To understand the appearance of a temperature-vs.-time graph, especially in the context of phase changes (like melting, boiling, etc.), it's essential to consider how temperature behaves during these processes.
1. During phase changes, such as melting or boiling, the temperature of the substance remains constant despite the continuous addition of heat. This is because the energy is used to alter the molecular structure (breaking bonds) rather than increasing kinetic energy. Therefore, the graph should show horizontal lines during these periods when the temperature remains constant.
2. Between these phase changes, when a substance either heats up or cools down without changing its state, the temperature changes at a steady rate. This results in upward-sloping lines for heating or downward-sloping lines for cooling.
Given these points, the correct description of the temperature-vs.-time graph during phase changes would be:
- Horizontal lines where the temperature is constant during phase changes.
- These horizontal lines are connected by upward-sloping lines where the temperature increases when heat is being added and there is no phase change.
So, the statement that accurately describes the appearance of the temperature-vs.-time graph is:
Horizontal lines where the temperature is constant during phase changes connect upward-sloping lines where the temperature increases.
Therefore, the answer is:
3. Horizontal lines where the temperature is constant during phase changes connect upward-sloping lines where the temperature increases.
