How does dilution affect molarity?

Hint: Molarity is defined as number of moles in one litre of solution. It means we are talking above that one litre of solution, on dilution we are adding more and more water to the solution that’s affecting the number of particles in that particular area. So, as the number of particles or we can also call them moles, as moles changes for that particular area, molarity changes.

Complete step-by-step answer:
Consider that you are holding a beaker having one mole of sucrose dissolved in one litre of solution, now if we write the whole equation of that beaker in a formula, it will be like this-
$Molarity\,\,Sucrose(M) = \frac{{Number\,of\,moles\,\,of\,sucrose}}{{Volume\,of\,solution\,in\,(L)}}$

If we start adding sucrose in it, we are increasing the concentration of sucrose in the solution and hence the solution is becoming more and more concentrated. Now if we do the opposite, that is in place of increasing the concentration we start decreasing its concentration.

Now how we will do that, we start mixing more and more water in it by which the amount of water increases in sucrose solution and the amount of sucrose decreases. It means we are diluting the solution, on dilution the number of moles remains same throughout but we are not only talking about moles, we are talking about the moles in one litre of solution (some part of solution).

So, on adding more and more water the number of moles in that particular area or volume start decreasing, that’s why overall change is that on dilution molarity changes. How it changes, it will start decreasing.

Note: Molarity and molality are both different. In molarity we are considering the number of moles in one litre of solution while in molality we are talking about solvent. On the other hand, molality can be changed when temperature changes. It depends upon temperature, molality doesn’t change.