Understanding the Concepts

* Solubility: The maximum amount of a solute (like KCl) that can dissolve in a given amount of solvent (like water) at a specific temperature.

* Equilibrium: A state where the rate of dissolving (forward reaction) equals the rate of crystallization (reverse reaction).

* Saturated Solution: A solution that contains the maximum amount of dissolved solute at a given temperature.

* Unsaturated Solution: A solution that can still dissolve more solute.

* Supersaturated Solution: A solution that holds more dissolved solute than it normally can at a given temperature, often created by carefully cooling a saturated solution.

Analysis

1. Dissolving: When you add KCl to water, it dissolves to form potassium ions (K+) and chloride ions (Cl-):

```

KCl(s) <=> K+(aq) + Cl-(aq)

```

2. Solubility Limit: The fact that only 10 g of KCl remains undissolved indicates that the water reached its solubility limit for KCl at that temperature.

3. Equilibrium: The 40 g of KCl that dissolved is in equilibrium with the remaining 10 g of solid KCl. This means:

* Forward Reaction: The dissolving of solid KCl continues at a specific rate.

* Reverse Reaction: The dissolved K+ and Cl- ions are recombining to form solid KCl at the same rate.

The Equilibrium Situation

The resulting solution is a saturated solution of KCl. Here's why:

* The solution holds the maximum amount of dissolved KCl possible at that temperature.

* The undissolved solid KCl is evidence of the equilibrium state.

* The system is in a dynamic equilibrium, with both the forward and reverse reactions occurring at equal rates.

Important Note: The exact solubility of KCl in water depends on the temperature. A higher temperature would allow more KCl to dissolve before reaching equilibrium.