Water is the chief chemical in oceans and seas. It usually exists in the form of a neutral chemical compound. However, water molecules are polar, which means that one end of the molecule is more electronegative than the other end. As a result, some water molecules are pulled apart by the electrical forces of neighboring water molecules, so that sea water contains small percentage of positive hydrogen ions (H+) and negative hydroxyl (OH-) ions. Hydrogen ions then may react with intact water molecules to form a further ion called the hydronium (H3O-) ion.
The principal seawater gases are carbon dioxide and oxygen. A small portion of these gaseous chemicals enter the water by diffusion from the atmosphere, but most come from photosynthesis or respiration of plants and animals. Oxygen is electrically neutral, but some carbon dioxide (CO2) reacts with water (H2O) to form carbonic acid (H2CO3). Because of the electrostatic action of polar water molecules, carbonic acid tends to dissociate and form positive hydrogen ions (H+) and negative bicarbonate ions (HCO3-) and doubly negative carbonate ions (CO3--). The latter precipitate quickly when they react with positive calcium ions.
Next to water itself, salt ions are the main chemical constituents of seawater. The principal salt ions in order of abundance are chloride, sodium, sulfate, magnesium, calcium, potassium, bicarbonate and bromide. When dry, such entities as table salt are electrically neutral because positive sodium ions (Na+) and negative chloride ions (Cl-) form a lattice in which the positive and negative components cancel each other out. However, when in the ocean, polar water molecules pull this lattice apart. The negative end of the water molecule attracts the sodium ions, while the positive end attracts the chlorine ions. Polar water molecules have the same effects on other salts also, so that seawater is rich in unbound salt ions. Accordingly, the reason why the main chemicals in seawater are positively or negatively charged is the polarity of the water molecule.
When an oil spill occurs, the oil does not mix with the sea water but floats on the top. Such materials that seem to repel water are called "hydrophobic." The polarity of water molecules explains the exclusion of hydrophobic substances from seawater. The electrostatic attraction between water molecules gives water a degree of cohesion. The water molecules forcefully draw as close as possible to one another and crowd out any electrically neutral non-polar molecule that happens to be in the way.