It means you should add enough of a particular liquid to the volumetric flask or other piece of glassware to reach the desired final volume.

Here's how it typically works:

* You'll have a solid or liquid solute that needs to be dissolved in a solvent.

* The recipe or protocol will specify a particular final volume.

* You'll add your solute to the glassware, then add enough solvent to bring the solution up to the final volume marking.

Example:

A recipe calls for 1 gram of sodium chloride (NaCl) dissolved in 100 ml of water.

1. You would add 1 gram of NaCl to a 100 ml volumetric flask.

2. You would then add water, slowly and carefully, until the bottom of the meniscus reaches the 100 ml mark on the flask. This is where the "QS" comes in - you add enough water (the solvent) to make the final volume "sufficient".

Important Notes:

* QS is often used with volumetric flasks, but it can also be used with graduated cylinders and other glassware.

* Always use distilled or deionized water for QS, to avoid introducing impurities.

* When adding solvent to QS, it's crucial to avoid overshooting the final volume. It's often a good idea to add the solvent in small increments near the end, to ensure accuracy.