Here, we have to find out the temperature at which 1 liter water weighs maximum. We know that, for ordinary substances density decreases with increase of temperature. But for water, volume decreases and density increases when temperature increases from 0°C to 4°C. From 4°C onwards the density decreases and volume increases. This is called anomalous expansion of water. Because of this water weighs maximum at 4°C (since mass = volume × density)