i. What happens to the internal energy of a substance during melting?
Answer: b) It increases
Explanation: During melting, the substance absorbs heat energy (latent heat of fusion). This energy is used to overcome the intermolecular forces holding the solid lattice together, converting it into a liquid. Since energy is absorbed, the internal energy of the substance increases, even though the temperature remains constant.
ii. Which process involves a solid turning directly into a gas?
Answer: c) Sublimation
Explanation: Sublimation is the phase transition where a solid changes directly into a gas without passing through the intermediate liquid phase. Examples include dry ice (solid CO₂) turning into CO₂ gas and iodine crystals vaporizing.
iii. Boiling occurs at:
Answer: c) A fixed temperature
Explanation: For a given external pressure, a pure liquid boils at a specific, fixed temperature known as its boiling point. At this temperature, the vapor pressure of the liquid equals the external atmospheric pressure. For water at 1 atm, this is 100°C.
iv. During condensation, the particles in a gas:
Answer: d) Lose energy and move slower
Explanation: Condensation is the phase change from gas to liquid. During this process, gas particles lose kinetic energy (heat is released to the surroundings). As they lose energy, they move slower and come closer together, forming liquid bonds.
v. Which law states that the volume of a gas is directly proportional to its temperature at constant pressure?
Answer: b) Charles's Law
Explanation: Charles's Law states that at constant pressure, the volume of a fixed amount of gas is directly proportional to its
absolute temperature (in Kelvin). The mathematical expression is V₁T₁ = V₂T₂.
vi. Evaporation differs from boiling because:
Answer: a) Evaporation occurs at the surface of a liquid at any temperature
Explanation: Evaporation is a surface phenomenon that occurs at any temperature, as high-energy molecules at the surface escape into the gas phase. Boiling is a bulk phenomenon that occurs at a specific boiling point throughout the entire liquid.
vii. According to kinetic particle theory, the pressure of a gas increases when:
Answer: d) The temperature increases
Explanation: According to the Kinetic Molecular Theory, increasing the temperature increases the average kinetic energy of gas particles. They move faster and collide with the container walls more frequently and with greater force, thereby increasing the pressure.
viii. Which factor affects the rate of diffusion of gases?
Answer: c) Both a and b
Explanation: The rate of diffusion is affected by molecular mass (Graham's Law: lighter gases diffuse faster) and temperature (higher temperature increases kinetic energy, thus increasing diffusion rate).
ix. Solid air fresheners use the process of:
Answer: c) Sublimation
Explanation: Solid air fresheners work by sublimation. The solid fragrance slowly changes directly into a gas, releasing the pleasant smell into the air without melting into a liquid first.
x. The diffusion of medicine in the body is important because:
Answer: b) It ensures the medicine reaches all parts of the body
Explanation: Diffusion allows medicine molecules to spread from the site of administration (e.g., bloodstream) to tissues and cells throughout the body, ensuring the therapeutic effect reaches the target areas.
i. Explain why the temperature remains constant during the phase change from ice to water.
Answer: During the phase change from ice to water (melting), the temperature remains constant because the heat energy supplied
(latent heat of fusion) is entirely used to overcome the intermolecular forces holding the ice lattice together. This energy breaks the rigid structure of the solid but does not increase the average kinetic energy of the molecules, which is what temperature measures. Once all the ice has melted, any further heat will increase the temperature of the water.
ii. What is the name given to the phase change when a solid is converted directly to a gas?
Answer: The phase change from a solid directly to a gas is called sublimation. Examples include dry ice (solid CO₂) becoming CO₂ gas and mothballs (naphthalene) vaporizing.
iii. What is the difference between evaporation and boiling?
Answer:
| Feature | Evaporation | Boiling |
| Location | Occurs at the surface of the liquid. | Occurs throughout the bulk of the liquid. |
| Temperature | Occurs at any temperature. | Occurs at a specific boiling point. |
| Bubbles | No bubbles are formed. | Bubbles of vapor are formed. |
| Energy Source | Uses ambient heat. | Requires continuous heat supply. |
iv. Interpret a heating curve for water, identifying key phase changes.
Answer: A heating curve for water is a graph of temperature versus time. It shows:
1. Solid Heating: Temperature rises as ice warms up.
2. Melting (Flat Region): Temperature remains constant at 0°C while ice melts to water (latent heat of fusion).
3. Liquid Heating: Temperature rises as water warms up.
4. Boiling (Flat Region): Temperature remains constant at 100°C while water boils to steam (latent heat of vaporization).
