The States of Matter – 40 MCQs

States of Matter – 40 MCQs

This section provides 40 multiple-choice questions covering key concepts from States of Matter, essential for your NEET and JEE Main preparation. Each question has four options, with only one correct answer.

Section 1: Multiple Choice Questions (MCQs)

Instructions: Choose the single best answer for each question.

  1. Which of the following conditions is most favorable for a gas to behave ideally? A) High pressure, low temperature B) Low pressure, high temperature C) High pressure, high temperature D) Low pressure, low temperature
  2. According to Boyle’s Law, at constant temperature, if the pressure of a given mass of gas is doubled, its volume will: A) Double B) Halve C) Remain unchanged D) Become four times
  3. The absolute zero of temperature is defined as the temperature at which: A) Water freezes B) All molecular motion ceases C) Volume of a gas becomes zero (theoretically) D) Pressure of a gas becomes zero (theoretically)
  4. Which of the following plots represents Charles’s Law? (V = Volume, T = Absolute Temperature, t = Celsius Temperature) A) V vs T (straight line through origin) B) V vs t (straight line through origin) C) P vs V (hyperbola) D) V vs 1/P (straight line through origin)
  5. The value of the universal gas constant (R) in SI units is: A) 0.0821 L atm mol-1 K-1 B) 8.314 J mol-1 K-1 C) 1.987 cal mol-1 K-1 D) 8.314 erg mol-1 K-1
  6. An ideal gas is enclosed in a container. If its pressure is reduced to half and its absolute temperature is doubled, the volume will: A) Remain unchanged B) Double C) Quadruple D) Halve
  7. According to Dalton’s Law of Partial Pressures, the total pressure of a mixture of non-reacting gases is equal to: A) The pressure of the gas with the highest partial pressure B) The pressure of the gas with the lowest partial pressure C) The average of the partial pressures D) The sum of the partial pressures of individual gases
  8. Which of the following gases will diffuse fastest under the same conditions of temperature and pressure? A) Oxygen (O2) B) Nitrogen (N2) C) Methane (CH4) D) Carbon Dioxide (CO2)
  9. What is the relationship between the root mean square (RMS) speed (u_rms), average speed (u_avg), and most probable speed (u_mp) of gas molecules at a given temperature? A) u_rms < u_avg < u_mp B) u_mp < u_avg < u_rms C) u_avg < u_rms < u_mp D) u_mp = u_avg = u_rms
  10. The average kinetic energy of gas molecules is directly proportional to: A) Pressure B) Volume C) Absolute temperature D) Molar mass
  11. Real gases show maximum deviation from ideal behavior at: A) High pressure and low temperature B) Low pressure and high temperature C) Low pressure and low temperature D) High pressure and high temperature
  12. The compressibility factor (Z) for an ideal gas is: A) Greater than 1 B) Less than 1 C) Equal to 1 D) Zero
  13. For hydrogen (H2) and helium (He) gases, the compressibility factor (Z) is usually greater than 1. This is because: A) Attractive forces are dominant B) Repulsive forces (volume of molecules) are dominant C) They are highly compressible D) They have strong intermolecular forces
  14. The van der Waals constant ‘a’ for a real gas accounts for: A) Volume of gas molecules B) Intermolecular attractive forces C) Kinetic energy of gas molecules D) Pressure exerted by the gas
  15. The critical temperature (Tc) of a gas is the temperature above which: A) It can be liquefied by applying pressure B) It cannot be liquefied, no matter how high the pressure C) Its volume becomes zero D) It starts behaving ideally
  16. Which of the following liquids would have the highest vapor pressure at a given temperature? A) Water (H2O) B) Ethanol (CH3CH2OH) C) Diethyl ether (CH3CH2OCH2CH3) D) Glycerine (C3H8O3)
  17. The boiling point of a liquid increases with: A) Decrease in external pressure B) Increase in intermolecular forces C) Decrease in surface tension D) Increase in volatility
  18. Surface tension of a liquid decreases with: A) Decrease in temperature B) Increase in intermolecular forces C) Increase in temperature D) Decrease in impurities
