Ocean
Sea Water
The term seawater refers to oceans and seas. An ocean is typically larger than a sea. A sea is commonly found where land meets the ocean so seas are partially enclosed by land. About 2.5% of salt water is salt, 96. 5% is plain water, and the remainder is other materials and compounds. Several elements are extracted from salt water: magnesium, bromine, and, of course, sodium chloride which is table salt. The amount of salt in salt water is determined by measuring its salinity. Salinity is grams of salt in every kilogram of water. It is expressed in parts per thousand. Oceans typically have 34 to 37 parts per thousand.
Pond vs. Lake
Basically, the difference between a pond and a lake is size. Lakes are generally larger in surface area and depth than ponds. A pond is all in a photic zone which means sunlight can reach the bottom. Aphotic zones are found in lakes so the sun does not reach all the way to the bottom of lakes. This means there is more vegetation on the floor of a pond than the floor of a lake. In addition, lakes typically have slightly larger waves than ponds. Ponds also have a more consistent temperature than lakes due to the depth.
Pools
A pool consists of a basin, a water pump, a filter, a chemical feeder, drains, and returns. There is a constant flow of water through a filter to keep the water clean. One type of filter is a sand filter to stop debris. Chemicals are added to pools to keep them clean and safe. A chlorine compound is used as a disinfectant to kill pathogens that thrive in water. In solid form, it is calcium hypochlorite and in liquid form, it is sodium hypochlorite. When they react with water it forms hypochlorous acid.
Ca(ClO)2 + H2O --> 2HClO + CaO 2NaClO + H2O --> 2HClO + Na2O
Hypochlorous acid attacks and destroys enzymes and structures inside a cell, plus the lipids in the cell wall of microorganisms like bacteria. The problem is hypochlorous acid is unstable especially under the sun so a stabilizer needs to be added. An example is cyanuric acid. Also, hypochlorous acid may react with other chemicals, like ammonia, to form chloramines which are ineffective sanitizers and are often irritating to a person's skin and eyes. To solve this problem, the pool can be shocked. To do this, an ungodly amount of chemicals are added to the pool at once.
So, chemicals are the reason why swimming pools are safe and clean.
