Q.The size of particles of colloids and those of true solutions.

Showing the 12 most recent of 24 on this concept.

1. CBSE 2026Set 56/3/11 markMCQ
   Q.The boiling point of an azeotropic mixture of water and ethanol is less than that of pure water and ethanol. The mixture shows : (A) positive deviation from Raoult's Law. (B) negative deviation from Raoult's Law. (C) no deviation from Raoult's Law. (D) that the solution is an ideal solution.

   ›Reveal solutionSolution

   A mixture with a boiling point lower than its pure components is a minimum boiling azeotrope, which occurs when the solution exhibits positive deviation from Raoult's Law due to higher vapor pressure.

   When we talk about solutions, especially liquid-liquid mixtures, their behavior is often described relative to Raoult's Law. This law provides a baseline for ideal solutions, and deviations from it tell us about the intermolecular interactions within the mixture.

   Understanding Raoult's Law and Deviations

   1. Raoult's Law: For an ideal solution, the partial vapor pressure of each volatile component in the solution is directly proportional to its mole fraction in the solution and the vapor pressure of the pure component.

      $P_A = x_A P_A^\circ$

      $P_B = x_B P_B^\circ$

      Where $P_A$ and $P_B$ are the partial vapor pressures of components A and B in the solution, $x_A$ and $x_B$ are their respective mole fractions, and $P_A^\circ$ and $P_B^\circ$ are the vapor pressures of the pure components.

   2. Ideal Solutions: These solutions perfectly obey Raoult's Law over the entire range of concentrations. In an ideal solution, the intermolecular forces between A-A, B-B, and A-B molecules are all very similar.

   3. Non-Ideal Solutions: Most real solutions do not behave ideally and show deviations from Raoult's Law. These deviations arise from differences in intermolecular forces.

      • Positive Deviation: This occurs when the intermolecular forces between A-B molecules are weaker than the average of A-A and B-B forces. This makes it easier for molecules to escape into the vapor phase, leading to a higher vapor pressure than predicted by Raoult's Law.

        $P_A > x_A P_A^\circ$ and $P_B > x_B P_B^\circ$.

        Examples: Ethanol-water, acetone-carbon disulfide.

      • Negative Deviation: This occurs when the intermolecular forces between A-B molecules are stronger than the average of A-A and B-B forces. This makes it harder for molecules to escape into the vapor phase, leading to a lower vapor pressure than predicted by Raoult's Law.

        $P_A < x_A P_A^\circ$ and $P_B < x_B P_B^\circ$.

        Examples: Acetone-chloroform, nitric acid-water.

   Connecting Vapor Pressure to Boiling Point

   The boiling point of a liquid is the temperature at which its vapor pressure equals the external atmospheric pressure.

   • If a solution has a higher vapor pressure at a given temperature, it will reach the atmospheric pressure at a lower temperature. Thus, a higher vapor pressure corresponds to a lower boiling point.
   • Conversely, if a solution has a lower vapor pressure, it will require a higher temperature to reach atmospheric pressure. Thus, a lower vapor pressure corresponds to a higher boiling point.

   Azeotropes

   Azeotropes are constant boiling mixtures that distill without change in composition. They are a specific type of non-ideal solution.

   • Minimum Boiling Azeotropes: These are formed by solutions showing positive deviation from Raoult's Law. At a specific composition, the solution has a vapor pressure higher than either pure component, leading to a boiling point lower than either pure component. The ethanol-water mixture is a classic example. Pure ethanol boils at $78.4^\circ\text{C}$, pure water boils at $100^\circ\text{C}$, but their azeotrope (approximately $95.6\%$ ethanol by mass) boils at $78.2^\circ\text{C}$.

   • Maximum Boiling Azeotropes: These are formed by solutions showing negative deviation from Raoult's Law. At a specific composition, the solution has a vapor pressure lower than either pure component, leading to a boiling point higher than either pure component. For example, the nitric acid-water azeotrope boils at $120.5^\circ\text{C}$, which is higher than pure nitric acid ($83^\circ\text{C}$) and pure water ($100^\circ\text{C}$).

   Step-by-step Solution

   1. Analyze the given information: The problem states that the boiling point of the azeotropic mixture of water and ethanol is less than that of pure water and pure ethanol.

      • Boiling point of mixture < Boiling point of pure water
      • Boiling point of mixture < Boiling point of pure ethanol

   2. Relate boiling point to vapor pressure: A lower boiling point implies that the mixture has a higher vapor pressure compared to its pure components at the same temperature. This is because less energy (lower temperature) is needed for its vapor pressure to reach the external atmospheric pressure.

