Dissolving is an example of a reversible change. For example, when salt is mixed with water it disappears because it dissolves in the water to make salty water. But we can get the salt can back again by boiling off the water.

Which best describes why a liquid needs a container when a solid does not? Solids particles do not expand to fill their containers, and liquid particles do. Solids particles expand to fill their containers, and liquid particles do not. Solids have definite shapes, and liquids fill their containers.

Dissolving a solid in liquid, such as table salt in water, is a physical change because only the state of the matter has changed. Physical changes can often be reversed. Allowing the water to evaporate will return the salt to a solid state. It has not combined with the water to cause a chemical reaction.

Water molecules pull the sodium and chloride ions apart, breaking the ionic bond that held them together. After the salt compounds are pulled apart, the sodium and chloride atoms are surrounded by water molecules, as this diagram shows. Once this happens, the salt is dissolved, resulting in a homogeneous solution.

Factors affecting solubility

• Temperature. Basically, solubility increases with temperature.
• Polarity. In most cases solutes dissolve in solvents that have a similar polarity.
• Pressure. Solid and liquid solutes.
• Molecular size.
• Stirring increases the speed of dissolving.

pH and the charge on protein The important point to remember is that in a pH condition below its isoelectric point, the protein will carry net positive charge and behave like a cation. In a pH condition above its isoelectric point, the protein will carry a net negative charge.