Q.When kept in water, raisin swells in size. Name and explain the phenomenon involved with the help of a diagram. Give three applications of the phenomenon.

The key idea is osmosis — the net movement of solvent molecules from a region of lower solute concentration to higher solute concentration through a semipermeable membrane.

Reasoning in steps:

1. A raisin is a dried grape with a high concentration of sugars and salts inside its cells. When placed in water, the water outside has a much lower solute concentration (hypotonic relative to the raisin's interior).
2. The raisin's outer skin acts as a semipermeable membrane — it allows water molecules to pass through but restricts the movement of larger solute molecules.
3. Water moves by osmosis from the surrounding water (higher water potential) into the raisin cells (lower water potential). This influx of water causes the cells to swell, making the raisin plump and larger in size.

Diagram (conceptual):

A simple sketch shows a raisin (drawn as a wrinkled oval) immersed in a beaker of water. Arrows point from the water into the raisin, labelled "Water molecules move in by osmosis." The raisin is shown becoming larger and smoother over time.

Three applications of osmosis: …

Concept and Intuition

Why does a dry raisin plump up when soaked in water? The key lies in osmosis — the net movement of solvent (water) from a region of lower solute concentration to a region of higher solute concentration through a semipermeable membrane.

A raisin is essentially a dehydrated grape. Its cells contain concentrated sugars, salts, and other solutes (the cell sap). The cell membrane acts as a semipermeable barrier: it lets water molecules pass through freely but blocks the larger solute molecules. When you place the raisin in pure water (which has almost zero solutes), the water outside is much more dilute than the sap inside. Nature tries to equalise concentrations, so water rushes into the cells. This inward movement of water is called endosmosis (from Greek endon = within + ōsmos = push). The cells swell, and the raisin as a whole becomes turgid and increases in size.

Step-by-Step Explanation

1. Identify the two solutions separated by the membrane.

   • Inside the raisin cells: A concentrated solution of sugars, organic acids, and minerals (cell sap).
   • Outside the raisin: Pure water (or very dilute water).
   • The membrane: The cell membrane (plasma membrane) of each raisin cell, which is selectively permeable.

2. Compare their water potentials.

   Water potential ($\Psi$) is a measure of the tendency of water to move. Pure water has the highest water potential (taken as 0 at standard conditions). A solution has a lower (more negative) water potential because solutes "tie up" water molecules.

   • $\Psi_{\text{outside}} \approx 0$ (pure water)
   • $\Psi_{\text{inside}} < 0$ (concentrated sap) Water always moves from a region of higher water potential to lower water potential — i.e., from outside to inside.

3. Describe the net movement of water.

   Water molecules diffuse through the semipermeable membrane into the cells. This is endosmosis. The cells swell as they take up water, exerting pressure against the cell wall (turgor pressure). The raisin becomes plump and firm.

4. Contrast with exosmosis.

   If you placed the raisin in a concentrated sugar solution (higher solute concentration outside), water would leave the cells — the raisin would shrink further. That outward movement is exosmosis.

Diagram

Below is a simple schematic of the process. (Imagine the raisin cell as a box with a dotted membrane.) …

Concept: Osmosis

Method: Demonstration of Endosmosis in a Raisin

Name of the method: Raisin Osmosis Experiment (or Endosmosis Demonstration)

Steps:

1. Take a dry raisin — note its small, shrivelled size and hard texture.
2. Place it in a beaker filled with water — ensure the raisin is fully submerged.
3. Observe after 2–3 hours — the raisin swells up, becomes soft, plump, and translucent.
4. Compare — the final size is much larger than the initial size.

Explanation of the Phenomenon

This is osmosis — the net movement of water molecules from a region of higher water concentration (pure water) to a region of lower water concentration (inside the raisin) through a semipermeable membrane (the raisin’s skin).

• The raisin’s cells contain dissolved sugars and salts, making the cell sap concentrated (low water concentration).
• Water from the beaker (high water concentration) moves into the raisin cells.
• This inward movement of water is called endosmosis.
• The cells swell, causing the raisin to increase in size.

Diagram description: picture a beaker of pure water with a shrivelled raisin submerged in it. The raisin's skin acts as the semipermeable membrane. Draw a single arrow crossing the skin, pointing from the surrounding water inward into the raisin -- this represents the net movement of water molecules by endosmosis, from the region of higher water concentration (outside) to the region of lower water concentration (inside the raisin's cell sap).

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Here are the common mistakes students make on this question about osmosis (the phenomenon where a raisin swells in water), along with how to avoid each.

Mistake 1: Confusing Osmosis with Diffusion or Imbibition

• The Error: Students often write "diffusion" or "imbibition" as the phenomenon. Diffusion is the movement of any substance from high to low concentration, while imbibition is absorption by solids (like seeds swelling). The raisin swells due to water moving through a semi-permeable membrane (the raisin's skin).
• How to Avoid: Remember the key condition: a selectively permeable membrane must separate two solutions of different concentrations. The raisin skin acts as this membrane. If the membrane isn't mentioned, it's not osmosis.
• Correct Answer: Osmosis — the movement of water molecules from a region of higher water concentration (pure water) to a region of lower water concentration (inside the raisin) through a semi-permeable membrane.

Mistake 2: Forgetting to Specify the Type of Osmosis

• The Error: Students just say "osmosis" without naming it as endosmosis. The examiner often expects this specific term.
• How to Avoid: Link the direction of water movement to the cell's state.
  • Endosmosis: Water enters the cell (cell swells).
  • Exosmosis: Water leaves the cell (cell shrinks).
• Correct Answer: The phenomenon is endosmosis (a type of osmosis where water enters the cell).

Mistake 3: Drawing a Poor or Incomplete Diagram

• The Error: Drawing a raisin with arrows pointing inward but no labels for water molecules, semi-permeable membrane, or concentration gradient.
• How to Avoid: Your diagram must clearly show:
  1. A circle (the raisin cell) with a dotted or thick line labeled "Semi-permeable membrane (raisin skin)".
  2. Small dots or "H₂O" outside the circle.
  3. Arrows pointing inward from outside to inside.
  4. Labels: "Pure water (High water concentration)" outside, "Concentrated cell sap (Low water concentration)" inside.
• Correct Diagram (describe in words): A simple cell with a membrane. Outside: many water molecules. Inside: fewer water molecules + solute particles. Arrows show water moving in.

Mistake 4: Giving Incorrect or Vague Applications

• The Error: Listing "cooking dal" or "soaking almonds" without explaining why osmosis is involved. Or giving applications that are actually about diffusion (e.g., "smell of perfume").
• How to Avoid: Each application must involve water moving through a membrane due to a concentration difference.
• Correct Applications (with explanation):
  • Preservation of meat/fish (salting): Salt outside draws water out of microbial cells (exosmosis), killing them.
  • Movement of water in plants: Water enters root hairs from soil (endosmosis) due to higher water concentration in soil. …