chemical analysis

What is a pure substance

A pure substance contains only one type of element or compound

How can you tell if a substance is pure

Pure substances have a sharp melting point. Impure substances melt over a range and at a lower temperature than the pure substance

Why is melting point data useful for testing purity

You can compare the measured melting point to the known value- if it matches exactly, the substance is pure

How does impurity affect boiling point

The boiling point of an impure substance is higher than the pure substance, and it makes it boil over a range of temperatures

How does impurity afect melting point

The melting point of an impure substance is lower than the pure substance, and it makes it melt over a range of temperatures

What is a formulation

A mixture that has been carefully designed to produce a useful product with a specific function.

Why are the proportions of components important in a formulation

Changing the proportion changes the properties of the product, so precise formulation is needed to make sure it works as intended.

Give an example of a formulation

Paint, fuels, fertilisers, alloys, cleaning products, medicines and foods

Why is it important for formulations(e.g. medicines) to use pure substances?

Impurities can change how the formulation works or cause side effects, so purity ensures safety and effectiveness

What is chromatography used for

To seperate mixtures and help identify substances

What is the mobile phase in paper chromatography

The solvent that moves through the paper (Eg water)

What is the stationary phase in paper chromatography

The chromatography paper

What is the rf value

The ratio of the distance moved by a compound (centre of spot from origin) to the distance moved by the solvent.

How to calculate rf value

Rf= distance moved by substance/ distance moved by solvent front

Why is the Rf value of a dye not affected by how far the solvent front is allowed to travel?

Because both the dye and the solvent front travel further, so their ratio stays the same, giving the same Rf value.

What other changes than type of paper could give differnt rf values. And why ?

Changing the solvent or temperature changes how the dye interacts with the stationary and mobile phases- this changes the distance it travels which gives different Rf values.

What does each spot on a chromatogram represent

Each spot represents a different substance in the mixture

How can you tell if a substance is pure from a chromatogram

A pure substance only produces one spot

Suggest two advantages of using gas chromatography rather than paper chromatography

It is more accurate, faster and more sensitive

Why can changing the type of chromatography paper give differnt Rf values for a dye

Different papers have different stationary phases. This changes how strongly the dye is attached to the paper. The distance travelled changes so the Rf value is different.

Why should the chromatogram be removed before the solvent front reaches the top

To ensure accurate measurement of solvent distance and prevent smudging Rf values