Water has maximum density at 4°C.

Answer

**Step-by-Step Solution:**

1. **Understanding Density**: Density is defined as the mass of a substance divided by its volume (Density = Mass/Volume). This means that for a given mass, if the volume decreases, the density increases.

2. **Behavior of Water at Different Temperatures**: Water behaves uniquely compared to most substances. When water freezes at 0°C, it forms ice, which has a more ordered structure due to hydrogen bonding. This structure causes ice to occupy more volume than liquid water, making ice less dense than water.

3. **Heating Ice**: As we gradually increase the temperature of ice, it begins to melt. During this process, the hydrogen bonds that hold the ice structure together start to break down.

4. **Increasing Thermal Energy**: With the increase in temperature, the thermal energy of the ice increases. This energy helps in breaking the hydrogen bonds, allowing the water molecules to move closer to each other.

5. **Volume and Density Changes**: As the water molecules come closer together, the volume of the water decreases. Since density is inversely related to volume (for a constant mass), as the volume decreases, the density of water increases.

6. **Maximum Density at 4°C**: This process continues until the temperature reaches 4°C, where the water molecules are packed in the most efficient arrangement. At this point, the density of water is at its maximum. Beyond 4°C, if the temperature continues to rise, the water molecules gain more energy and start to move apart, leading to a decrease in density.

**Conclusion**: Water has its maximum density at 4°C because, at this temperature, the arrangement of water molecules is such that they occupy the least volume, resulting in the highest density.

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