Question

One sample of air is found to have 0.03% carbon dioxide and another sample 0.02%. This illustrates that:
A ) Air is a compound.
B ) air is an element.
C ) air does not follow the law of constant proportions.
D ) air is a mixture.

Answer 1

Hint: According to the law of constant proportions, the composition of the compound remains the same irrespective of the nature of the source from which it is obtained. Here composition refers to the ratio of different constituent elements.

Complete answer:
> An element contains atoms of the same element. For example a molecule of hydrogen contains two atoms of hydrogen and a molecule of ozone contains three atoms of oxygen.
> A compound contains atoms from two or different elements. Thus water is an example of a compound. One molecule of water contains two molecules of hydrogen and one molecule of oxygen. Similarly, methane is a compound. One molecule of methane contains one carbon atom and four hydrogen atoms.
> A mixture contains a combination of several elements or compounds. Thus sugar solution is the mixture containing sugar and water. In sugar solution, the ratio of sugar to water is not fixed. Similarly salt solution is the mixture containing salt and water. In the salt solution, the ratio of salt to sugar is not fixed.
> The ratio of the atoms of various elements is fixed for a compound.
However for the mixture, the ratio of constituent elements or compounds is not fixed and can vary.
Since the amount of carbon dioxide gas in two samples of air is different, air is a mixture.

Hence, the option D ) is the correct option.

Note: Air is a mixture of several gases. Nitrogen (78%) and oxygen (21%) are the major components. Carbon dioxide gas is a minor component. Air also has small amounts of other gases like argon, neon etc. It also contains water vapours.