Q.Assertion: Molarity of a solution in liquid state changes with temperature. Reason: The volume of a solution changes with change in temperature. Choose the correct option:

The key idea here is that molarity depends on the volume of the solution, and volume is temperature-dependent.

Step 1: Molarity is defined as moles of solute per litre of solution. Since volume appears in the denominator, any change in volume directly changes the molarity.

Step 2: When temperature rises, most liquids expand — their volume increases. This means the same number of moles now occupy a larger volume, so molarity decreases. Conversely, cooling contracts the volume and raises molarity. …

The core idea: why molarity is temperature-sensitive

Molarity is defined as moles of solute per litre of solution. The "litre" is a volume unit, and volume of any liquid changes when temperature changes — liquids expand when heated, contract when cooled. So if the volume changes but the number of moles of solute stays fixed, the molarity must change. That's the entire logic.

Let's break it down step by step.

1. What the assertion says

Assertion: Molarity of a solution in liquid state changes with temperature.

Molarity ($M$) is:

The numerator (moles of solute) depends only on the mass of solute and its molar mass — neither of which changes with temperature. So the only way molarity can change is if the denominator (volume) changes. And volume does change with temperature for nearly all liquids. So the assertion is correct.

2. What the reason says

Reason: The volume of a solution changes with change in temperature.

This is a basic physical fact. For most substances, increasing temperature increases molecular motion, pushing molecules slightly farther apart — hence volume increases. Cooling does the opposite. So the reason is also correct.

3. Does the reason explain the assertion?

The assertion says molarity changes with temperature. The reason says volume changes with temperature. Since molarity = moles / volume, a change in volume directly causes a change in molarity (moles being constant). So yes — the reason is the mechanism behind the assertion. It is a correct explanation. …

Concept: Temperature Dependence of Molarity

Molarity ($M$) is defined as:

Since volume changes with temperature (liquids expand on heating and contract on cooling), molarity changes — even though the number of moles of solute remains constant.

Method: Cause-Effect Analysis

Step 1 — Check the Assertion

"Molarity of a solution in liquid state changes with temperature."

• Molarity depends on volume.
• Volume changes with temperature.
• Therefore, assertion is correct.

Step 2 — Check the Reason

"The volume of a solution changes with change in temperature."

• This is a fundamental property of liquids (thermal expansion/contraction).
• Therefore, reason is correct. …

Here’s a breakdown of the common mistakes students make on this Assertion-Reason question, along with how to avoid each.

The Correct Answer First

• Assertion: Correct. Molarity ($M$) is defined as moles of solute per liter of solution. Since volume changes with temperature, molarity changes.
• Reason: Correct. The volume of a liquid solution does expand or contract with temperature.
• Link: The reason is the correct explanation for the assertion.

Correct option: (i) Assertion and reason both are correct statements and reason is correct explanation for assertion.

Common Mistake #1: Confusing Molarity with Molality

The Mistake: Students think molarity is temperature-independent because they confuse it with molality ($m$), which is moles of solute per kilogram of solvent. Since mass does not change with temperature, molality is temperature-independent.

Why it happens: Both terms sound similar and are used for concentration. In a hurry, a student might apply the property of molality to molarity.

How to Avoid:

• Remember the definitions by their denominators:
  • Molarity ($M$) = moles / liter (volume → temperature-sensitive).
  • Molality ($m$) = moles / kilogram (mass → temperature-insensitive).
• Mnemonic: "Molarity melts with heat" (volume expands, molarity drops). "Molality is mass-ive and stable."

Common Mistake #2: Thinking Volume Change is Negligible

The Mistake: Students accept that volume changes with temperature but argue it’s "too small to matter," so they mark the assertion as wrong or the reason as not a valid explanation.

Why it happens: In daily life, a glass of water doesn't look bigger when it's hot. Students underestimate the effect of thermal expansion on concentration calculations.

How to Avoid:

• Recall the formula: $M = \frac{n}{V}$. If $V$ changes, $M$ must change — even if the change is small, it is real.
• Exam tip: In chemistry, "changes with temperature" means any change, not just a large one. The assertion is about the relationship, not the magnitude.
• Example: If a solution is prepared at $25^\circ\text{C}$ and heated to $50^\circ\text{C}$, its volume increases by roughly 1–2%. That 1% change in $M$ is significant in precise titrations or pharmaceutical preparations.

Common Mistake #3: Misreading the Assertion-Reason Link

The Mistake: Students correctly identify both statements as true, but then choose option (ii) — "both are correct but reason is not the correct explanation."

Why it happens: They think the reason is too obvious or general. They look for a "deeper" reason, like "molarity depends on moles, and moles don't change with temperature."

How to Avoid:

• Ask yourself: "Does the reason directly cause the assertion to be true?"
  • Assertion: Molarity changes with temperature.
  • Reason: Volume changes with temperature.
  • Link: Molarity = moles/volume. If volume changes, molarity changes. That is a direct, causal link.
• Rule of thumb: If removing the reason makes the assertion seem less obvious, then the reason is the correct explanation. Here, without the volume change, you'd have no reason for molarity to change.

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