  19. Which of the following properties of liquids is related to the internal friction between its layers? A) Vapour pressure B) Boiling point C) Surface tension D) Viscosity
  20. Which of the following is an example of an amorphous solid? A) Quartz (SiO2) B) Sodium Chloride (NaCl) C) Glass D) Diamond
  21. A solid that has a sharp melting point and is anisotropic is classified as: A) Amorphous solid B) Crystalline solid C) Supercooled liquid D) Pseudo solid
  22. Which type of crystalline solid would typically have the lowest melting point? A) Ionic solid B) Metallic solid C) Covalent (Network) solid D) Non-polar molecular solid
  23. The unique property of ice being less dense than liquid water is due to: A) Strong covalent bonds in water B) Extensive hydrogen bonding in ice forming an open cage-like structure C) Ionic character of H2O molecules D) High kinetic energy of water molecules
  24. Which of the following exhibits only London Dispersion Forces as its primary intermolecular force? A) HCl B) NH3 C) He D) H2O
  25. In which of the following processes is energy absorbed (endothermic)? A) Freezing B) Condensation C) Melting D) Deposition
  26. The conditions for hydrogen bonding to occur are: A) Hydrogen bonded to Carbon, attracted to Oxygen. B) Hydrogen bonded to F, O, or N, attracted to F, O, or N. C) Hydrogen bonded to any non-metal, attracted to any other non-metal. D) Only between polar molecules.
  27. Which of the following statements about real gases is CORRECT? A) They always obey PV=nRT. B) Their molecules have negligible volume. C) They have significant intermolecular forces. D) They never condense into liquids.
  28. If the density of a gas at STP is 1.429 g/L, its molar mass is approximately: A) 16 g/mol B) 28 g/mol C) 32 g/mol D) 44 g/mol
  29. Two gases, A and B, have molar masses of 4 g/mol and 16 g/mol respectively. What is the ratio of their rates of diffusion (rA/rB)? A) 1:2 B) 2:1 C) 1:4 D) 4:1
  30. The critical temperature of CO2 is 30.98°C. This means that CO2 can be liquefied below this temperature, but not above it, by applying pressure. This is because: A) Intermolecular forces are too strong above Tc. B) Kinetic energy of molecules is too high above Tc to be overcome by pressure. C) Molecules break down above Tc. D) The gas behaves ideally above Tc.
  31. Which of the following is true for the van der Waals constant ‘b’? A) It accounts for attractive forces. B) It is larger for smaller molecules. C) It represents the effective volume of gas molecules. D) Its units are atm L2 mol-2.
  32. The process of solid CO2 (dry ice) directly changing into gas is called: A) Evaporation B) Condensation C) Sublimation D) Melting
  33. Which of the following is a characteristic of a covalent (network) solid? A) Low melting point B) Good electrical conductivity (except graphite) C) Held by strong covalent bonds forming a continuous network D) Soft and malleable
  34. A gas is heated from 27°C to 127°C at constant pressure. Its volume will become: A) Double B) Halve C) (4/3) times D) (3/4) times
  35. The pressure of a gas is observed to be 2.5 atm in a 5 L container. What will be the pressure if the volume is reduced to 2.5 L at constant temperature? A) 1.25 atm B) 2.5 atm C) 5.0 atm D) 10.0 atm
  36. Which of the following postulates of KMT is NOT true for real gases? A) Gases consist of tiny particles. B) Collisions are perfectly elastic. C) Average kinetic energy is proportional to absolute temperature. D) There are no significant forces between molecules.
  37. The SI unit for surface tension is: A) Dyne/cm B) N/m C) J/m2 D) Both B and C
  38. Which of the following pairs of properties is characteristic of amorphous solids? A) Sharp melting point, anisotropic B) Melt over a range, isotropic C) Sharp melting point, isotropic D) Melt over a range, anisotropic
  39. The viscosity of water is lower than that of glycerine. This is primarily due to: A) Water molecules are smaller than glycerine. B) Water has less extensive hydrogen bonding than glycerine. C) Water has stronger London forces. D) Glycerine has a higher vapor pressure.
  40. At what point do solid, liquid, and gas phases of a substance coexist in equilibrium? A) Boiling point B) Melting point C) Triple point D) Critical point

