### Correct Option: B. Fall Steadily ### Detailed Explanation: When water is heated from 0°C to 30°C, its density changes due to the thermal expansion of the water molecules. Let's break down the process step-by-step: 1. **Understanding Density**: - Density (\( \rho \)) is defined as mass (\( m \)) per unit volume (\( V \)): \[ \rho = \frac{m}{V} \] - For a given mass of water, if the volume increases, the density will decrease. 2. **Behavior of Water with Temperature**: - Water has a unique property compared to most substances: it reaches its maximum density at around 4°C. - Below 4°C, as water cools, it becomes denser until it reaches this temperature. However, as it continues to cool below 4°C, it starts to expand, leading to a decrease in density. 3. **Heating Water from 0°C to 30°C**: - When water is heated from 0°C to 4°C, it continues to become denser until it reaches 4°C. - After reaching 4°C, as the temperature increases from 4°C to 30°C, the water expands, and its density decreases steadily. 4. **Conclusion on Density Change**: - Therefore, when heating water from 0°C to 30°C, the density of water will initially increase as it approaches 4°C, but after that point, it will decrease steadily as the temperature rises to 30°C. ### Why Other Options Are Incorrect: - **Option A: Rise Steadily**: This option suggests that the density increases continuously as the temperature rises. This is incorrect because, while density does increase up to 4°C, it decreases after that point as the water expands. - **Option C: Fall Then Rise**: This option implies that the density decreases initially and then increases again. This is incorrect because, after reaching the maximum density at 4°C, the density does not rise again as the temperature increases to 30°C. - **Option D: Rise Then Fall**: This option suggests that density increases and then decreases. While it correctly identifies that density decreases after 4°C, it incorrectly states that density rises before falling, which is misleading since the density only increases up to 4°C and then decreases. ### Common Pitfalls: - **Misunderstanding Water's Unique Properties**: Many students may not realize that water behaves differently than most substances when it comes to density and temperature. It is crucial to remember that water reaches its maximum density at 4°C. - **Ignoring the Temperature Range**: Students might overlook the specific temperature range (0°C to 4°C) where density increases, leading to confusion about the overall trend. ### Revision Summary: - Water's density increases as it cools down to 4°C and decreases as it heats up beyond that point. - The maximum density of water occurs at 4°C. - When heating water from 0°C to 30°C, the density initially increases and then decreases steadily. - Understanding the unique properties of water is essential for solving related problems in thermodynamics and fluid mechanics.