Which processes are represented by stages X and Y?

Here's the breakdown for question 19:

Question 19: The diagram shows a heating curve for water, plotting temperature against heat added.

• Stage X: This is a horizontal segment on the graph at $0^\circ\text{C}$. During this stage, heat is being added, but the temperature remains constant. This indicates a phase change from solid (ice) to liquid (water). This process is melting.
• Stage Y: This is another horizontal segment on the graph at a higher temperature (typically $100^\circ\text{C}$ for water at standard pressure). Here, heat is added, but the temperature remains constant. This indicates a phase change from liquid (water) to gas (steam). This process is boiling.

Comparing this with the given options:

• A. melting, evaporation
• B. evaporation, boiling
• C. evaporation, melting
• D. melting, boiling

The correct processes are melting for stage X and boiling for stage Y.

The final answer is $\boxed{\text{D}}$

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Here's the breakdown for question 20:

Question 20: This question describes water waves refracting as they move from deep to shallow water.

• Frequency: The frequency of a wave is determined by its source and does not change when the wave passes from one medium to another (or from one depth to another in the case of water waves). So, the frequency remains unchanged.
• Speed: The speed of water waves depends on the depth of the water. Water waves travel faster in deeper water and slower in shallower water. Therefore, as waves move from deep to shallow water, their speed decreases.

Comparing this with the given options:

• A. frequency: decrease, speed: increase
• B. frequency: increase, speed: decrease
• C. frequency: no change, speed: decrease
• D. frequency: no change, speed: increase

The correct row shows no change in frequency and a decrease in speed.

The final answer is $\boxed{\text{C}}$

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Here's the breakdown for question 21:

Question 21: The question asks for the term that describes the maximum displacement of a particle from its rest position in a wave.

• Amplitude: This is defined as the maximum displacement or distance moved by a point on a vibrating body or wave measured from its equilibrium (rest) position.
• Frequency: This is the number of complete oscillations or cycles of a wave that pass a point per unit time.
• Wavefront: This is a surface connecting points of constant phase on a wave.
• Wavelength: This is the spatial period of a periodic wave, the distance over which the wave's shape repeats.

Based on the definitions, the term that describes the maximum displacement from the rest position is amplitude.

The final answer is $\boxed{\text{A}}$