The correct option is Both A and R are true and R is the correct explanation of A
Explanation
Molarity is a quantitative measure of the concentration of a solute in a solution. It is defined as the number of moles of solute dissolved per litre of solution.
Analysis:
- Assertion (A) is Correct: The molarity of a solution is sensitive to thermal changes. It varies inversely with temperature.
- Reason (R) is Correct: Molarity ($M$) is mathematically expressed as:
$$M = \frac{\text{Moles of Solute}}{\text{Volume of Solution (in Litres)}}$$
The volume of a liquid is a function of temperature; liquids generally expand (volume increases) when heated and contract (volume decreases) when cooled. Since molarity is inversely proportional to the volume of the solution, any change in temperature that alters the volume will necessarily alter the molarity.
Conclusion:
Because the temperature dependence of volume is the fundamental reason why molarity changes with temperature, the Reason (R) is the correct explanation for the Assertion (A).
Key Takeaway:
Concentration terms involving volume (e.g., Molarity, Normality) are temperature-dependent. In contrast, terms based solely on mass (e.g., Molality, Mole Fraction, Mass percentage) are temperature-independent.