   3. Connect higher vapor pressure to Raoult's Law deviation: When a solution exhibits a vapor pressure higher than what would be predicted by Raoult's Law for an ideal solution, it is said to show positive deviation from Raoult's Law. This is characteristic of minimum boiling azeotropes.

   4. Conclusion: Since the ethanol-water azeotrope has a boiling point lower than its pure components, it is a minimum boiling azeotrope, which is a direct consequence of showing positive deviation from Raoult's Law.

   Watch out

   Do not confuse the boiling point of the azeotrope with the boiling points of the pure components. A minimum boiling azeotrope has a boiling point lower than both pure components, not just lower than one of them.

   The mixture shows positive deviation from Raoult's Law.

   ✓Final answer

   The mixture shows (A) positive deviation from Raoult's Law.

2. CBSE 2026Set 56/1/11 markMCQ
   Q.Assertion (A) : It is not possible to separate the components of an azeotrope by fractional distillation. Reason (R) : Components of an azeotrope have the same composition in liquid and vapour phase and boil at a constant temperature. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Assertion (A) is false, but Reason (R) is true.

   ›Reveal solutionSolution

   An azeotrope is a constant-boiling mixture whose vapour has the same composition as the liquid, making separation by simple fractional distillation impossible. Both the assertion and the reason are true, and the reason correctly explains the assertion.

   Concept and Intuition

   Fractional distillation works because, for most liquid mixtures, the vapour is richer in the more volatile component. As you boil the mixture, the vapour is condensed and reboiled repeatedly, gradually enriching the distillate in the lower-boiling component. This process relies on a difference in composition between the liquid and vapour phases at equilibrium.

   An azeotrope is a special mixture that defies this. At a specific composition (the azeotropic composition), the liquid and vapour have exactly the same mole fractions. The mixture boils at a constant temperature — either higher or lower than the boiling points of the pure components — and the vapour that comes off is identical in composition to the liquid left behind. No amount of distillation can change the composition of either phase; the mixture simply boils away unchanged.

   Therefore, the reason (R) correctly states the defining property of an azeotrope, and the assertion (A) correctly states the consequence: you cannot separate the components by fractional distillation.

   Step-by-step reasoning

   1. Understand the assertion.

      Assertion (A) says: “It is not possible to separate the components of an azeotrope by fractional distillation.” This is a well-known fact in chemistry. For example, ethanol and water form a minimum-boiling azeotrope at about 95.6% ethanol by mass. If you try to distill a mixture that is already at that composition, the distillate will also be 95.6% ethanol — you cannot get pure ethanol by simple fractional distillation.

   2. Understand the reason.

      Reason (R) says: “Components of an azeotrope have the same composition in liquid and vapour phase and boil at a constant temperature.” This is the precise definition of an azeotrope. At the azeotropic point, the boiling point is either a maximum or a minimum, and the vapour-liquid equilibrium curve touches the diagonal line (where $x_{\text{liquid}} = y_{\text{vapour}}$). Because the compositions are equal, there is no driving force for separation by distillation.

   3. Check if the reason correctly explains the assertion.

      The reason directly explains why fractional distillation fails: if the vapour has the same composition as the liquid, then no enrichment occurs during boiling or condensation. The constant boiling temperature is a consequence of this fixed composition. Thus, the reason is the correct explanation for the assertion.

   4. Evaluate the options.

      • Option (A): Both true, and (R) is the correct explanation of (A). This matches our analysis.
      • Option (B): Both true, but (R) is not the correct explanation — this is false because (R) directly explains (A).
      • Option (C): (A) true, (R) false — false because (R) is true.
      • Option (D): (A) false, (R) true — false because (A) is true.

   Watch out

   A common mistake is to think that azeotropes can be separated by simple distillation if you change the pressure. While pressure-swing distillation can sometimes break an azeotrope, the question specifically says “fractional distillation” under ordinary conditions. At a given pressure, the azeotropic composition is invariant, so fractional distillation alone cannot separate the components.

   ✓Final answer

   The correct option is (A) — both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).

3. CBSE 2026Set ANNUAL1 mark
   Q.Write the solute and solvent in the gaseous solution of chloroform in nitrogen gas.

   ›Reveal solutionSolution

   In any solution, the component present in a larger proportion is the solvent, and the one present in a smaller proportion is the solute - this also applies to gas-in-gas solutions.

   In a gaseous solution of chloroform vapour in nitrogen gas, nitrogen (the bulk carrier gas) is the solvent, and chloroform (present in a smaller, dilute amount) is the solute.