Section 2: Answer Key

  1. B
  2. B
  3. C
  4. A
  5. B
  6. C
  7. D
  8. C
  9. B
  10. C
  11. A
  12. C
  13. B
  14. B
  15. B
  16. C
  17. B
  18. C
  19. D
  20. C
  21. B
  22. D
  23. B
  24. C
  25. C
  26. B
  27. C
  28. C
  29. B
  30. B
  31. C
  32. C
  33. C
  34. C
  35. C
  36. D
  37. D
  38. B
  39. B
  40. C

Section 3: Detailed Explanations

  1. B) Low pressure, high temperature
    • Explanation: Real gases behave most ideally at low pressures (where intermolecular distances are large, and molecular volume is negligible) and high temperatures (where kinetic energy is high, and intermolecular forces are minimal).
  2. B) Halve
    • Explanation: According to Boyle’s Law, P1V1 = P2V2. If P2 = 2P1, then P1V1 = 2P1V2, which simplifies to V2 = V1/2. The volume will halve.
  3. C) Volume of a gas becomes zero (theoretically)
    • Explanation: Charles’s Law states that volume is directly proportional to absolute temperature. Extrapolating the V-T graph to V=0, the corresponding temperature is -273.15°C or 0 K, which is absolute zero. This is a theoretical concept as gases liquefy or solidify before reaching this temperature.
  4. A) V vs T (straight line through origin)
    • Explanation: Charles’s Law states V ∝ T (absolute temperature). Plotting V against T (in Kelvin) yields a straight line passing through the origin.
  5. B) 8.314 J mol-1 K-1
    • Explanation: This is the SI unit value of the universal gas constant (R). The other options are also correct values of R but in different units.
  6. C) Quadruple
    • Explanation: Use the combined gas law: (P1V1)/T1 = (P2V2)/T2. Given P2 = P1/2 and T2 = 2T1. (P1V1)/T1 = ((P1/2) * V2) / (2T1) V1 = (V2 / 4) => V2 = 4V1. The volume will quadruple.
  7. D) The sum of the partial pressures of individual gases
    • Explanation: Dalton’s Law of Partial Pressures states that the total pressure exerted by a mixture of non-reacting gases is the sum of the partial pressures of the individual gases.
  8. C) Methane (CH4)
    • Explanation: According to Graham’s Law of Diffusion, the rate of diffusion is inversely proportional to the square root of the molar mass (r ∝ 1/M​). The gas with the lowest molar mass will diffuse fastest.
      • O2: 32 g/mol
      • N2: 28 g/mol
      • CH4: 16 g/mol
      • CO2: 44 g/mol Methane (CH4) has the lowest molar mass, so it will diffuse fastest.
  9. B) u_mp < u_avg < u_rms
    • Explanation: At any given temperature, the most probable speed (speed of maximum molecules) is the lowest, followed by the average speed, and then the root mean square speed (which reflects the average kinetic energy and is highest). The approximate ratio is 1 : 1.128 : 1.224.
  10. C) Absolute temperature
    • Explanation: According to the Kinetic Molecular Theory of Gases, the average kinetic energy of gas molecules is directly proportional to the absolute temperature (KE_avg = (3/2)kT).
  11. A) High pressure and low temperature
    • Explanation: Deviations from ideal behavior are most significant when molecules are close together (high pressure) and moving slowly (low temperature), allowing their finite volume and intermolecular forces to become influential.
  12. C) Equal to 1
    • Explanation: For an ideal gas, PV = nRT, so the compressibility factor Z = PV/nRT = 1.
  13. B) Repulsive forces (volume of molecules) are dominant
    • Explanation: For H2 and He, attractive forces are very weak due to their small size. At normal and even moderately high pressures, the dominant factor contributing to deviation from ideal behavior is the finite volume occupied by the molecules themselves (repulsive forces), leading to Z > 1.
  14. B) Intermolecular attractive forces
    • Explanation: In the van der Waals equation, the constant ‘a’ corrects for the intermolecular attractive forces between gas molecules. A larger ‘a’ value indicates stronger attractive forces.
