Temperature and salinity of ocean waters

Salinity is the concentration of dissolved salts in water, and it is commonly expressed in parts per thousand, written as ‰ or ppt. A mean ocean salinity of 35 means that approximately 35 units by mass of dissolved salts are present in 1,000 units by mass of seawater; the remaining mass is mainly water. Thus option A correctly interprets the measurement. Option B is incorrect because salinity does not measure sand or other solid sediments. Options C and D incorrectly treat the number as an interval of temperature or time. The value is a concentration, not a rule about currents or freezing. Natural salinity varies with evaporation, precipitation, river inflow, ice processes and mixing.

The word thermohaline combines two scientific roots: “thermo,” relating to heat or temperature, and “haline,” relating to salt or salinity. Thermohaline circulation is the large-scale movement of seawater driven by density differences that arise mainly from variations in temperature and salinity. Cooling can make water denser, while increased salinity can also raise density; dense water may sink and contribute to deep-ocean circulation. Hence option A correctly identifies both terms. Option B refers to other forces that can influence ocean movement but does not explain the word itself. Depth and coast are locations or dimensions, not the meanings of the roots, and sand and soil are unrelated to the terminology.

River water is generally fresh compared with seawater, so its entry at an estuary or river mouth dilutes the salt concentration of nearby surface water. Lower salinity usually means lower density, provided other factors such as temperature and suspended sediment do not outweigh that effect. Because freshwater commonly spreads over denser seawater, the surface layer can become lighter and less dense. Thus option A is correct. Option B reverses the usual dilution process; river inflow does not always raise salinity. Options C and D are not physical explanations: water cannot turn temperature into stone, and river discharge does not close the sea. Local conditions may modify the result, but the fundamental relationship is river inflow, reduced salinity and potentially lower surface-water density.

The governing concept is the unequal penetration of solar energy and the contrast between surface and deep ocean layers. Solar radiation enters mainly through the upper water, so the surface receives direct heating during the day and loses heat through radiation and turbulent exchange at night. This produces a noticeable daily range. Deep water is shielded from these rapid changes because sunlight becomes very weak with depth, while mixing and the large heat capacity of the ocean moderate temperature variation. Therefore option A is correct. Option B is opposite to reality, option C is an absolute and false claim, and option D is physically incorrect because the surface is covered by water.

The governing concept is seasonal variation in the solar-energy budget of the ocean. Because Earth is tilted on its axis and revolves around the Sun, the angle of incoming rays and the length of daylight change through the year. These changes alter the amount of insolation received by the sea surface, causing seasonal warming and cooling. Ocean currents, winds, cloud cover, and mixing can modify the local response, but the basic cause is the seasonal change in solar radiation. Thus option A is correct. The other options do not provide a physical explanation: soil formation is irrelevant, zero salinity is not a normal seasonal condition, and rivers do not universally stop flowing.

The governing concept is dilution of seawater by freshwater input. Heavy rainfall places relatively salt-free water on the ocean surface, rivers transport freshwater from land into coastal and open-water areas, and melting ice adds water with very little dissolved salt. If the added freshwater mixes with seawater, the same dissolved salts are distributed through a larger amount of water, so salinity decreases, assuming other conditions remain comparable. Thus option A is correct. Option B describes the opposite effect, option C is not a necessary result of these processes, and option D has no physical connection with salinity. Local circulation can modify the strength of the effect but not its basic direction.

The governing concept is salinity, meaning the amount or concentration of dissolved salts in water. Seawater contains substantial quantities of dissolved salts, so drinking it directly adds more salt than the human body can safely excrete with the available water. This can worsen dehydration rather than satisfy thirst; therefore seawater normally requires desalination before it is used as drinking water. Option A gives the correct everyday connection. B is the opposite of reality. Salinity does not physically form roads, so C is unrelated. Although seawater properties may affect wider environmental processes, salinity does not directly indicate cloud height, making D incorrect.

The density of seawater is governed mainly by temperature and salinity. Colder water is generally denser because its molecules are packed more closely, while water with greater dissolved salt content is also denser. Therefore, differences in heating, cooling, evaporation, freezing, and freshwater addition can create density contrasts between ocean-water masses. These contrasts help drive vertical sinking and rising and are important in thermohaline circulation and deep ocean currents. Option A is correct because it names both controlling factors. Coastal buildings, the number of fishers, and the colour of ships do not significantly determine seawater density; they are unrelated surface or human features.