Which of the following associated with isotonic solutions is not correct

Options

A. They will have the same osmotic pressure
B. They have the same weight concentrations
C. Osmosis does not take place when the two solutions are separated by a semipermeable membrane
D. They will have the same vapour pressure

Answer

To determine which statement associated with isotonic solutions is not correct, let's analyze the concept of isotonic solutions step by step.

### Step 1: Understand the Definition of Isotonic Solutions
Isotonic solutions are solutions that have the same osmotic pressure. This means that when two solutions are separated by a semi-permeable membrane, there will be no net movement of solvent molecules between them.

**Hint:** Remember, isotonic solutions have equal osmotic pressure, leading to no net movement of solvent.

### Step 2: Explore Osmotic Pressure
Osmotic pressure (π) can be calculated using the formula:
\[ \pi = C \cdot R \cdot T \]
where:
- \( C \) = concentration of the solution
- \( R \) = universal gas constant
- \( T \) = temperature in Kelvin

For two solutions to be isotonic, their osmotic pressures must be equal, which can happen if their concentrations are equal at the same temperature.

**Hint:** The formula for osmotic pressure is crucial in understanding why two solutions can be isotonic.

### Step 3: Conditions for Osmosis
Osmosis occurs when there is a difference in concentration across a semi-permeable membrane. The solvent moves from a region of lower solute concentration to a region of higher solute concentration until equilibrium is reached.

**Hint:** Osmosis requires a semi-permeable membrane to occur.

### Step 4: Analyze the Statements
Now, let's analyze the statements given in the question. We need to identify which statement is incorrect regarding isotonic solutions.

1. Isotonic solutions have the same osmotic pressure.
2. Isotonic solutions have the same weight concentration.
3. Osmosis occurs when two solutions are isotonic.
4. Isotonic solutions do not cause any net movement of solvent.

Among these statements, the third one is incorrect. Osmosis does not occur between isotonic solutions because there is no concentration gradient to drive the movement of solvent.

**Hint:** Look for the statement that contradicts the definition of osmosis.

### Conclusion
The statement that is not correct regarding isotonic solutions is that "osmosis occurs when two solutions are isotonic." This is false because isotonic solutions do not promote osmosis due to equal osmotic pressures.

**Final Answer:** The third option is not correct.