- Describe paper chromatography
Chromatography is a process used to separate and identify two or more substances from a mixture. This method depends on the solubility of the tested substances. Chromatography, for instance, is also used to find out the number of components in a drink.
Let’s say we want to find the number of colored dyes present in black ink. First we get a piece of filter paper or chromatography paper. We draw a line, in pencil, at the bottom of the paper. This line is called the base line, and the reason it is drawn in pencil is because pencil is insoluble so it won’t interfere with the solubility of the ink. Then we place a spot of the black ink on the base line. The chromatography paper is now put with its bottom soaked in a suitable solvent, which is in our case water. The chromatography paper is going to absorb the solvent, which moves upwards. When the solvent reaches the base line, the spot of black ink will dissolve in it. The solvent will keep moving upwards taking with it the black ink. The more soluble the contents of the ink the higher it will move until it can’t anymore.
Let’s say we want to find the number of colored dyes present in black ink. First we get a piece of filter paper or chromatography paper. We draw a line, in pencil, at the bottom of the paper. This line is called the base line, and the reason it is drawn in pencil is because pencil is insoluble so it won’t interfere with the solubility of the ink. Then we place a spot of the black ink on the base line. The chromatography paper is now put with its bottom soaked in a suitable solvent, which is in our case water. The chromatography paper is going to absorb the solvent, which moves upwards. When the solvent reaches the base line, the spot of black ink will dissolve in it. The solvent will keep moving upwards taking with it the black ink. The more soluble the contents of the ink the higher it will move until it can’t anymore.
Sometimes the substance we are testing is in solid form. In this case we have to crush and dissolve it in water and filter it. We then take the filtrate and evaporate some of it water to get the most concentrated sample. Then we are ready to do the experiment.
When dealing with ethanol in concentrating the sample. We have to heat it in a water bath because it is flammable. And when we use it a solvent in chromatography, it has to be performed in a covered beaker because ethanol is volatile.
The solvent front is the furthest distance travelled by the solvent.
Sometimes, the sample is separated into colorless spots. In this case the chromatography paper is sprayed with a locating agent to that locates the spots. The number of spots indicates the number of components in the sample.
To identify the substances which were formed when the sample was separated, we measure what’s called
the Rf Value.
The RfValue is the rate of the distance travelled by the solute (the spot) to the distance travelled by the solvent line.
It’s calculated by measuring the distance travelled by the spot (Distance1) from the base line, measuring the distance from the base line to the solvent front (Distance2), and dividing Distance1 by Distance2.
This value is always less than one because the distance travelled by the solvent is always larger than the distance travelled by the spot. Each substance has a different Rf Value.
Chromatography can be used to test purity of substances. If a substance gives only one spot, it means it is pure because it contains one substance.
If two spots have the same Rf value they are made of the same substance.
When dealing with ethanol in concentrating the sample. We have to heat it in a water bath because it is flammable. And when we use it a solvent in chromatography, it has to be performed in a covered beaker because ethanol is volatile.
The solvent front is the furthest distance travelled by the solvent.
Sometimes, the sample is separated into colorless spots. In this case the chromatography paper is sprayed with a locating agent to that locates the spots. The number of spots indicates the number of components in the sample.
To identify the substances which were formed when the sample was separated, we measure what’s called
the Rf Value.
The RfValue is the rate of the distance travelled by the solute (the spot) to the distance travelled by the solvent line.
It’s calculated by measuring the distance travelled by the spot (Distance1) from the base line, measuring the distance from the base line to the solvent front (Distance2), and dividing Distance1 by Distance2.
This value is always less than one because the distance travelled by the solvent is always larger than the distance travelled by the spot. Each substance has a different Rf Value.
Chromatography can be used to test purity of substances. If a substance gives only one spot, it means it is pure because it contains one substance.
If two spots have the same Rf value they are made of the same substance.
- Identify substances and assess their purity from melting point and boiling point information
Melting and boiling point
Chemist use some complex methods to check purity but we can use one simple method to find the purity of a substance, we can check the melting and the boiling point.
Water melts at 0°C and boils at 100°C.
Chemist use some complex methods to check purity but we can use one simple method to find the purity of a substance, we can check the melting and the boiling point.
- A pure substance has a definite, sharp, melting point and boiling point.
- When a substance contains an impurity, the melting point decreases and the boiling point increases.
Water melts at 0°C and boils at 100°C.
Important of Purity in Substances
Does purity matter?
Often it does not matter if a substance is not pure. We wash in tap water; without thinking too much about what is in it, but sometimes purity is very important. If you are making a new medical drug, or a flavouring for food, you must
make sure it contains nothing that could harm people.
An unwanted substance, mixed with the substance you want, is called an impurity.
Does purity matter?
Often it does not matter if a substance is not pure. We wash in tap water; without thinking too much about what is in it, but sometimes purity is very important. If you are making a new medical drug, or a flavouring for food, you must
make sure it contains nothing that could harm people.
An unwanted substance, mixed with the substance you want, is called an impurity.