Problem 44 - Advanced Problems in Physical Chemistry for Competitive Examinations

Question

The process of getting fresh water from sea water is known as (a) osmosis (b) filtration (c) pressure distillation (d) reverse osmosis

Step-by-Step Solution

Answer

The process of getting fresh water from sea water is known as reverse osmosis (d).

Step 1: Understand the Concept of Obtaining Fresh Water from Sea Water

To solve this exercise, it's important to understand the different processes that can be used to remove salt and other impurities from sea water to produce fresh water. The options given are osmosis, filtration, pressure distillation, and reverse osmosis.

Step 2: Define Each Process

Osmosis is the movement of water from a region of lower solute concentration to a region of higher solute concentration through a semi-permeable membrane. Filtration is a physical process where impurities are removed by passing the water through filters. Pressure distillation involves heating sea water until it vaporizes, then condensing the steam back into liquid water, leaving the salts behind. Reverse osmosis is the process of forcing sea water through a semi-permeable membrane that allows water molecules to pass while blocking salts.

Step 3: Identify the Correct Process

Based on the definitions, we can determine that reverse osmosis is the correct answer. It is a widely used method of desalination for producing fresh water from sea water by applying pressure to push water molecules through a semi-permeable membrane.

Key Concepts

Osmosis

Osmosis is a naturally occurring process vital to the survival of plants, animals, and humans. It refers to the movement of solvent molecules, typically water, across a semi-permeable membrane from a region of lower solute concentration to one of higher solute concentration. This process aims to equalize the solute concentrations on the two sides of the membrane. While essential for biological mechanisms like cell hydration, osmosis is not the method used to obtain fresh water from sea water on an industrial scale because it does not facilitate the removal of salts from water.

Filtration

Filtration represents one of the simplest methods for separating substances. It's a mechanical or physical operation, which is used for the separation of solids from fluids (liquids or gases) by interposing a medium through which only the fluid can pass. Oversize solids in the fluid are caught by the filter and removed. In the context of sea water, filtration processes can remove particulate matter but are not typically efficient enough to remove dissolved salts to produce fresh water. However, they are often used as a preliminary treatment step before desalination to ensure the longevity and efficiency of more advanced treatments like reverse osmosis.

Pressure Distillation

Pressure distillation, also known as thermal desalination, is a process that mimics the natural water cycle. It involves boiling sea water under low pressure, which lowers the boiling point, to produce steam. The steam, which leaves the salts and other impurities behind, is then condensed back into liquid form, resulting in fresh, distilled water. This method is energy-intensive due to the heat required to vaporize water but is one of the oldest and simplest methods for desalination when powered by abundant heat sources.

Reverse Osmosis

Reverse osmosis is an advanced and efficient method for desalinating sea water. It is essentially osmosis in reverse, where water is forced through a semi-permeable membrane against the natural osmotic gradient. This is achieved by applying pressure greater than the osmotic pressure exerted by the water's dissolved solutes, typically salts. The membrane's minute pores allow water molecules to pass through while blocking larger salt ions and other impurities. The resultant product is fresh, potable water. Reverse osmosis is highly effective and uses less energy compared to thermal distillation methods, making it a popular choice for large-scale desalination facilities worldwide.