   ✓Final answer

   Solute: chloroform; Solvent: nitrogen gas.

4. CBSE 2026Set ANNUAL1 markMCQ
   Q.Which of the following is the correct example of a solid solution in which the solute is in gas phase?

   (a) Copper dissolved in gold

   (b) Camphor in nitrogen gas

   (c) Hydrogen in palladium

   (d) All of the above

   ›Reveal solutionSolution

   Solid solutions are classified by the physical state of the solute dissolved in the solid solvent; hydrogen dissolved in solid palladium is the classic gas-in-solid example.

   A solid solution can have solute particles that are themselves solid, liquid, or gaseous, dispersed uniformly in a solid solvent.

   • Copper dissolved in gold — both are solids, so this is a solid-in-solid solution (an alloy).

   • Camphor in nitrogen gas — this is actually a solid dissolved in a gas (or, read the other way, describes a gas as solvent), not a solid solution with a gaseous solute.

   • Hydrogen in palladium — hydrogen gas molecules occupy interstitial spaces within the solid palladium lattice. Here the solvent is solid (Pd) and the solute is a gas (H₂). This is the standard NCERT example of a solid solution in which the solute is in the gas phase.

   • Hence "All of the above" is incorrect since only one option actually fits the gas-in-solid description.

   ✓Final answer

   (c) Hydrogen in palladium — this is a solid solution of a gas (solute) dissolved in a solid (solvent).

5. CBSE 2026Set ANNUAL1 markMCQ
   Q.The carbonated water is an example of _____.

   (a) solid in liquid solution

   (b) liquid in liquid solution

   (c) gas in liquid solution

   (d) liquid in gas solution

   ›Reveal solutionSolution

   Carbonated water is CO$_2$ gas dissolved in water — a gas-in-liquid solution.

   Carbonated water is prepared by dissolving carbon dioxide gas ($CO_2$) under pressure in water. Here the solute (CO$_2$) is a gas and the solvent (water) is a liquid, so carbonated water is a classic example of a gas-in-liquid solution.

   ✓Final answer

   (c) gas in liquid solution.

6. CBSE 2025Set 56/6/11 markMCQ
   Q.A solution of acetone in ethanol : (A) obeys Raoult's law. (B) forms an ideal solution. (C) shows a positive deviation from Raoult's law. (D) shows a negative deviation from Raoult's law.

   ›Reveal solutionSolution

   Acetone–ethanol mixtures form hydrogen bonds weaker than those in pure ethanol, making escape easier and raising vapor pressure above the ideal prediction. The solution shows a positive deviation from Raoult's law — option (C).

   Types of Solutions: Understanding Deviations from Ideality

   Raoult's law predicts that the partial vapor pressure of each component in a solution is proportional to its mole fraction: $P_i = x_i P_i^0$. This holds perfectly only for ideal solutions, where the intermolecular forces between unlike molecules (A–B) are identical in strength to those between like molecules (A–A and B–B). When these forces differ, the solution deviates from Raoult's law.

   Positive deviation occurs when A–B interactions are weaker than the average of A–A and B–B interactions. Molecules escape more easily into the vapor phase, so the total vapor pressure exceeds the ideal prediction.

   Negative deviation occurs when A–B interactions are stronger than those in the pure components. Molecules are held more tightly in the liquid, reducing vapor pressure below the ideal value.

   The key is to compare the intermolecular forces in the mixture with those in the pure liquids.

   ---

   Step-by-Step Analysis

   1. Identify the intermolecular forces in pure ethanol

      Ethanol ($\ce{C2H5OH}$) has a hydroxyl group that forms strong hydrogen bonds between molecules. These O–H···O interactions are quite robust, holding ethanol molecules together tightly in the liquid phase.

   2. Identify the intermolecular forces in pure acetone

      Acetone ($\ce{CH3COCH3}$) is a polar molecule with a carbonyl group, but it cannot donate hydrogen bonds (no O–H or N–H). It can only accept hydrogen bonds. In pure acetone, the dominant forces are dipole–dipole interactions, which are weaker than hydrogen bonds.

   3. Examine the acetone–ethanol interaction

      When acetone and ethanol mix, ethanol can donate a hydrogen bond to the oxygen of acetone's carbonyl group, forming an O–H···O=C interaction. However, this hydrogen bond is weaker than the O–H···O–H hydrogen bonds present in pure ethanol, because the carbonyl oxygen is a less effective hydrogen-bond acceptor than the hydroxyl oxygen.

