Pure substances can or can not be chemically combined.
Pure substances can be either elements or compounds, but not mixture. Mixture are different substances mixing together without being chemically combined, such as air, which is a mixture of oxygen, carbon dioxide, water etc. Mixture can be separated by physical methods, like filtration or decantation.
Meanwhile, elements are the substances that cannot be further separated by any means. No matter physical or chemical methods. Examples of elements are oxygen, hydrogen, neon and all the other ones from the periodic table. Compounds are basically elements joining together, but they’re chemically combined which means their electrons (kind of subatomic particle) are either shared or given away. These elements can only be separated by chemical methods like electrolysis or heating.
Therefore, as long as the substance cannot be separated by physical methods, it can be considered as a pure substance. We can now conclude that pure substance can be (element) or can not be (compound) chemically combined.
Answer: Option (a) is the correct answer.
Explanation:
It is known that elements which have completely filled octet when they are present in neutral state then they do not need to gain or lose an electron. Hence, the element becomes unreactive in nature.
For example, neon is a noble gas whose atomic number is 10 and its electronic distribution is 2, 8. As they have completely filled orbitals so they do not react with other elements.
As a result, they are rarely found in combination with other elements.
Thus, we can conclude that the Group 8A(18) elements are unreactive and are rarely found in combination with other elements.
A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. A chemical reaction is usually accompanied by easily observed physical effects, such as the emission of heat and light, the formation of a precipitate, the evolution of gas, or a color change. Photosynthesis is an example from daily life
Energy may only be transformed from one sort to another. hope this helps :)
