homogeneous
Summary
Homogeneous means uniform in composition and properties throughout, as in a solution, which is a single phase. The MCAT also applies it to equilibria and catalysts in which everything is in one phase. In a heterogeneous equilibrium, pure solids and liquids are left out of K.
Detail
Meaning: A homogeneous mixture has the same composition and properties throughout, so no separate parts can be seen. Solutions such as salt water, air, and brass are examples; solute particles are dispersed as individual molecules or ions. A heterogeneous mixture, such as sand in water, has distinct regions or phases.
The word also describes reactions. In a homogeneous equilibrium, all species are in one phase, such as all gases or all aqueous. In homogeneous catalysis, the catalyst is in the same phase as the reactants, such as dissolved acid in an aqueous reaction. A heterogeneous catalyst is in a different phase, such as a solid metal with gas reactants.
On the MCAT: You may classify a sample as a solution or a suspension, write an equilibrium expression, or decide whether a catalyst is homogeneous. In a heterogeneous equilibrium such as CaCO₃(s) ⇌ CaO(s) + CO₂(g), K = [CO₂] because pure solids and liquids are left out.
Common confusion: Homogeneous does not mean pure. Salt water is a mixture of several substances yet is one phase. Color or clarity does not decide it; uniformity does.
Sources
- Chemistry: The Central Science
- General Chemistry: Principles and Modern Applications
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.