- A substance is tested in a laboratory. It cannot be separated by filtration, chromatography, or distillation. When an electric current is passed through the molten substance, two new substances are formed at the electrodes. What is the classification of the original substance?
Answer: A compound
The substance cannot be separated by physical means (filtration, chromatography, distillation), which indicates it is a pure substance, not a mixture. The fact that it can be broken down into simpler substances by a chemical process (electrolysis) means it is a compound. An element is a pure substance that cannot be broken down by chemical means.
- Two identical balloons are filled with equal moles of gas at the same temperature. Balloon X contains helium (M=4.00 g/mol), and Balloon Y contains carbon dioxide (M=44.0 g/mol). After several hours, Balloon X has noticeably deflated while Balloon Y remains nearly unchanged. Which particulate explanation best accounts for this observation?
Answer: Helium atoms have smaller molecular diameter and higher average velocity, increasing their probability of escaping through microscopic openings in the balloon.
Correct answer C: At the same temperature, lighter helium atoms have higher average velocities than heavier CO₂ molecules (Graham's law). Combined with their smaller size, helium atoms more easily effuse through tiny pores in the balloon material. A is wrong because both gases have the same average kinetic energy at the same temperature. B is wrong because gas escape is due to effusion through pores, not intermolecular attractions with balloon material. D is wrong because pressure depends on particle collisions per unit time and area, not just momentum.
- A student separates a mixture of two miscible liquids with boiling points of 85 °C and 115 °C using distillation. Which statement correctly describes what happens in the distillation flask as the temperature first reaches 85 °C?
Answer: The vapor phase will consist of a mixture of both liquids, but will be enriched in the more volatile component.
When a mixture of miscible liquids is heated, both components will exert a vapor pressure and vaporize to some extent at any given temperature. The liquid with the lower boiling point (the more volatile component) will vaporize more readily. Therefore, the vapor above the liquid will contain molecules of both substances, but it will be richer (have a higher mole fraction) in the component with the lower boiling point (85 °C). This enrichment allows for separation by condensation. A is an oversimplification. D is only true for a pure substance, not a mixture.
- A student observes that when equal masses of aluminum powder and aluminum foil are separately added to hydrochloric acid, the powder reacts much more rapidly than the foil. The student concludes that "breaking aluminum into smaller pieces changes its chemical properties." Which analysis of this conclusion is most accurate?
Answer: The conclusion is incorrect; the chemical properties remain identical, but smaller particles provide greater surface area for contact with acid molecules.
This question tests your understanding of the difference between chemical and physical properties, particularly how particle size affects reaction rates without changing the fundamental nature of a substance.
The key insight is that aluminum's chemical properties—its electron configuration, bonding behavior, and intrinsic reactivity—remain identical whether it's in powder or foil form. What changes dramatically is the surface area available for reaction. When you break aluminum into smaller pieces, you expose more surface atoms to the hydrochloric acid molecules. Since chemical reactions occur at the interface between reactants, more surface area means more simultaneous collision sites, leading to faster reaction rates. This is a physical effect, not a chemical one.
Option A correctly identifies this distinction: the chemical properties are unchanged, but increased surface area accelerates the reaction. Option B incorrectly claims that particle size alters electronic configuration—electrons in aluminum atoms remain the same regardless of how you divide the metal. Option C mentions kinetic energy changes, but smaller particles at the same temperature don't inherently possess higher kinetic energy than larger ones. Option D suggests that mechanical subdivision creates new, more reactive bonds, which is false—breaking aluminum doesn't change the nature of metallic bonding.
When you encounter reaction rate questions on the IB exam, always distinguish between factors that change intrinsic chemical properties (like temperature affecting molecular energy) versus those that change physical conditions (like surface area or concentration). Particle size is a classic physical factor that affects reaction kinetics without altering chemical identity.
- Which of the following processes involves only a physical change?
Answer: The separation of a crude oil fraction into gasoline and kerosene by heating.
A physical change is a change in the form of a substance, but not its chemical composition. C describes fractional distillation, a physical process that separates components of a mixture based on differences in boiling points. A is a chemical reaction producing hydrogen gas. B is a precipitation reaction, which is a chemical change. D is an enzymatic oxidation reaction, a chemical change.
- A pure, colourless liquid, substance Q, is heated in an open container. It begins to boil at 110 °C and produces two different colourless gases, R and S. The boiling point remains constant at 110 °C until all of Q has vaporized. How should Q, R, and S be classified?
Answer: Q is a compound that decomposes upon boiling; R and S could be elements or compounds.