  15. B) It cannot be liquefied, no matter how high the pressure
    • Explanation: Critical temperature (Tc) is the highest temperature at which a gas can be liquefied by simply increasing pressure. Above this temperature, the kinetic energy of the molecules is too high for the intermolecular forces to bring them together into the liquid state, regardless of pressure.
  16. C) Diethyl ether (CH3CH2OCH2CH3)
    • Explanation: Vapour pressure is inversely related to the strength of intermolecular forces.
      • Water, ethanol, and glycerine all exhibit strong hydrogen bonding.
      • Diethyl ether has dipole-dipole forces and London dispersion forces, but no hydrogen bonding. Therefore, it has the weakest intermolecular forces among the options and thus the highest vapor pressure (most volatile).
  17. B) Increase in intermolecular forces
    • Explanation: Boiling point is the temperature at which vapor pressure equals external pressure. Stronger intermolecular forces require more energy to overcome, leading to a higher vapor pressure at a given temperature, or a higher temperature required to reach a specific vapor pressure.
  18. C) Increase in temperature
    • Explanation: Surface tension decreases with increasing temperature because increased kinetic energy of molecules weakens the intermolecular forces at the surface, reducing the net inward pull.
  19. D) Viscosity
    • Explanation: Viscosity is a measure of a liquid’s resistance to flow, which arises from the internal friction between its moving layers. This friction is primarily due to intermolecular forces.
  20. C) Glass
    • Explanation: Glass is a classic example of an amorphous solid. It lacks a long-range, ordered crystalline structure and melts over a range of temperatures, rather than a sharp melting point. Quartz (SiO2) in its pure form is crystalline, as are NaCl and diamond.
  21. B) Crystalline solid
    • Explanation: Crystalline solids are characterized by a highly ordered, long-range arrangement of particles, leading to sharp melting points and anisotropy (properties vary with direction). Amorphous solids are supercooled liquids, melt over a range, and are isotropic.
  22. D) Non-polar molecular solid
    • Explanation: The strength of intermolecular forces determines the melting point.
      • Ionic, metallic, and covalent (network) solids have very strong intramolecular or strong metallic bonds, leading to high melting points.
      • Molecular solids are held by weaker intermolecular forces. Non-polar molecular solids (e.g., solid H2, CO2, or CH4) are held only by weak London dispersion forces, giving them the lowest melting points among crystalline solids.
  23. B) Extensive hydrogen bonding in ice forming an open cage-like structure
    • Explanation: In ice, water molecules form a highly ordered, open, cage-like structure stabilized by extensive intermolecular hydrogen bonding. This open structure results in a lower density compared to liquid water, where molecules are more closely packed despite hydrogen bonding.
  24. C) He
    • Explanation:
      • HCl, NH3, and H2O are all polar molecules. HCl has dipole-dipole forces. NH3 and H2O exhibit strong hydrogen bonding (a special type of dipole-dipole).
      • Helium (He) is a noble gas, which is non-polar. The only intermolecular forces present between non-polar molecules (or noble gas atoms) are weak London Dispersion Forces.
  25. C) Melting
    • Explanation: Melting (solid to liquid) is an endothermic process, meaning it requires heat energy to break the intermolecular forces holding the particles in fixed positions. Freezing, condensation, and deposition are exothermic processes (release heat).
  26. B) Hydrogen bonded to F, O, or N, attracted to F, O, or N.
