What is salinity of sea water?
Salinity is the term used to define the total content of dissolved salts in sea water. All waters in nature, whether rain water or ocean water, contain dissolved mineral salts.
It is calculated as the amount of salt (in gm) dissolved in 1,000 gm (1 kg) of seawater. Salinity is usually expressed as parts per thousand (°/°00) or ppt. A salinity of 24.7°/°00 has been considered as the upper limit to demarcate 'brackish water'.
Explanation
The question asks for a definition of salinity. The textbook context provides a clear definition in the first paragraph: salinity is the total content of dissolved salts in sea water. The answer also includes how it is calculated (gm of salt per 1,000 gm of seawater) and its unit of expression (parts per thousand or ppt). The mention of the brackish water limit adds completeness to the answer for a 2-mark question.