8.1 - Purity, Formulations & Chromatography

What is a pure substance?

A single element or compound, not mixed with any other substance.

What is a formulation and how is it made? Give examples.

• A mixture of compounds in measured quantities that has been designed as a useful product.

• They include fuels, cleaning agents, paints, medicines, alloys, fertilizers, and foods.

• E.g. alloys are mixture of metals; they are harder than pure metals, so have a particular purpose.

Describe paper chromatography.

1. A start line is drawn near the bottom of the paper. The mixture is spotted on the line.

2. A beaker is filled with small amount of solvent (it cannot touch or go above the start line when paper is placed in a beaker).

3. Paper is hung on a rod and placed in a beaker.

4. Solvent travels up the paper, thus separating the components.

5. Before solvent level reaches the end, the paper is taken out and the finish line is marked. The paper is dried.

6. The procedure works when different compounds have different affinities for the solvent/paper. Stronger attraction for the paper - travels slowly with the solvent etc.

7. Paper is called the stationary phase - it doesn’t move. Solvent is the mobile phase.

How is R_f calculated?

Distance moved by the spot/distance moved by solvent.

In a paper chromatography experiment, a compound A was found to have an R_f value of 0.85 - what does it tell you about the compound?

It has a higher affinity for the solvent than for the paper.