Polar molecules have slight positive and negative charges that allow them to form structures like water molecules. Solvents dissolve other substances, like sugar dissolving in water. The three states of matter are solids (molecules tightly packed and vibrating), liquids (molecules moving freely but staying together), and gases (molecules moving very fast and spread apart). Evaporation and condensation describe how water cycles between liquid and gas forms. Transpiration and precipitation further contribute to the water cycle through plants and rain respectively. Runoff, erosion, water tables, and percolation describe how water moves through the ground. Pollution from human, animal, and industrial waste negatively impacts water and air quality.