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When we talk about 'parts per million' or ppm, we're discussing a way of expressing very dilute concentrations of substances. Imagine taking a million parts of a whole and then specifying how many parts of a specific substance are found within those million total parts. In other words, if you have 1 ppm of a substance in water, it means that out of one million particles of that solution, one particle is the substance of interest.

In our exercise, the salinity of ocean water, which is the amount of salt dissolved in water, is presented as a whole number, but for scientific precision and for certain applications, it's necessary to convert this number into ppm. This aids in understanding the tiny concentrations and comparisons in different water bodies or solutions. The use of ppm is widely accepted in fields like chemistry, environmental science, and engineering, where accurate measurements of concentration are critical.

Seawater is notoriously known for its high salinity, which is due to the various salts dissolved in it. These salts come from the erosion of rocks on land, volcanic emissions, and from the ocean's floor. When we measure salinity, we’re essentially measuring the concentration of these dissolved salts.

The primary salts are sodium chloride, magnesium sulfate, and calcium carbonate, among others, which give seawater its distinct saline taste. The concentration of dissolved salts in seawater, as in the exercise, is crucial for marine life, oceanic chemistry, and for various human activities such as desalination. The calculated 350,000 ppm from the exercise outlines the massive difference in salt concentration between seawater and fresh water, and it is this very salinity that supports certain organisms and affects the water's buoyancy and density.

To make seawater drinkable or usable for most agricultural and many industrial processes, the dissolved salts concentration must be drastically reduced — in essence, converting seawater to freshwater. This process is known as desalination, and it involves several methodologies, including distillation and reverse osmosis.

The aim is to get the salt concentration down to under 500 ppm, which is the general maximum concentration allowed for water to be considered 'freshwater'. According to the exercise, approximately 99.86% of the dissolved salts in seawater need to be removed to reach this level. This indicates just how saline the ocean is and the extent of purification required. Desalination technology has been a cornerstone in providing fresh water in arid regions, and understanding the concentrations involved helps us appreciate the challenges of this critical process.