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Which of the following statements about colloids is/are INCORRECT? (A) Collodion is a 4% solution of nitrocellulose in a mixture of alcohol and an ester. (B) A milk-and-water mixture appears red when viewed by reflected light and blue when viewed by transmitted light. (C) Sols of starch, gum, clay, and methylene blue are all negatively charged colloidal sols. (D) When the ionic strength of a colloidal solution is increased, the thickness of the diffuse double layer decreases and the colloid gets precipitated.

1. (A) only
2. (B) only
3. (C) only
4. (D) only

Correct answer: (C) only

Solution

Statement A is correct (collodion composition). Statement B is debatable but commonly stated as blue in transmitted and reddish in reflected (for gold sols); for milk-water it is more complex. Statement C is clearly incorrect: starch, gum, and clay are negatively charged, but methylene blue is a cationic dye and its colloidal sol is positively charged. Statement D is correct — higher ionic strength compresses the double layer (Debye length decreases) causing coagulation.

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