do those categories have sub-categories? Explain
- 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)
- 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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