Since Q is a pure substance that produces two different substances (R and S) upon heating, it must be undergoing a chemical change (decomposition), not just a phase change. This means Q is a compound. A pure substance has a sharp, constant boiling point, but if it decomposes upon boiling, it's a chemical reaction occurring at that temperature. The products, R and S, could be simpler compounds or elements (e.g., 2H₂O₂ (l) → 2H₂O (g) + O₂ (g)). Choice B is incorrect because a mixture would typically boil over a range of temperatures. Choice C is incorrect as an element cannot decompose into different substances. Choice D is incorrect because a simple phase change would only produce gaseous molecules of Q, not two different substances R and S.
- An insulated container holds 50 g of liquid water at 25 °C. An identical insulated container holds 50 g of steam at 100 °C. The containers are cooled at the same constant rate. Which statement is correct regarding the temperature change over the first minute of cooling?
Answer: The temperature of the steam will remain at 100 °C for a period of time.
When the steam is cooled, it must first undergo a phase change from gas to liquid (condensation). This is an exothermic process that releases a large amount of latent heat of vaporization. As heat is removed from the container at a constant rate, this latent heat must be removed first before the temperature of the resulting liquid can begin to drop. Therefore, the temperature of the steam will plateau at 100 °C until all of it has condensed to liquid water. The liquid water at 25 °C, however, will begin to cool immediately as heat is removed.
- A sample of helium gas in a rigid, sealed container is heated from 150 K to 300 K. Which statement correctly describes the effect of this temperature change?
Answer: The average kinetic energy of the helium atoms doubles.
Temperature in Kelvin is directly proportional to the average kinetic energy of the particles. Since the Kelvin temperature is doubled (from 150 K to 300 K), the average kinetic energy of the helium atoms also doubles. A is incorrect because average speed is proportional to the square root of the absolute temperature, so it would increase by a factor of √2, not 2. B is incorrect because for a fixed volume and amount of gas, pressure is directly proportional to the Kelvin temperature (P ∝ T), so the pressure would double. D is incorrect because with higher speeds, the atoms will collide with the walls and with each other more frequently.
- A student is given a mixture of sand (insoluble in water), ammonium chloride (soluble in water), and naphthalene (insoluble in water but soluble in hot ethanol). Which sequence of techniques would be most effective for separating the three components to recover each as a pure, dry solid?
Answer: Add water to the mixture and filter; evaporate the filtrate to dryness; add hot ethanol to the residue, filter while hot, and then cool the new filtrate.
The correct procedure involves using solubility differences. Step 1: Add water to dissolve the ammonium chloride. Filter to separate the insoluble sand and naphthalene (residue) from the aqueous ammonium chloride solution (filtrate). Step 2: Evaporate the water from the filtrate to recover solid ammonium chloride. Step 3: Add hot ethanol to the residue to dissolve the naphthalene, leaving the sand. Filter this hot mixture to separate the sand. Step 4: Cool the ethanol filtrate to cause the naphthalene to crystallize out, which can then be collected. Choice A correctly outlines this logic.
- Two identical, sealed containers hold krypton (Kr) and neon (Ne) gas at the same temperature. The pressure in the container with krypton is twice the pressure in the container with neon. Which statement is correct?
Answer: The number of krypton atoms is approximately twice the number of neon atoms.
Since both gases are at the same temperature, their atoms have the same average kinetic energy (A is incorrect). Because krypton atoms are much more massive than neon atoms, for their kinetic energies (½mv²) to be equal, the neon atoms must have a higher average speed (B is incorrect). According to the ideal gas law (PV=nRT), pressure is directly proportional to the number of moles (n) when volume (V) and temperature (T) are constant. Since the pressure of Kr is twice the pressure of Ne, the container must contain twice the number of moles (and therefore atoms) of Kr (C is correct). Since there are twice as many Kr atoms and each Kr atom is much heavier than a Ne atom, the mass of krypton gas will be significantly more than twice the mass of neon gas (D is incorrect).
- Which of the following is a characteristic property of all homogeneous mixtures?
Answer: The proportions of their components can be varied.
A defining characteristic of a mixture, whether homogeneous or heterogeneous, is that the components are not in a fixed ratio by mass and their proportions can be varied. In contrast, compounds have a fixed composition. A is incorrect; mixtures typically boil over a range of temperatures, unlike pure substances. B describes how compounds can be broken down; mixtures are separated by physical means. D is characteristic of chemical reactions forming compounds, whereas forming a mixture typically involves a minimal energy change.
- The temperature of a substance is increased from -73 °C to 27 °C. What is the percentage increase in the average kinetic energy of its particles?
Answer: 50%
Average kinetic energy is directly proportional to the absolute temperature in Kelvin. First, convert the Celsius temperatures to Kelvin: T₁ = -73 °C + 273 = 200 K. T₂ = 27 °C + 273 = 300 K. The absolute increase in temperature is 300 K - 200 K = 100 K. The percentage increase is calculated relative to the initial temperature: (Change / Initial) × 100 = (100 K / 200 K) × 100 = 50%.
