How does the density of water change as its temperature decreases from warm to cold?

• A. The density decreases as water cools.
• B. The density first increases, then decreases as water cools.
• C. The density increases as water cools.
• D. The density remains constant regardless of temperature.

Answer: (C)

Verified step by step guidance

1. Understand that density is defined as mass per unit volume, given by the formula \(\text{density} = \frac{\text{mass}}{\text{volume}}\).
2. Recall that for most substances, as temperature decreases, the molecules move closer together, causing the volume to decrease and density to increase.
3. Recognize that water exhibits an unusual behavior: as it cools from warm temperatures down to about 4°C, its density increases because the molecules pack more tightly.
4. Note that below 4°C, water begins to expand due to the formation of a more open hydrogen-bonded structure, causing the density to decrease as it approaches freezing.
5. Conclude that the density of water first increases as it cools from warm temperatures to 4°C, then decreases as it cools further toward 0°C.