Which of the following describes a colloid?

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The correct answer is Option C: Particles scatter a beam of light. A colloid is a heterogeneous mixture where particle sizes range between $1 	ext{ nm}$ and $1000 	ext{ nm}$. Because these particles are large enough to reflect or scatter light, colloids exhibit the Tyndall effect.

Step-by-Step Solution & Concepts:
A colloid is a heterogeneous mixture where particle sizes range between $1 	ext{ nm}$ and $1000 	ext{ nm}$. Because these particles are large enough to reflect or scatter light, colloids exhibit the Tyndall effect.

Working Principle: The Tyndall effect occurs when light waves hit suspended colloidal particles whose size is comparable to the wavelength of the light. This causes the light to scatter in various directions, making the path of the beam visible within the medium.

Key Formulas: $1 	ext{ nm}

Key Points: Colloids appear homogeneous but are heterogeneous at the microscopic level.; Colloidal particles do not settle under the influence of gravity.; Colloidal particles can pass through ordinary filter paper but are retained by ultra-filters.; The Tyndall effect is a characteristic property used to distinguish colloids from true solutions.

Advantages: Used in medicine for targeted drug delivery; Used in water purification (coagulation)

Disadvantages: Colloids can undergo coagulation or flocculation when electrolytes are added; They are sensitive to temperature and concentration changes

Applications: Milk and blood; Fog, smoke, and clouds

Additional Info: True solutions are transparent; suspensions settle over time; colloids are stable and translucent or opaque.. Option A describes a suspension, Option B describes both solutions and colloids (making it ambiguous, but less specific than C), and Option D is false as many colloids are opaque.

Answer: C is correct — Particles in a colloid are large enough to scatter a beam of light passing through the mixture, a phenomenon known as the Tyndall effect.

Exam Tip: In exams, always look for the most specific characteristic. While colloids can pass through filter paper, the Tyndall effect is their primary diagnostic feature.

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Option A: Particles settle down over time

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Option B: Particles pass through filter paper

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Option D: The mixture is always transparent

Feature	True Solution	Colloid
Particle Size	< 1 nm	1 - 1000 nm
Tyndall Effect	No	Yes

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