5. Gas Heating: Temperature rises as steam warms up.
v. Explain the effect of increasing temperature on the pressure of a gas in a sealed container.
Answer: In a sealed container (constant volume), increasing the temperature increases the average kinetic energy of the gas molecules. They move faster and collide with the container walls more frequently and with greater force. This results in an increase in the pressure exerted by the gas on the container walls. This relationship is described by Gay-Lussac's Law (P [source symbol unavailable] T).
vi. How does increasing the external pressure affect the boiling point of a liquid?
Answer: Increasing the external pressure raises the boiling point of a liquid. This is because a higher external pressure requires a higher vapor pressure to be achieved for the liquid to boil. More heat energy is needed to raise the vapor pressure to match the higher external pressure. This is why a pressure cooker cooks food faster (higher boiling point).
vii. Describe how molecular mass influences the rate of diffusion.
Answer: According to Graham's Law of Diffusion, the rate of diffusion of a gas is inversely proportional to the square root of its
molecular mass. This means that lighter gases (lower molecular mass) diffuse faster than heavier gases (higher molecular mass). For example, hydrogen (H₂, mass 2) diffuses much faster than oxygen (O₂, mass 32).
viii. Give an example of sublimation and explain its practical application.
Answer: An example of sublimation is dry ice (solid carbon dioxide).
Practical Application: Dry ice is used as a refrigerant for transporting perishable goods (like ice cream) because it sublimes directly to CO₂ gas without melting into a liquid, leaving no messy residue. It is also used in fog machines for theatrical effects.
ix. Why is the diffusion of gases faster at higher temperatures?
Answer: At higher temperatures, gas molecules have greater kinetic energy and move at higher speeds. This increased molecular motion leads to more frequent and energetic collisions, causing the gas molecules to spread out and mix with other gases more rapidly. Thus, the rate of diffusion increases with temperature.
x. How does Avogadro's law relate to the volume and number of moles of gas?
Answer: Avogadro's Law states that equal volumes of all gases, at the same temperature and pressure, contain the same number of molecules (or moles). Mathematically, the volume of a gas is directly proportional to the number of moles of gas present (V [source symbol unavailable] n). This means that if you double the number of moles of gas while keeping temperature and pressure constant, the volume will also double.
i. Describe in detail the kinetic particle theory and how it explains the phase changes of melting, freezing, boiling, and condensation.
Answer: The Kinetic Particle Theory (KPT) states that all matter is made up of tiny particles (atoms, molecules, ions) that are in constant random motion. The state of matter depends on the balance between the kinetic energy of the particles and the intermolecular forces of attraction between them.
1. Melting (Solid to Liquid):
Explanation: When a solid is heated, its particles gain kinetic energy and vibrate more vigorously. At the melting point, the energy is sufficient to overcome the rigid intermolecular forces holding the particles in fixed positions. The particles break free from their lattice structure and begin to slide past one another, forming a liquid.
2. Freezing (Liquid to Solid):
Explanation: When a liquid is cooled, its particles lose kinetic energy and move slower. At the freezing point, the particles no longer have enough energy to overcome the attractive forces between them. They settle into a regular, fixed lattice arrangement, forming a solid.
3. Boiling (Liquid to Gas):
Explanation: When a liquid is heated, its particles gain kinetic energy. At the boiling point, the vapor pressure of the liquid equals the external pressure. Particles throughout the liquid have enough energy to overcome the intermolecular forces completely and escape into the gaseous phase as bubbles.
4. Condensation (Gas to Liquid):
Explanation: When a gas is cooled, its particles lose kinetic energy and move slower. The intermolecular forces of attraction between the particles become strong enough to hold them together, causing them to come closer and form a
liquid.
In summary, phase changes are the result of changes in the kinetic energy of particles and the competition between kinetic energy and intermolecular forces.
ii. Interpret a cooling curve, identifying and explaining the significance of the flat regions on the curve.
Answer: A cooling curve is a graph of temperature versus time as a substance cools from a gas to a solid.
Regions of the Curve:
1. Gas Cooling: The temperature decreases as the gas loses heat to the surroundings.
2. Condensation (Flat Region): The temperature remains constant at the condensation point. This is because the heat energy being removed is used to overcome the latent heat of vaporization as gas particles come together to form a liquid. The potential energy decreases while kinetic energy (temperature) remains constant.
3. Liquid Cooling: The temperature decreases as the liquid loses heat.
4. Freezing (Flat Region): The temperature remains constant at the freezing point. This is because the heat energy being removed is used to overcome the latent heat of fusion as liquid particles settle into a solid lattice. Again, potential energy changes while kinetic energy remains constant.
