Reverse osmosis (RO) is a water purification process that utilizes the principles of osmosis and applies external pressure to overcome the natural osmotic pressure. The science behind reverse osmosis involves the selective movement of water molecules across a semi-permeable membrane to separate contaminants from water. Here’s a breakdown of the science involved:

1. Osmosis: Osmosis is a natural process where solvent molecules (in this case, water) move from an area of lower solute concentration to an area of higher solute concentration through a semi-permeable membrane. The goal of reverse osmosis is to reverse this process.
2. Semi-permeable membrane: The reverse osmosis membrane is a specialized membrane with extremely small pores that allow the passage of water molecules while blocking the majority of dissolved solids, contaminants, and particles. The membrane acts as a barrier, selectively allowing the smaller water molecules to pass through while rejecting larger molecules and impurities.
3. Applying pressure: In reverse osmosis, external pressure is applied to the water to overcome the natural osmotic pressure and force the water molecules through the membrane. The pressure helps to counteract the tendency of water to flow from lower solute concentration to higher solute concentration, allowing the purified water to be separated from the impurities.
4. Rejecting contaminants: The semi-permeable membrane in an RO system is designed to reject a wide range of contaminants, including dissolved salts, minerals, organic compounds, bacteria, viruses, heavy metals, and other impurities present in the feed water. These contaminants are typically larger in size or have different properties that prevent them from passing through the membrane.
5. Permeate and concentrate streams: As water is forced through the membrane, two streams are formed: the permeate and the concentrate. The permeate stream contains the purified water, while the concentrate stream carries the concentrated impurities and is typically discharged or treated separately.
6. Pre-filtration and post-treatment: Reverse osmosis systems often incorporate pre-filtration stages to remove larger particles, sediments, and chlorine, which can harm the RO membrane. Additionally, post-treatment processes such as activated carbon filtration or remineralization may be included to enhance the taste and quality of the purified water.

By utilizing these scientific principles, reverse osmosis effectively removes a wide range of impurities and contaminants from water, producing clean, purified water for various applications. It is a widely used and effective water treatment technology in residential, commercial, and industrial settings.