- Which statement best explains why a gas is much more compressible than a liquid?
Answer: The volume occupied by the particles themselves is negligible compared to the total volume of a gas.
Compressibility is the ability to reduce volume under pressure. In a gas, the particles are very far apart, and most of the volume of the container is empty space. This allows the particles to be forced much closer together. In a liquid, the particles are already in close contact. While the forces between gas particles are weaker (B), the primary reason for compressibility is the large amount of empty space between them (D). A is incorrect as liquid particles are not in a fixed lattice. C is not necessarily true; a gas and a liquid can exist at the same temperature (e.g., water and steam at 100 °C) and have the same average kinetic energy.
- When a small amount of potassium permanganate (KMnO4) is placed in one corner of a large container of still air, the purple color gradually spreads throughout the container over several hours. A student claims this demonstrates that "air molecules are moving randomly and colliding with the potassium permanganate particles." What is the most accurate evaluation of this claim?
Answer: The claim is partially correct about random motion, but the spreading occurs because KMnO4 molecules sublime and then move independently through kinetic motion.
Correct answer C: The observation involves sublimation of solid KMnO₄ into gaseous molecules, which then diffuse through random kinetic motion. While air molecules do move randomly, the primary mechanism is the random motion of the KMnO₄ molecules themselves after sublimation. A is wrong because it overemphasizes Brownian motion over sublimation and diffusion. B is wrong because it doesn't account for the phase change from solid to gas. D is wrong because the primary mechanism is molecular diffusion, not convection. - In a paper chromatography experiment, a solvent is used to separate a mixture of two amino acids, alanine and lysine. Alanine is less polar than lysine. The stationary phase is cellulose (paper), which is polar. Which statement correctly predicts the result?
Answer: Alanine will have a higher Rƒ value because it is less strongly adsorbed by the polar stationary phase.
In paper chromatography, the stationary phase (paper) is polar. Polar molecules will adsorb more strongly to the polar stationary phase and thus travel more slowly up the paper. Less polar molecules will be more soluble in a typical non-polar or moderately polar mobile phase and adsorb less strongly to the paper, allowing them to travel further. Since alanine is less polar than lysine, it will be less strongly adsorbed by the polar paper and will travel further with the mobile phase, resulting in a higher Rƒ value. Lysine, being more polar, will interact more strongly with the paper and have a lower Rƒ value.
- A sealed flask contains both liquid water and water vapor at equilibrium at 60°C. A student claims that "molecules are constantly moving between liquid and gas phases, but we can't observe any macroscopic changes because the rates of evaporation and condensation are equal." Which aspect of particulate theory does this claim most directly illustrate?
Answer: The concept that equilibrium represents a dynamic balance where opposing microscopic processes occur simultaneously at equal rates.
When you encounter questions about equilibrium systems, focus on the fundamental concept that equilibrium is dynamic, not static. The student's observation perfectly captures this principle—molecules are continuously switching between liquid and gas phases, but the system appears unchanged because evaporation and condensation occur at identical rates.
This scenario directly illustrates dynamic equilibrium, where opposing microscopic processes balance each other perfectly. At 60°C, water molecules in the liquid phase gain enough kinetic energy to escape into the vapor phase (evaporation), while simultaneously, vapor molecules lose energy and return to the liquid phase (condensation). The equal rates create the illusion of a motionless system at the macroscopic level, even though intense molecular activity continues.
Answer A correctly identifies this dynamic balance concept. Answer B is fundamentally wrong—molecular motion never ceases at equilibrium; it's the constant motion that maintains equilibrium. Answer C misrepresents intermolecular forces, which remain significant and actually determine the equilibrium position; they don't become negligible. Answer D incorrectly suggests uniform kinetic energies prevent phase changes, when in reality, the distribution of kinetic energies (following Maxwell-Boltzmann distribution) enables continuous phase transitions in both directions.
Study tip: Remember that "equilibrium = dynamic balance" is a core theme throughout IB Chemistry. Whether you're dealing with phase equilibria, chemical equilibria, or solubility equilibria, the system always involves continuous molecular-level processes occurring at equal rates, creating apparent stability at the observable scale. - To determine the mass of dissolved salt in a sample of seawater, a student heats a known volume of the sample in an evaporating basin. Which is the best procedure to ensure an accurate result?
Answer: Heat the basin gently until most of the water has evaporated, then heat to a constant mass.