5. Solid Cooling: The temperature decreases as the solid loses heat.
Significance of Flat Regions: The flat regions indicate that a phase change is occurring. During a phase change, the temperature does not change because the energy is used to alter the potential energy (arrangement of particles) rather than the kinetic energy (speed of particles).
iii. Explain how diffusion works in gases and discuss the factors affecting the rate of diffusion, including molecular mass and temperature.
Answer: How Diffusion Works in Gases: Diffusion in gases is the spontaneous, random mixing of gas particles due to their constant, rapid motion. Gas particles move in straight lines until they collide with other particles or the container walls. These collisions cause them to change direction, leading to a net movement of particles from an area of higher concentration to an area of lower concentration.
Factors Affecting the Rate of Diffusion:
1. Molecular Mass (Graham's Law):
The rate of diffusion is inversely proportional to the square root of the molecular mass of the gas.
Explanation: Lighter gas molecules move faster at a given temperature (they have higher average speeds) and therefore diffuse more quickly than heavier gas molecules. For example, hydrogen gas (H₂) diffuses about four times faster than oxygen gas (O₂).
2. Temperature:
The rate of diffusion increases with increasing temperature.
Explanation: Higher temperatures provide gas particles with more kinetic energy, increasing their average speed. Faster- moving particles collide more frequently and spread out more quickly, leading to a higher rate of diffusion.
In summary, diffusion in gases is driven by the random motion of particles and is influenced by molecular mass (lighter is faster) and temperature (hotter is faster).
1. Analyze the impact of altitude on the boiling point of water and how it affects cooking times. Provide a detailed explanation based on kinetic particle theory and external pressure.
Answer: Impact of Altitude on Boiling Point: At higher altitudes, the atmospheric pressure is lower than at sea level. According to the Kinetic Particle Theory, for a liquid to boil, its vapor pressure must equal the external atmospheric pressure. Since the external pressure is lower at high altitudes, the vapor pressure required to boil is also lower. This means water boils at a lower temperature (e.g., around 90°C at high altitudes compared to 100°C at sea level).
Effect on Cooking Times: Cooking generally relies on the heat energy transferred to the food. At higher altitudes, because water boils at a lower temperature, the maximum temperature the water can reach is lower. This lower temperature means that chemical reactions in food (like the denaturation of proteins) occur more slowly. Therefore, cooking times are longer at high altitudes.
Practical Solutions: Pressure cookers are used to counteract this by increasing the internal pressure, thereby raising the boiling point and reducing cooking time.
2. Discuss the importance of understanding diffusion rates in the development of pharmaceuticals.
Answer: Understanding diffusion rates is crucial in pharmaceutical development for several reasons:
Drug Delivery: The rate at which a drug diffuses across cell membranes determines how quickly it reaches its target site and how long its effects last. This affects the design of dosage forms (e.g., tablets, patches, injections). Bioavailability: Diffusion plays a key role in the absorption of drugs from the gastrointestinal tract into the bloodstream. Formulations must be designed to ensure optimal diffusion rates for effective absorption. Controlled Release: Many modern drugs use controlled-release technology that relies on diffusion through polymers to release the drug slowly over time. Toxicity: If a drug diffuses too quickly, it may reach toxic levels in the body. Understanding diffusion helps in designing
safe and effective dosing regimens.
3. Evaluate the environmental and practical implications of using sublimation in various industries, such as air fresheners and 3D printing, considering both benefits and potential drawbacks.
Answer: Benefits:
Air Fresheners: Sublimation allows for the slow, consistent release of fragrance without the need for heat or electricity. It is convenient and long-lasting. 3D Printing: Sublimation is used in specific 3D printing techniques (like sublimation printing) to transfer designs onto materials, offering high-quality, durable prints. Freeze-Drying (Lyophilization): Sublimation is used to preserve food and pharmaceuticals by removing water at low temperatures, preserving flavor and nutritional value.
Drawbacks:
Air Fresheners: The fragrance is released continuously, regardless of need, and may diminish over time. Some synthetic fragrances may release VOCs (volatile organic compounds) into the air, potentially affecting indoor air quality. 3D Printing: Sublimation printing requires specialized equipment and materials, and the process may generate fumes if not properly ventilated. Environmental Impact: The release of certain sublimated chemicals into the environment can contribute to air pollution if not managed properly.
In summary, sublimation has valuable applications but requires responsible management to minimize environmental impact.
