Question 1: Define melting and describe the process occurring at the molecular level during melting of ice.

Answer: Melting is the change of a substance from solid to liquid when it absorbs heat. At the molecular level, the molecules gain energy, vibrate more vigorously, and overcome the forces holding them in a fixed position, resulting in a liquid.

Question 2: What is sublimation? Provide an example of a substance that sublimates under normal conditions.

Answer: Sublimation is the direct transition from a solid to a gas without passing through the liquid state. An example is dry ice (solid carbon dioxide).

Question 3: Explain the difference between vaporization and condensation.

Answer: Vaporization is the process of a liquid turning into a gas, which includes boiling and evaporation. Condensation is the change of a gas into a liquid when it cools or pressure increases.

Question 1: A 50 g sample of ice at -10°C is heated until it completely melts and then boils. How much heat energy is required to melt the ice? (Specific heat of ice = 2.1 J/g°C, heat of fusion of ice = 334 J/g).

Answer: Q = m * (C * ΔT) + m * Hf = 50 g * (2.1 J/g°C * 10°C) + 50 g * 334 J/g = 50 * 21 + 16700 = 1050 + 16700 = 17750 Joules

Question 2: Calculate the energy needed to convert 100 g of water at 100°C into steam at the same temperature. (Heat of vaporization of water = 2260 J/g).

Answer: Q = m * Hv = 100 g * 2260 J/g = 226000 Joules

Question 1: Describe a simple experiment to demonstrate the process of condensation. Include the variables that need to be controlled.

Answer: A simple experiment involves breathing onto a cold mirror or glass surface. As warm, moist air contacts the cold surface, water vapor condenses into tiny droplets. Variables to control include the temperature of the surface and humidity levels to ensure consistent results.

Question 2: List three safety precautions to observe when heating substances during phase change experiments.

Answer: 1. Always wear safety goggles and heat-resistant gloves. 2. Use a controlled heat source to prevent overheating or accidents. 3. Conduct experiments in a well-ventilated area or under a fume hood.

Question 1: A sample of water is heated from 25°C to 100°C, and the temperature remains constant for 5 minutes at 100°C before boiling begins. Explain this observation in terms of phase change.

Answer: The temperature remains constant at 100°C because the energy supplied is used for the phase change from liquid to gas (vaporization). This is called the heat of vaporization, during which temperature does not increase until the entire sample has boiled.

Question 2: During an experiment, a substance's temperature drops from 150°C to 100°C while releasing heat. Identify the phase change occurring and explain the process.

Answer: The substance is undergoing condensation or cooling during the phase change from vapor to liquid, releasing heat to the surroundings as molecules lose energy and come closer together.

Question 1: Explain how understanding sublimation is important in the dry ice cleaning industry.

Answer: Understanding sublimation allows the dry ice to sublimate directly from solid to gas, removing dirt and contaminants without using liquids. This makes it effective for cleaning delicate surfaces and electronic components.

Question 2: Describe a technological application that relies on phase changes to generate energy or perform work, and explain how it works.

Answer: Steam turbines in power plants utilize the phase change of water from liquid to vapor to generate energy. Water is heated to produce high-pressure steam, which then spins the turbine blades, converting thermal energy into mechanical energy, and subsequently into electrical energy.