The only natural substance found in all three physical states, water is referred to as the "universal solvent" because more substances dissolve in water than any other liquid, which means that wherever it goes, it takes whatever has dissolved in it along. Water has a neutral pH of seven, and the atoms in water molecules are in constant motion, allowing other substances to dissolve in it.
All molecules are in constant random motion according to the theory of passive transport. The energy of their motion is called kinetic energy. The glass of cold water contains water molecules that are in motion. Adding a drop of food coloring to the water will move water molecules upwardly while the food coloring molecules sink to the bottom. The molecules of food coloring also have kinetic energy and will slowly spread out among all of the water molecules until evenly distributed, where the ratio of water to coloring is that same at any given point in the glass. This is referred to as an isotonic solution.
When food coloring is added to the water and begins to dissolve, its component molecules move from an area of high concentration to an area of low concentration. At the beginning point where the solute (food coloring) is still in a state of higher concentration in some parts of the water and lower concentration everywhere else, the solution is referred to as hypotonic. As diffusion occurs and the solute dissolves, the solution will eventually become isotonic.
Adding heat to anything causes its component molecules, already in motion, to start moving more quickly. If adding food coloring to hot water, the food coloring will diffuse much faster than in cold water, where the water molecules are moving more slowly. Watching food coloring dissolve in cold water is infinitely more interesting because the cloudy shapes that the food coloring makes while diffusing will last longer, growing and stretching before your eyes.