When a sample of water is heated from 4°C to 100°C at standard atmospheric pressure, which of the following occurs?

• A. The water remains at constant volume and density.
• B. The water expands and its density decreases.
• C. The water freezes and its density increases.
• D. The water contracts and its density increases.

Answer: (B)

Verified step by step guidance

1. Understand the physical properties of water as it is heated from 4°C to 100°C at standard atmospheric pressure. Water has a unique behavior near 4°C where it reaches its maximum density.
2. Recall that at 4°C, water is at its maximum density, meaning it occupies the least volume for a given mass. When heated above 4°C, water molecules gain kinetic energy and move apart, causing expansion.
3. Recognize that as water expands, its volume increases. Since density is defined as mass divided by volume, an increase in volume with constant mass results in a decrease in density.
4. Note that water does not freeze when heated; freezing occurs when temperature decreases below 0°C. Therefore, freezing and density increase are not applicable in this heating scenario.
5. Conclude that heating water from 4°C to 100°C causes it to expand and its density to decrease, which matches the correct answer choice.