What does it mean to say that two solutions are isotonic?

Question

What does it mean to say that two solutions are isotonic?

Answer 1

Isotonic means:

One solution contains the same concentration of solute as an another solution.

Where a solute is:

A substance that is dissolved by  a solvent to form a solution.

For example salt dissolved by water.

In the biology world isotonic is often discussed in regards to cells.

So imagine a cell with a cell wall. The cell wall is a semi-permeable membrane (a barrier that will allow certain molecules  to pass through it while blocking others.)  

Generally speaking for cells it is the water molecule that can pass through the cell wall.

So in effect we have two solutions, one within the cell one outside the cell.

The dynamic element is Osmosis, (Osmosis is the diffusion or movement of the solvent through the cell wall.)

The direction of movement is from the solution with the lower concentration of solute to the solution with the higher concentration of solute.

Because, Nature tries for balance or equilibrium

Three relationships are possible between these two solutions,  hypertonic, hypotonic, and isotonic.

If the cell has a higher concentration of solute, it is considered to be the hypertonic solution in relation to the external solution. So water flows into the cell.

If the cell has a lower concentration of solute, it is considered to be the hypotonic solution in relation to the external solution. So water flows out of the cell.

Now for your isotonic solution.

If the two solutions have the same concentration of solute they are isotonic with respect to each other.

They are in balance  For any water flow out of the cell there is an equal water flow into the cell

Answer 2

It implies that they have equal solute concentrations and thus are in dynamic equilibrium (since there is no concentration gradient).