General

To put it simply, tonicity refers to the movement of water into or out of a test subject depending on the concentration of the solutes in both the solution being used as well as the test subject. The whole process is done in order to bring balance to the solutes in each, that is the solution and the test subject.

On a side note you don't really need to know the in depth explanation of that you just need to know that it's to bring about balance of solutes, in each of the three given solutions. In an isotonic there is already balance of solutes in both hence no movement of water while in a hypotonic as well as a hypertonic the solute balance must be obtained.

As i said that's not necessary to know just know that the solutes do NOT move only the water molecules move in the process of osmosis.

Edit: Refer to above poster for advanced explanation of solute balance

knowing what the prefixes mean will help you understand these concepts. hyper- means over, hypo- means under, and iso- means equal. tonicity refers to the concentration of solutions between two biological subjects, such as the interior and exterior of a cell. it is used as a relative term, so when you say something is hyper/iso/hypo-tonic, you are always comparing it to something else. for example, saying that a cell is hypertonic to blood means that the cell has a higher concentration of certain solutes compared to blood. isotonic means that the concentrations are equal, and hypotonic means that the cell has a smaller concentration of solutes compared to blood. so why is this concept important? well, differing concentrations means that diffusion will occur to try to achieve equilibrium, that is, the subjects will try to achieve the same solute concentration inside the cell and in the blood depending on the concentration gradient.

for example, if a cell is hypertonic to blood, water from the blood will enter the cell and decrease the concentration of solutes inside the cell, concentration being roughly defined as the amount of solute per unit volume. if you increase the volume and the amount of solute remains unchanged, then the concentration decreases like it does in this example until solute concentration is equal inside and outside of the cell. if a cell is hypotonic to blood, water diffuses across the cell membrane and the concentration increases until the solute concentration is equal to the solute concentration in the blood.

@As3an omg thanks! I'm a bit confused though. So the tonicity determines the movement of water right? Not the solute? Cause I always thought it was the solute

Test subject: Leaf (Example)

Isotonic solution: When the leaf is placed into a solution which has the same concentration as the cell contents of the leaf there is no movement of water into or out of the leaf causing it to remain in it original form. Osmosis does not occur as a result.

Hypotonic solution: When the leaf is placed into a solution which is more concentrated than the cell contents of the leaf there is a movement of water from the solution into the leaf since its concentration is higher. Osmosis is the movement of water molecules from a region of higher concentration to lower concentration. As a result the water molecules move from the region of higher concentration (liquid in container which leaf is placed into) to a region of lower concentration (the cell contents of the leaf) causing a movement of water into the leaf. The leaf increases in size as a result.

Hypertonic solution: When the leaf is placed into a solution which is less concentrated than the cell contents of the leaf there is a movement of water from within the leaf to the solution since the concentration within the leaf is higher. Osmosis similarly occurs where the water molecules move from the region of higher concentration (the cell contents of the leaf) to a region of lower concentration ( liquid in container which leaf is placed into) causing the movement of water from within the leaf to the solution. The leaf decreases in size as a result.

Hence an isotonic solution is one which has the same concentration as the cell contents of the given test subject while a hypotonic solution is one which has a higher concentration rather than the given test subject and a hypertonic solution is one which has a lower concentration rather than the given test subject.