Paper chromatography is a technique used to separate mixtures of substances based on their different affinities for a stationary phase (the paper) and a mobile phase (the solvent). It works because different components of a mixture travel at different speeds across the paper.

Apparatus:

• A strip of chromatography paper (usually cellulose-based).
• A suitable solvent (e.g., water, ethanol, or a mixture of solvents).
• A container (e.g., a test tube or beaker) to hold the solvent.
• A pencil for drawing a line on the paper.
• A capillary tube or dropper for applying the sample.
• A clip or tape to hold the paper in the container.

Procedure:

1. Draw a light pencil line about 1-2 cm from the bottom of the chromatography paper. This is where the sample will be applied.
2. Dissolve the coloured mixture in a suitable solvent.
3. Using a capillary tube, carefully apply a small spot of the coloured solution onto the pencil line. Allow the solvent to evaporate completely.
4. Place the chromatography paper in the container, ensuring that the bottom edge of the paper is just above the solvent level. The solvent level should not touch the sample spot.
5. Cover the container with a lid or paper to prevent evaporation of the solvent.
6. Allow the solvent to travel up the paper by capillary action. As the solvent moves, it carries the different components of the mixture with it.
7. As the solvent reaches a certain height, remove the paper from the container and allow it to dry.
8. The separated components of the mixture will appear as distinct spots on the paper. The distance each component travels depends on its solubility in the solvent and its affinity for the paper. Components with higher solubility in the solvent will travel further.

The choice of solvent is crucial. It must be able to dissolve the components of the mixture but not the stationary phase (paper). A suitable solvent is one that allows for a good separation of the components.

A suitable method involves using a transparent reaction vessel with a marked scale to measure the distance the precipitate travels. The experiment should be conducted at each temperature (20°C and 40°C) separately.

The procedure would be as follows:

1. Set up the reaction vessel at the desired temperature (either 20°C or 40°C). This can be achieved using a water bath.
2. Add a known volume of the reactants to the vessel.
3. Start the timer as soon as the reactants are mixed.
4. At regular time intervals (e.g., every 10 seconds), record the distance the precipitate has travelled using the marked scale on the vessel.
5. Repeat the measurements for a sufficient period to obtain a reliable dataset.

The rate of formation is calculated for each temperature using the equation R_f = (distance travelled by substance) / (distance travelled by solvent). The distance travelled by the substance (precipitate) is measured from the point of formation to a defined point on the scale. The distance travelled by the solvent is measured by observing the change in the solvent level over the same time period. The rate of formation is then calculated as the ratio of these two distances.

A potential source of error is parallax error when reading the scale on the reaction vessel. This can be minimised by ensuring that the eye is directly in line with the scale being read. Using a ruler placed against the vessel can also help to improve accuracy.

Answer:

To identify substances A and B, we need to compare their Rf values to the Rf values of the known substances (aniline and benzene). The Rf value is calculated as (distance travelled by the substance) / (distance travelled by the solvent).

From the chromatogram, substance A has an Rf value of approximately 0.65 and substance B has an Rf value of approximately 0.40.

Aniline has an Rf value of 0.55 and benzene has an Rf value of 0.30. Therefore:

• Substance A has an Rf value of 0.65, which is closer to the Rf value of aniline (0.55) than to benzene (0.30). Therefore, substance A is likely to be aniline.
• Substance B has an Rf value of 0.40, which is closer to the Rf value of benzene (0.30) than to aniline (0.55). Therefore, substance B is likely to be benzene.

Reasoning: Substances with similar chemical properties will travel the same distance on the chromatography paper because they interact with the stationary phase (paper) and the mobile phase (solvent) in a similar way. Therefore, substances with similar Rf values are likely to be the same.