The mass number can vary without changing the identity of an atom whereas change in atomic number may not be possible.
Atomic number of an atom is the number of protons or electrons in an atom. So the atomic number is an essential criteria for the identification of an element. No two elements will have the same atomic number. Thus atomic number cannot be changed for a particular atom. This is because the number of protons cannot be changed.
Mass number is the sum of the number of protons and neutrons of an atom. This can be varied without changing the identity of atom. We call them isotopes. Some elements have more than one kind of atoms with different mass number but same atomic number. For example, Hydrogen has three isotopes. These are formed by the difference in the number of neutrons in an atom.
heated crude oil enters a tall fractionating column , which is hot at the bottom and gets cooler towards the top. vapours from the oil rise through the column. vapours condense when they become cool enough. liquids are led out of the column at different heights.
Water takes longer to cool down and heat up because it detains temperature longer. So in the summer, coastal areas are cooler and the in the winter, coastal ares are warmer.