    • Explanation: Hydrogen bonding occurs specifically when a hydrogen atom is covalently bonded to a highly electronegative atom (Fluorine, Oxygen, or Nitrogen) and is attracted to another highly electronegative atom (F, O, or N) in the same or another molecule.
  27. C) They have significant intermolecular forces.
    • Explanation: Real gases deviate from ideal behavior because, unlike ideal gases, their molecules occupy a finite volume and experience significant intermolecular forces, especially at high pressures and low temperatures. They do not always obey PV=nRT.
  28. C) 32 g/mol
    • Explanation: Use the density formula for ideal gases at STP: d = PM/RT. At STP, P = 1 atm, T = 273.15 K, R = 0.0821 L atm mol-1 K-1. M = (dRT) / P = (1.429 g/L * 0.0821 L atm mol-1 K-1 * 273.15 K) / 1 atm M ≈ 32 g/mol. This corresponds to Oxygen (O2).
  29. B) 2:1
    • Explanation: According to Graham’s Law, rA/rB = MB​/MA​​. rA/rB = 16/4​=4​=2/1. The ratio of their rates of diffusion is 2:1.
  30. B) Kinetic energy of molecules is too high above Tc to be overcome by pressure.
    • Explanation: Above the critical temperature, the kinetic energy of the gas molecules is so high that the intermolecular attractive forces, no matter how much pressure is applied, are insufficient to bring the molecules close enough to form a liquid phase.
  31. C) It represents the effective volume of gas molecules.
    • Explanation: The van der Waals constant ‘b’ (co-volume or excluded volume) corrects for the finite volume occupied by the gas molecules themselves. A larger ‘b’ value indicates larger molecular size.
  32. C) Sublimation
    • Explanation: Sublimation is the direct conversion of a solid into a gas without passing through the liquid phase. Dry ice (solid CO2) is a common example.
  33. C) Held by strong covalent bonds forming a continuous network
    • Explanation: Covalent (network) solids consist of atoms held together by a continuous network of strong covalent bonds. This leads to very high melting points, extreme hardness, and they are typically insulators (except graphite).
  34. C) (4/3) times
    • Explanation: Use Charles’s Law: V1/T1 = V2/T2. Convert temperatures to Kelvin: T1 = 27 + 273 = 300 K; T2 = 127 + 273 = 400 K. V2 = V1 * (T2/T1) = V1 * (400/300) = V1 * (4/3). The volume will become (4/3) times the original.
  35. C) 5.0 atm
    • Explanation: Use Boyle’s Law: P1V1 = P2V2. 2.5 atm * 5 L = P2 * 2.5 L P2 = (2.5 * 5) / 2.5 = 5.0 atm.
  36. D) There are no significant forces between molecules.
    • Explanation: This is a postulate of KMT for ideal gases. However, for real gases, especially at high pressures and low temperatures, intermolecular forces become significant and cannot be ignored.
  37. D) Both B and C
    • Explanation: Surface tension (γ) is defined as force per unit length (N/m). It can also be defined as energy per unit area (J/m2). Since 1 J = 1 N m, then 1 J/m2 = 1 (N m)/m2 = N/m. Both units are correct. Dyne/cm is a CGS unit.
  38. B) Melt over a range, isotropic
    • Explanation: Amorphous solids soften gradually and melt over a range of temperatures, unlike crystalline solids which have sharp melting points. They are isotropic, meaning their physical properties are the same in all directions due to the random arrangement of particles.
  39. B) Water has less extensive hydrogen bonding than glycerine.
    • Explanation: Viscosity is directly related to the strength of intermolecular forces. Glycerine (C3H8O3) has three -OH groups per molecule, allowing for much more extensive hydrogen bonding than water (which has two H atoms and two lone pairs, allowing 4 H-bonds per molecule on average). More hydrogen bonding leads to higher internal friction and thus higher viscosity.
  40. C) Triple point
    • Explanation: The triple point is the unique temperature and pressure at which the solid, liquid, and gas phases of a substance coexist in thermodynamic equilibrium.

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