Physical Science Flashcards

A student creates a supersaturated solution of sodium acetate in water at 80°C and allows it to cool undisturbed to 20°C. The solution remains a clear liquid. Which of the following events would most likely trigger rapid crystallization?

Answer: Adding a single, tiny crystal of solid sodium acetate to the solution.

A supersaturated solution is in a metastable state, meaning it contains more dissolved solute than it can normally hold at that temperature. Crystallization is thermodynamically favored but requires a nucleation site to begin. Adding a 'seed' crystal of the solute provides a template for other solute particles to arrange themselves onto, initiating a rapid chain reaction of crystallization.

The phenomenon of sonoluminescence involves converting sound energy into light. A gas bubble trapped in a liquid by an acoustic standing wave collapses violently, emitting a flash of light. What is the primary energy transformation responsible for the light emission?

Answer: The kinetic energy of the collapsing bubble wall is converted into thermal energy, causing incandescence.

In sonoluminescence, a standing sound wave traps a bubble and causes it to oscillate, expand, and then collapse violently. This rapid implosion superheats the gas and plasma inside the bubble to extreme temperatures, causing it to emit a brief, bright flash of light. The core process is the conversion of the kinetic energy from the collapsing liquid into immense thermal energy, leading to light emission.

A passenger on a stationary jet bridge hears the sound from the two engines of a taxiing aircraft. They notice the total sound intensity fluctuates, becoming periodically louder and quieter. This phenomenon is a direct result of:

Answer: The superposition of sound waves from the two engines, which have slightly different frequencies, causing alternating constructive and destructive interference.

When two sound waves of similar but not identical frequencies interfere, they alternate between being in phase (constructive interference, causing a louder sound) and out of phase (destructive interference, causing a quieter sound). This pattern of fluctuation in amplitude is called 'beats'. The passenger is hearing the beats created by the superposition of the sound waves from the two separate jet engines.

In the context of nuclear reactions, how does the energy released per nucleon in a typical fusion reaction (e.g., deuterium-tritium) compare to a typical fission reaction (e.g., uranium-235)?

Answer: Fusion releases significantly more energy per nucleon than fission.

While a single fission event of a heavy nucleus like uranium releases more total energy than a single fusion event of light nuclei like hydrogen isotopes, the change in binding energy per nucleon is much greater in fusion. Fusion converts a larger fraction of the mass of the reacting particles into energy. Therefore, on a per-nucleon or per-unit-mass basis, fusion releases significantly more energy than fission.

Consider the reversible reaction for the synthesis of ammonia (Haber process), which is exothermic: N₂(g) + 3H₂(g) ⇌ 2NH₃(g) + heat. An industrial chemist wants to maximize the yield of ammonia at equilibrium. According to Le Chatelier's principle, which set of conditions would be most effective?

Answer: Low temperature and high pressure

According to Le Chatelier's principle, the equilibrium will shift to counteract any change. To favor the forward (exothermic) reaction, heat should be removed, meaning a lower temperature is ideal. The forward reaction also results in fewer moles of gas (1+3=4 moles of reactants vs. 2 moles of product). Increasing the pressure will cause the equilibrium to shift to the side with fewer gas moles to relieve the pressure. Therefore, low temperature and high pressure maximize the ammonia yield.

A student observes that when two identical wave pulses are sent towards each other along a spring, there is a moment when the spring appears completely flat and undisturbed. Immediately after this moment, the two pulses reappear and continue traveling in their original directions. This observation is a clear demonstration of:

Answer: Total destructive interference.

This scenario describes total destructive interference. When two identical waves (same amplitude) that are perfectly out of phase (one is a crest, the other a trough) meet, their displacements cancel each other out for a moment, making the medium appear flat. Importantly, the waves themselves are not destroyed; they pass through each other and continue on, which is a key property of wave superposition.