   4. Compare net intermolecular forces

      • In pure ethanol: strong H-bonds (O–H···O–H)
      • In the mixture: weaker H-bonds (O–H···O=C) plus dipole–dipole forces
      • The average A–B interaction is weaker than the ethanol–ethanol interaction.

   5. Predict the deviation

      Because the unlike interactions (acetone–ethanol) are weaker than the average of the like interactions (especially the strong ethanol–ethanol H-bonds), molecules in the mixture are held less tightly. They escape into the vapor phase more readily, raising the vapor pressure above what Raoult's law predicts.

   Watch out

   A common mistake is to assume that any hydrogen bonding between components means negative deviation. The critical question is whether the new H-bonds are stronger or weaker than those in the pure liquids. Here, acetone–ethanol H-bonds are weaker than ethanol–ethanol H-bonds.

   ---

   Experimental Evidence

   The acetone–ethanol system is a textbook example of positive deviation. The total vapor pressure of the mixture is higher than the ideal value at all compositions, and the system forms a minimum-boiling azeotrope (a hallmark of positive deviation).

   ✓Final answer

   The correct option is (C) — the solution shows a positive deviation from Raoult's law.

7. CBSE 2025Set X11 markMCQ
   Q.Camphor in nitrogen gas, is an example of ______.

   (a) liquid solution

   (b) solid solution

   (c) gaseous solution

   (d) aqueous solution

   ›Reveal solutionSolution

   Solvent = nitrogen gas, solute = solid camphor → a solid-in-gas solution, classified as a gaseous solution.

   A solution is classified by the physical state of the solvent (the component present in larger amount / the medium). Here the medium is nitrogen gas, so the solution is a gaseous solution, with solid camphor as the solute dispersed in it.

   This is the standard NCERT example of a solid solute in a gaseous solvent:

   Solute	Solvent	Type	Example
   solid	gas	gaseous solution	camphor in nitrogen gas

   ✓Final answer

   (c) gaseous solution

8. CBSE 2025Set JZ1 markMCQ
   Q.Give an example of such solid solution in which solute is gas.

   (a) Oxygen dissolved in water.

   (b) Solution of camphor in nitrogen.

   (c) Solution of hydrogen in palladium.

   (d) Glucose dissolved in water.

   ›Reveal solutionSolution

   A solid solvent holding a gaseous solute → hydrogen dissolved in palladium, option (c).

   Concept. A solution is a homogeneous mixture; it is classified by the physical state of the solvent (the component in excess), and separately by the state of the solute. A solid solution therefore has a solid as its solvent. The question asks for a solid solution in which the solute is a gas.

   Checking the options:

   • (a) Oxygen (gas) in water (liquid) → this is a liquid solution, not solid.
   • (b) Camphor (solid) in nitrogen (gas) → solvent is a gas, so this is a gaseous solution.
   • (c) Hydrogen (gas) in palladium (solid) → solvent is solid, solute is gas → a solid solution with a gaseous solute. Palladium famously occludes large volumes of H$_2$ in the interstitial spaces of its lattice.
   • (d) Glucose (solid) in water (liquid) → a liquid solution.

   Only (c) has a solid solvent and a gaseous solute.

   ✓Final answer

   (c) Solution of hydrogen in palladium — a solid (Pd) solvent with a gaseous (H$_2$) solute.

9. CBSE 2025Set A1 markMCQ
   Q.Homogeneous mixtures of two or more than two components is —

   (a) Solution

   (b) Liquid solute

   (c) Liquid solvent

   (d) Solvent

   ›Reveal solutionSolution

   A solution is a homogeneous mixture of two or more non-reacting substances.

   A mixture is homogeneous when its composition and properties are uniform throughout — no individual component can be distinguished by eye, and any sample taken from any part of it is identical. A solution (made of a solute dissolved in a solvent) is exactly this kind of mixture. Options (b) 'liquid solute', (c) 'liquid solvent' and (d) 'solvent' each name only ONE ingredient of a solution, not the mixture as a whole, so none of them can be the definition being asked for.

   ✓Final answer

   (a) Solution — the homogeneous mixture of two or more components.

10. CBSE 2024Set 56/2/11 markMCQ
    Q.An azeotropic solution of two liquids has boiling point lower than either of them when it : (A) is saturated (B) shows positive deviation from Raoult's law (C) shows negative deviation from Raoult's law (D) shows no deviation from Raoult's law

    ›Reveal solutionSolution

    An azeotrope with a boiling point lower than either pure component forms when the mixture shows positive deviation from Raoult's law, because the components "want to escape" the liquid phase more readily than ideal behavior predicts. The answer is (B).

