Equilibrium is reached when the sugar dissolves completely in the water, meaning that the rate at which sugar molecules enter the solution is equal to the rate at which they exit the solution. This occurs when the sugar is fully saturated in the water, and no more sugar can dissolve at that specific temperature and pressure.
When sugar is added to water, it starts to dissociate into individual molecules, spreading throughout the liquid. As more sugar is dissolved, there will come a point where the solution becomes saturated. At this point, if more sugar is added, it will not dissolve; instead, it will remain as solid precipitate at the bottom. The dynamic between the dissolved sugar molecules and those that are not yet dissolved continues until saturation is achieved.
In summary, the equilibrium in the sugar and water mixture is reached when the concentration of dissolved sugar remains constant over time, indicating that the dissolution and precipitation processes are balanced.