Q1.
Will water work as a solvent in every case?
Answer
No. Water works only when the components of the mixture actually dissolve in water. In many cases a different solvent, or a mixture of solvents, must be used.
| Mixture to be separated | Suitable mobile phase | Reason |
|---|---|---|
| Black or coloured sketch-pen ink | Water, or 2 % m/v salt solution | The dyes are water-soluble |
| Green pigments of spinach leaves | Alcohol, or a solvent mixture | Chlorophyll is insoluble in water but dissolves in alcohol |
| Coloured pigments of flower petals | Alcohol or a water–alcohol mixture | Depends on which pigment the petal contains |
Why the choice of solvent matters: A component can only move up the paper if the mobile phase can dissolve it and carry it. If the solvent cannot dissolve a component at all, that component simply stays on the starting line and nothing is separated. The solvent must also not dissolve every component equally well, or all of them would race up together and stay as one spot. A good solvent is one that separates the components by treating each of them a little differently.