In Simple words chemical analysis is finding out the substance present and the amount of the substance present in a sample. Finding out the substances present is called qualitative analysis and the amount of the substances presence is called qualitative analysis.
Example : Look at a water bottle available for sale.
Does the label give the composition of the ions/elements present as percentages ?
Yes. The manufacture of this water brand has done a sample testing for quality and
qualitative analysis.
If you consider the above example , each and every bottle carries this label. Do you think he has checked each and every water bottle ? No. It is practically impossible (Although it should been done.) What must have he done ? He must have checked some water bottles (samples) of the production. This is called sampling. He has taken few samples from a batch of bottles or a population of bottles. Let us see what these terms really mean.
Population and Sample
What is a population ?
It is an actual or conceptual collection of individual items/ observations / measurements , which may be finite of infinite.
Example : 1. Students in a university
2. Inland waters of Sri Lanka
3. Soil in the Anuradhpura District area
4. The effluent of a paint factory
5. Blood of a patient
6. Results of a tit-ration
What is a Sample ?
A sample is a small part of the population, taken for analysis.
Example : A water sample of a well be of about 1 L.
When the size of the sample or number of samples is large, its properties will be very much closer to the population. Therefore, if you want test your well water it is better to take at least about three samples of 1 L each.
Population and a sample
In the determination of a characteristic of population, taking measurements of the whole population is not possible. Therefore, we have take samples from the population for the determination.
Let's explain this more by trying to answer the following question.
Can you find the average weight of children of age five in the Colombo district?
You can . What should you do in Oder to find out the average weight ? You have to take the weight of each child and average it. Is it an easy task? How much of time and how many people should be employed for this ? Here, the population is the total number of children of age five in the district of Colombo. It is not an easy task. This is because we can't take measurements form all units/ individuals of due to various reasons. It may be due to lack of time, energy, money etc.... If so, is there another way to find the average weight of children of age five in the Colombo district ?
Yes. There is . We have to take a sample of children having a number we can cope with. then we can take the weight of the children in the sample and take the average. The sample should not be biased. What does this mean? Samples should be randomly selected in order to be true representatives of the population.
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