At what temperature is the density of pure water at atmospheric pressure greatest?

• A. 25~^ext{o}ext{C}
• B. 4~^ext{o}ext{C}
• C. 0~^ext{o}ext{C}
• D. 100~^ext{o}ext{C}

Answer: (B)

Verified step by step guidance

1. Recall that the density of a substance is defined as its mass divided by its volume, \(\rho = \frac{m}{V}\). For water, density changes with temperature because volume changes as water expands or contracts.
2. Understand that water exhibits an unusual behavior compared to most substances: it reaches its maximum density at a temperature above its freezing point, not at the lowest temperature.
3. Recognize that at 0~^\text{o}C, water is at its freezing point and begins to form ice, which is less dense than liquid water, so the density is not at its maximum there.
4. At 100~^\text{o}C, water is boiling and turning into steam, so the density of liquid water is much lower due to increased volume from thermal expansion.
5. Therefore, the temperature at which pure water has its greatest density at atmospheric pressure is approximately 4~^\text{o}C, where water is most compact before it starts expanding as it cools further toward freezing.