Among the following, the incorrect statement about colloids is
(a) They can scatter light
(b) They a

Q: Why do colloids have a high molar mass?

Colloidal particles are not usually single, simple molecules. They are often aggregates of many smaller molecules clumped together, or they are massive macromolecules like proteins and synthetic polymers. Because a single colloidal particle contains so much mass, its effective molar mass is very high.

Q: What exactly is the Tyndall effect?

The Tyndall effect is the scattering of light by particles in a colloid or in a very fine suspension. Because colloidal particles are roughly the same size as the wavelength of visible light, they scatter the light rays, making the path of the light beam visible (like headlights in fog).

Q: Why is the osmotic pressure of a colloid lower than a true solution?

Osmotic pressure is a colligative property, meaning it depends only on the number of particles. If you take 10 grams of a substance to make a true solution, you might have a billion individual molecules. If you take 10 grams to make a colloid, those molecules clump together into groups of, say, 1000. Now you only have a million particles. Fewer particles mean a lower osmotic pressure.

Q: Are colloids considered homogeneous or heterogeneous?

Colloids are considered heterogeneous mixtures. Even though they might look uniform and clear to the naked eye, under a powerful microscope, you can distinguish the dispersed phase particles from the dispersion medium.

Q: What is the exact size range for colloidal particles?

The generally accepted size range for the diameter of colloidal particles is between $1 \text{ nm}$ and $1000 \text{ nm}$ ($10^{-9} \text{ m}$ to $10^{-6} \text{ m}$).

Question

Among the following, the incorrect statement about colloids is

Options:

(A) They can scatter light

(B) They are larger than small molecules and have high molar mass

(C) The osmotic pressure of a colloidal solution is of higher order than the true solution at the same concentration

(D) The range of diameters of colloidal particles is between 1 and 1000 nm

Sigma Percentile · Accepted Answer

### The Nature of Colloids

To identify the incorrect statement, we first need to understand what makes a colloidal solution unique. Colloids are heterogeneous mixtures where the dispersed phase particles are larger than those in true solutions but smaller than those in suspensions. Specifically, their diameters range from $1 	ext{ nm}$ to $1000 	ext{ nm}$.

Because of this specific size range, colloidal particles are large enough to scatter a beam of visible light passing through them. This beautiful phenomenon is known as the **Tyndall effect**. Furthermore, colloidal particles are often aggregates of hundreds or thousands of smaller molecules, or they might be giant macromolecules themselves (like proteins or polymers). Consequently, they possess a very high molar mass. This confirms that statements (a), (b), and (d) are perfectly accurate descriptions of colloids.

### The Secret of Colligative Properties

Now, let's investigate statement (c), which discusses osmotic pressure. Osmotic pressure is a **colligative property**. The defining characteristic of any colligative property—whether it's relative lowering of vapor pressure, elevation in boiling point, depression in freezing point, or osmotic pressure—is that it depends *exclusively* on the **number of solute particles** present in a given volume of the solution, regardless of their chemical identity or size.

### The Final Verdict

Imagine we have a true solution and a colloidal solution, both prepared with the same mass of solute per liter (same concentration). In the true solution, the solute exists as individual, tiny molecules or ions. However, in the colloidal solution, these individual molecules aggregate to form much larger colloidal particles.

Because many small molecules clump together to form a single colloidal particle, the **total number of particles** in the colloidal solution is significantly less than the number of particles in the true solution. Since colligative properties are directly proportional to the number of particles, the osmotic pressure (and all other colligative properties) of a colloidal solution will be of a **lower order** compared to a true solution at the same concentration.

Statement (c) claims the exact opposite—that the osmotic pressure is of a higher order. Therefore, statement (c) is the incorrect statement we were looking for.

Animated Solution for Chemistry - Surface Chemistry: Among the following, the incorrect statement about colloids is

Select Answer:

Visualized Solution

$General Properties of Colloids$

$Colligative Properties$

$Comparing Osmotic Pressure$

The Sigma Insight: Colloids, Micelles and and Emulsions

The Nature of Colloids

The Secret of Colligative Properties

The Final Verdict

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