A solute is added to water and a portion of the solute remains un-dissolved. When equilibrium between the dissolved and un-dissolved solute is reached, the solution must be
saturated?
saturated.
To understand why, let's break it down step by step. When a solute is added to water, it can either dissolve completely or remain undissolved. The solute particles interact with the water molecules, and if the forces between them are strong enough, the solute will dissolve to form a homogeneous solution.
However, in some cases, the forces holding the solute particles together are stronger than the forces between the solute and water molecules. As a result, some portion of the solute remains undissolved.
When this happens, the system will reach a point where the rate of dissolution of solute particles into the solution is equal to the rate at which solute particles are re-precipitating or coming out of the solution. This point is known as equilibrium.
At equilibrium, the concentration of the dissolved solute remains constant. Any further dissolution of the solute is balanced by the re-precipitation of solute back into its solid form. It is important to note that equilibrium does not mean that both dissolved and undissolved solute are of equal quantities, but rather that the rates at which solute is dissolving and precipitating are equal.
Now, coming back to the question, when equilibrium is reached and the solute has not completely dissolved, it means that the maximum amount of solute that can dissolve in the given amount of water at that particular temperature and pressure has been reached. This maximum solute concentration is known as the solubility of the solute in that specific solvent (in this case, water).
Therefore, the solution in this scenario is said to be saturated because it contains the maximum possible amount of solute that can be dissolved in the solvent under the given conditions.