    Understanding Azeotropes and Deviations from Ideality

    An azeotrope is a constant-boiling mixture where the vapor has the same composition as the liquid. This means you cannot separate the components by simple distillation at that composition. The key to understanding which type of azeotrope forms lies in how the solution deviates from Raoult's law.

    Raoult's law for an ideal solution states that the partial vapor pressure of each component is proportional to its mole fraction:

    $P_i = x_i P_i^0$

    where $P_i^0$ is the vapor pressure of the pure component. Real solutions often deviate from this ideal behavior due to intermolecular forces between unlike molecules differing from those between like molecules.

    The Connection Between Deviation and Boiling Point

    The boiling point of a liquid is reached when its total vapor pressure equals atmospheric pressure. If a mixture has a higher total vapor pressure than predicted by Raoult's law, it will boil at a lower temperature. Conversely, if the vapor pressure is lower than ideal, the boiling point rises.

    Step-by-Step Analysis

    1. Positive Deviation from Raoult's Law

       When $A$-$B$ interactions are weaker than $A$-$A$ and $B$-$B$ interactions, molecules escape the liquid phase more easily than in an ideal solution. The total vapor pressure is:

    $P_{\text{total}} > x_A P_A^0 + x_B P_B^0$

    This increased vapor pressure means the mixture reaches atmospheric pressure at a lower temperature than either pure component. The azeotrope formed is a minimum-boiling azeotrope.

    2. Negative Deviation from Raoult's Law

       When $A$-$B$ interactions are stronger than $A$-$A$ and $B$-$B$ interactions (hydrogen bonding, for example), molecules are held more tightly in the liquid phase:

    $P_{\text{total}} < x_A P_A^0 + x_B P_B^0$

    The reduced vapor pressure means a higher boiling point than either pure component. This forms a maximum-boiling azeotrope.

    3. No Deviation (Ideal Solution)

       An ideal solution follows Raoult's law exactly. No azeotrope forms because the vapor composition continuously changes with liquid composition during distillation. Complete separation is possible.

    4. Saturation

       The term "saturated" refers to a solution containing the maximum amount of solute at a given temperature. This concept is unrelated to azeotrope formation or boiling point behavior.

    Tip

    A quick mnemonic: Positive deviation → Molecules repel → Higher vapor pressure → Lower boiling point. Think of ethanol-water (positive deviation, minimum-boiling azeotrope at 78.2°C, lower than water's 100°C).

    Watch out

    Don't confuse the direction of deviation with the direction of boiling point change. Positive deviation (higher vapor pressure) gives a lower boiling point, which can seem counterintuitive at first.

    Classic Examples

    System	Deviation Type	Azeotrope Type	Boiling Point
    Ethanol + Water	Positive	Minimum-boiling	78.2°C (lower than both)
    Acetone + Chloroform	Negative	Maximum-boiling	Higher than both
    Nitric acid + Water	Negative	Maximum-boiling	120.5°C (higher than both)

    ✓Final answer

    The correct option is (B) — an azeotropic solution has a boiling point lower than either component when it shows positive deviation from Raoult's law.

11. CBSE 2024Set ANNUAL1 mark
    Q.Define saturated solution.

    ›Reveal solutionSolution

    A saturated solution has reached the maximum solute concentration possible at that temperature; dissolution and crystallisation occur at equal rates.

    When a solute is added to a solvent, it keeps dissolving until a point is reached where the solution can hold no additional solute (at a fixed temperature and pressure) - at this point a dynamic equilibrium is established between the solute molecules going into solution and those coming out of solution (if excess undissolved solute is present).

    Such a solution, where the rate of dissolution equals the rate of crystallisation (precipitation), is called a saturated solution. Its concentration at that temperature equals the solubility of the solute in that solvent.

    ✓Final answer

    A saturated solution is one in which no more solute can dissolve at that temperature - the dissolved and undissolved solute are in dynamic equilibrium.

12. CBSE 2024Set ANNUAL1 mark
    Q.Write names of solute and solvent present in sodium amalgam solution.

    ›Reveal solutionSolution

    Sodium amalgam is a solution of sodium metal dissolved in mercury; since mercury is present in the larger amount and is the medium, it is the solvent, while sodium (the dissolved component) is the solute.

    An amalgam is an alloy/solution of a metal dissolved in mercury.

    In sodium amalgam, solid sodium dissolves into liquid mercury; mercury, being the medium into which sodium is dissolved (and normally present in larger proportion), is the solvent, and sodium is the solute.

    ✓Final answer

    Solute: Sodium (Na); Solvent: Mercury (Hg).