To get an accurate mass of the dissolved solids, all the water must be removed. Heating to a constant mass is the standard procedure to ensure this. The process involves heating, cooling in a desiccator, and weighing, repeating until two consecutive weighings are the same. Gentle heating prevents spitting and loss of solid. A is inaccurate because it doesn't confirm all water (especially hydrated water) is gone, and weighing a hot object is inaccurate. C is inappropriate as there is no solid to filter until the water is gone. D is slow and may not remove all water of crystallization, and the sample could absorb atmospheric moisture.
- Ammonia (NH₃) and hydrogen chloride (HCl) are volatile substances that react to form a white solid, ammonium chloride (NH₄Cl). If cotton wool plugs soaked in aqueous solutions of these two substances are placed at opposite ends of a sealed glass tube, a white ring of NH₄Cl forms. Given the relative molecular masses Mᵣ(NH₃) = 17 and Mᵣ(HCl) = 36.5, what can be deduced?
Answer: The white ring will form closer to the hydrogen chloride end.
At the same temperature, gas molecules have the same average kinetic energy. Since kinetic energy is ½mv², lighter molecules must have a higher average speed. Ammonia (Mᵣ = 17) is lighter than hydrogen chloride (Mᵣ = 36.5), so NH₃ molecules will diffuse faster down the tube than HCl molecules. Therefore, the ammonia gas will travel a greater distance in the same amount of time, and the white ring will form where the gases meet, which will be closer to the end where the slower-moving HCl was placed. D is incorrect because at the same temperature, the average kinetic energies are equal.
- At 25°C, liquid water molecules have an average kinetic energy, yet the water doesn't spontaneously boil. However, some water molecules at the surface do evaporate. Which statement best explains this apparent contradiction using principles of particulate behavior?
Answer: Individual molecules have a range of kinetic energies around the average, and surface molecules with sufficient energy can overcome intermolecular attractions.
This question tests your understanding of kinetic molecular theory and phase transitions at the particle level. The key insight is recognizing that "average" kinetic energy doesn't mean all molecules have identical energies.
At any given temperature, molecules in a liquid have a distribution of kinetic energies around the average value - this is called the Maxwell-Boltzmann distribution. While most molecules have energies near the average, some have significantly higher or lower energies due to random collisions. For evaporation to occur, surface molecules need enough kinetic energy to overcome the intermolecular forces holding them in the liquid phase. Even though the average energy isn't sufficient for boiling, the highest-energy molecules in the distribution can still escape as vapor. This explains why evaporation happens at temperatures below the boiling point.
Option A incorrectly suggests surface molecules gain extra energy from air - temperature equilibrium means air and liquid surface have the same average kinetic energy. Option B correctly notes that surface molecules experience weaker forces (fewer neighbors), but wrongly claims all molecules have identical kinetic energies rather than a distribution. Option C describes chemical decomposition rather than the physical process of evaporation, where no bonds break within H2O molecules.
Option D correctly identifies both crucial factors: the energy distribution around the average (some molecules have much higher energies) and the fact that only surface molecules with sufficient energy can overcome intermolecular attractions to evaporate.
Remember: whenever you see questions about phase changes below boiling/melting points, think about energy distributions rather than average values alone. - A sealed container holds a mixture of nitrogen gas and solid iodine crystals at room temperature. Purple iodine vapor gradually appears in the gas phase until equilibrium is reached. Based on particulate behavior, what can be concluded about the energy distribution among iodine molecules in the solid phase?
Answer: Most iodine molecules have similar kinetic energies, but some molecules have sufficient energy to escape intermolecular attractions in the solid phase.
This question tests your understanding of kinetic molecular theory and phase equilibria, specifically how molecular energy distribution enables phase transitions like sublimation.
When you observe purple iodine vapor gradually appearing from solid crystals, you're witnessing sublimation at equilibrium. This occurs because molecules in any phase have a distribution of kinetic energies - not all molecules move at the same speed. According to Maxwell-Boltzmann distribution, most molecules cluster around an average energy, but there's always a fraction with much higher energies. Those high-energy molecules can overcome the intermolecular forces holding the crystal together and escape to the gas phase.
Option D correctly describes this reality: most iodine molecules have similar kinetic energies insufficient for escape, but some have enough energy to sublime. This explains why sublimation occurs gradually until equilibrium, not all at once.
Option A is wrong because it claims all molecules have identical energies, which contradicts kinetic molecular theory - molecular energies always follow a distribution. Option B incorrectly suggests kinetic energies are negligible; while potential energy matters, it's the kinetic energy distribution that determines which molecules can escape. Option C is fundamentally flawed because it states solid molecules have no kinetic energy, when in reality they're constantly vibrating and have thermal motion even in the solid phase.
Remember: whenever you see phase transitions occurring gradually at constant temperature, think energy distribution. Not all molecules have the same energy - it's the high-energy "tail" of the distribution that drives sublimation, evaporation, and other phase changes.
