In everyday chemistry, which one of the following substances is an example of a natural colloid rather than a true solution: cane sugar solution, blood, sodium chloride solution or urea solution?

Introduction / Context:
This question examines your understanding of colloidal systems versus true solutions in chemistry. Colloids are mixtures where the dispersed particles are intermediate in size between those in true solutions and suspensions. Many important biological and industrial materials are colloids. Knowing which common substances are colloidal helps you connect classroom theory with real life examples.

Given Data / Assumptions:
- Four common mixtures are listed: cane sugar solution, blood, sodium chloride solution and urea solution.
- We assume all solutions mentioned are prepared in water.
- You are asked to identify which one behaves as a natural colloid rather than as a true solution of small particles.

Concept / Approach:
In a true solution, solute particles are extremely small (typically less than 1 nanometre), completely dissolved and cannot be seen even under a powerful microscope. They do not scatter a beam of light significantly. In a colloid, dispersed particles are larger (about 1 to 1000 nanometres), do not settle under gravity and can scatter light (Tyndall effect). Blood is a classic natural colloid, consisting of plasma as the dispersion medium and cells and proteins as dispersed particles. In contrast, cane sugar, sodium chloride and urea dissolved in water form true solutions with molecular or ionic size particles.

Step-by-Step Solution:
Step 1: Consider cane sugar solution. Sugar molecules dissolve completely in water and form a homogeneous true solution with very small particles. Step 2: Consider sodium chloride solution. Sodium and chloride ions are fully dissolved and form an ionic true solution where particles are at the ionic scale. Step 3: Consider urea solution. Urea molecules also dissolve completely, giving another example of a true molecular solution. Step 4: Consider blood. It contains red blood cells, white blood cells, platelets and large proteins suspended in plasma. These dispersed particles are much larger than ordinary solute molecules and give blood colloidal properties. Step 5: Conclude that blood is the natural colloid among the four substances.

Verification / Alternative check:
You can think about practical observations. When a beam of light passes through a strong sugar solution, you do not see the beam path clearly, indicating a true solution. However, when light passes through milk or blood, the path of the beam becomes visible due to scattering by colloidal particles. Blood also shows settling behaviour only under centrifugation rather than under normal gravity. These are characteristic features of colloidal systems, confirming that blood is a colloid.

Why Other Options Are Wrong:
Cane sugar solution: sugar molecules are small and fully dissolved, so this is a true solution, not a colloid.
Sodium chloride solution: the dissolved ions form a typical ionic solution, again not colloidal.
Urea solution: urea forms a homogeneous molecular solution with no colloidal particle size range.

Common Pitfalls:
Learners sometimes classify any cloudy liquid as a colloid and any clear liquid as a true solution without considering particle size and behaviour. Although blood appears opaque and red, the reason lies in suspended cells rather than simple dissolution. Another pitfall is to think only of milk as a natural colloid and to forget that blood is an equally important biological colloidal system. Remember that colloids can be gas liquid, liquid liquid or solid liquid systems and that many body fluids fall into this category.

Final Answer:
The natural colloid among the options is blood.