In oceanography and environmental science, what is the typical salt concentration (salinity) of average sea water, expressed in parts per thousand and approximately equivalent to a few percent by mass?

Introduction / Context:
Sea water is not pure water; it contains dissolved salts, mainly sodium chloride, along with many other ions. The overall salt content is usually expressed as salinity, measured in parts per thousand. Knowing the typical salinity of sea water is important in marine biology, oceanography, climate studies, and even in everyday discussions about drinking water and desalination. This question asks you to choose the normal salinity range for average sea water.

Given Data / Assumptions:

• We are referring to open ocean sea water, not fresh water lakes or special inland seas.
• Salinity is measured in parts per thousand, often written as ppt.
• The given options are broad ranges such as 10–20 or 30–35 parts per thousand.
• Typical values reported in textbooks and references should be used.

Concept / Approach:
Average ocean salinity is usually quoted as about 35 parts per thousand, which means 35 grams of salt per 1000 grams of sea water. This value can vary slightly from place to place due to evaporation, rainfall, river inflow, and ice formation, but it generally stays near this range. In percentage terms, 35 parts per thousand is about 3.5 percent by mass. Therefore, the correct range among the given options is the one that includes 30–35 parts per thousand, covering the typical value of around 35.

Step-by-Step Solution:
Step 1: Recall from basic geography or chemistry that normal sea water has a salinity close to 35 parts per thousand. Step 2: Interpret parts per thousand as grams of salt per 1000 grams of sea water. Step 3: Compare the typical value of about 35 parts per thousand with the ranges given in the options. Step 4: Note that the range 30–35 parts per thousand comfortably includes the commonly quoted value. Step 5: Select 30–35 parts per thousand as the most accurate choice for average sea water salinity.

Verification / Alternative check:
As an additional check, remember that river water and fresh drinking water have very low salinity, close to zero parts per thousand. Highly saline bodies such as the Dead Sea can reach values well above 200 parts per thousand. The open ocean, which lies between these extremes, is consistently described in school and competitive exam material as having salinity around 35 parts per thousand. This matches the 30–35 range and rules out the much higher ranges in the other options.

Why Other Options Are Wrong:
The range 10–20 parts per thousand is too low for typical sea water and would correspond more to brackish water rather than normal ocean water. The ranges 40–50 and 60–70 parts per thousand are too high for average seas and are more comparable to special highly saline environments. These values do not match standard reference values for global ocean salinity and are therefore not correct for normal sea water conditions.

Common Pitfalls:
Students sometimes confuse parts per thousand with percent and may think 35 parts per thousand means 35 percent, which would be extremely salty. Another pitfall is to remember only that sea water is salty and to pick an extreme range without recalling the actual typical value. To avoid these mistakes, keep in mind that 35 parts per thousand is about 3.5 percent, and that most oceans stay close to this figure. This makes the 30–35 range clearly the most reasonable choice.

Final Answer:
The typical salinity of average sea water is about 30–35 parts per thousand, which corresponds to roughly 3–3.5 percent salt by mass.