Distillation method. The sea water is heated to turn the water into steam, and then the steam is condensed. Since the salt dissolved in seawater will not come out with steam, the water obtained is distilled water that is almost pure water. However, both heating and condensing require energy. It takes 2.3 kilojoules of energy to evaporate 1 gram of water. So this method can be an emergency, but it is not a long-term solution.
ion exchange method. The salts dissolved in water are in the form of cations and anions. If there is something that can remove these ions, then the water will be purified. Ion exchange resins have this ability. Ion exchange resin is a water-insoluble polymer material with a network structure. It is like a big tree with many branches on it, and each branch has a pocket. When sea water passes through, the cations will exchange the hydrogen ions in the cation exchange resin "pocket", and the anions will exchange the hydroxide ions in the anion exchange resin "pocket", and the hydrogen ions and hydroxide ions will meet. Turned into water. However, the ion exchange resin will reach saturation after being used for a period of time. How can the ion exchange resin be used continuously? Since ion exchange is reversible, it can be renewed by reverse exchange with acid and base separately, so that the ion exchange resin can be used repeatedly. However, the ion exchange method has limited processing capacity and requires a large amount of acid and alkali to "regenerate" the resin. If a large amount is used for seawater desalination, the cost is too high. Therefore, this technology is currently mainly used in the further purification of water.
reverse osmosis method. Penetration is a very common natural phenomenon in nature. For example, plants rely on the penetration of their roots to absorb water, and the balance of penetration is also extremely important for human life activities. Permeation is achieved by a material called semi-permeable membrane. The semi-permeable membrane allows water to pass freely, but other chemicals in the water cannot pass. If the left side of the semi-permeable membrane is pure water and the right side is the solution, the water passing through the semi-permeable membrane from the pure water side will be much more than the water passing through the semi-permeable membrane on the right side of the solution. This is because the saturated vapor pressure above the pure water is greater than the saturated vapor pressure above the solution. When both the pure water and the top of the solution are at a pressure of 1.01×105 Pa (1 standard atmosphere), the power that drives the movement of water molecules is determined by the saturated vapor pressure. Therefore, the liquid level of pure water drops, and the liquid level of the solution rises. When a certain level difference is reached, equilibrium is reached. This is the phenomenon of osmosis. The epidermis of plant roots is such a semi-permeable membrane.