Final answer:
The dotted line between water molecules represents a hydrogen bond, which is a weak bond occurring between the oppositely charged parts of polar molecules like water.
Step-by-step explanation:
The dotted line between water molecules represents a hydrogen bond, which forms between the partially positive hydrogen atom of one water molecule and the partially negative oxygen atom of another water molecule. This is due to the polar nature of water, where the oxygen atom is more electronegative and attracts electrons more strongly than the hydrogen atoms, making the oxygen end of the water molecule slightly negative and the hydrogen end slightly positive. Hydrogen bonds are relatively weak compared to covalent bonds, and are significant in determining the physical properties of water.