Q.Give an example of a material used for making semipermeable membrane for carrying out reverse osmosis.

The key idea is reverse osmosis, where pressure greater than the osmotic pressure is applied to force solvent molecules from a concentrated solution to a pure solvent side through a semipermeable membrane.

Reasoning:

1. A semipermeable membrane allows only solvent (water) molecules to pass, not solute ions or molecules.
2. For reverse osmosis, the membrane must withstand high applied pressures and resist chemical attack. …

Why This Material Works

Reverse osmosis (RO) is the opposite of natural osmosis — you apply pressure greater than the osmotic pressure to force water from a concentrated solution to a dilute one, against the concentration gradient. The heart of the system is the semipermeable membrane. It must be:

• Highly permeable to water
• Nearly impermeable to dissolved salts and organic molecules
• Chemically stable under high pressure and in contact with water

Cellulose acetate was the first commercially successful RO membrane material. It's a polymer made from cellulose (plant fibre) treated with acetic acid. Its structure contains hydrophilic (water-loving) hydroxyl groups that attract water molecules, while the polymer chains are tightly packed enough to exclude larger hydrated ions like Na⁺ and Cl⁻.

Step-by-Step Reasoning

1. Identify the requirement: The question asks for a material used to make the semipermeable membrane in reverse osmosis. This is a factual recall from chemistry or environmental science — no calculation needed.

2. Recall the classic example: In textbooks and exam syllabi (especially for CBSE, ICSE, and state boards), the standard example given is cellulose acetate. It's mentioned in the context of desalination of seawater by reverse osmosis. …

Concept: Reverse Osmosis (RO) and Semipermeable Membranes

Reverse osmosis is a water purification process where pressure is applied to overcome natural osmotic pressure, forcing water molecules to move against their concentration gradient through a semipermeable membrane. The membrane allows water to pass but blocks dissolved salts, ions, and larger molecules.

Method: Pressure-Driven Membrane Filtration (Reverse Osmosis)

Steps:

1. Select a suitable semipermeable membrane material — must allow water molecules to pass while rejecting dissolved solutes (salts, ions, organic molecules).

2. Apply pressure greater than the osmotic pressure of the solution on the concentrated side (typically 15–70 bar for seawater desalination).

3. Water molecules pass through the membrane pores (typically 0.1–1 nm in diameter) by diffusion and capillary flow.

4. Dissolved ions and larger molecules are retained on the high-pressure side due to size exclusion and electrostatic repulsion.

5. Collect purified water (permeate) on the low-pressure side.

Example Material for RO Membrane

Cellulose acetate (a polymer derived from cellulose) is a commonly used material for making semipermeable membranes in reverse osmosis.

Why it works: …

Here is a breakdown of the common mistakes students make on this specific question, along with how to avoid them.

The Core Concept

First, let's clarify the concept. Reverse Osmosis (RO) is a water purification process. It uses pressure to force water molecules through a semipermeable membrane against their natural osmotic pressure. The membrane acts like a very fine filter, allowing water to pass through but blocking dissolved salts, bacteria, and other impurities.

The question asks for a material used to make this membrane.

Common Mistake #1: Naming the Process or Apparatus Instead of the Material

• The Mistake: Students often write answers like "Reverse Osmosis" or "RO plant" or "a semipermeable membrane." These are not materials.
• Why it happens: The student confuses the concept (RO) or the device (the membrane itself) with the substance it is made from.
• How to Avoid It: Read the question carefully. It explicitly asks for a "material". Think of materials like plastic, metal, glass, or rubber. The membrane is a thin sheet made of a specific polymer (plastic-like material).

Common Mistake #2: Giving a Vague or Incorrect Material

• The Mistake: Writing answers like "plastic," "polymer," or "cellulose." While technically not entirely wrong, they are too vague for an exam. "Cellulose" is also incorrect for modern RO membranes.

• Why it happens: The student remembers the membrane is a polymer but doesn't recall the specific, exam-accurate name.

• How to Avoid It: Memorize the two most common and exam-accurate examples:

  1. Cellulose Acetate (an older, but still valid, material).
  2. Polyamide (the most common material in modern RO membranes, often in a thin-film composite).

  Best answer for exams: Polyamide or Cellulose Acetate.

Common Mistake #3: Confusing RO Membranes with Dialysis Membranes

• The Mistake: Writing "cellophane" or "parchment paper." These are used in simple laboratory experiments for dialysis (separating colloids from crystalloids), not for industrial reverse osmosis.
• Why it happens: Students mix up the two similar concepts of "semipermeable membrane" and "dialysis."
• How to Avoid It: Create a clear mental distinction: …