Which of the following best describes benzenediazonium chloride?

Colourless crystals

The correct answer is option 3. Colourless crystals.

Benzenediazonium chloride (\(\text{C}_6\text{H}_5\text{N}_2\text{Cl}\)) is an important intermediate in organic chemistry, particularly in the synthesis of azo dyes and other aromatic compounds. Here's a detailed explanation of its properties:

Physical State and Appearance

Colourless Crystals: Benzenediazonium chloride is commonly found as colorless crystalline solids. These crystals are formed under specific conditions, typically in aqueous solutions at low temperatures (around 0°C to 5°C) to maintain their stability.

Stability and Reactivity

Temperature Sensitivity: The diazonium group (\(\text{N}_2^+\)) in benzenediazonium chloride is highly reactive and can decompose easily, especially at higher temperatures. This decomposition can release nitrogen gas and form phenyl radicals, which can lead to various side reactions.

OIly liquid: Due to its reactivity, benzenediazonium chloride is usually ps is incorrect because benzenediazonium chloride is not an oily liquid but rather a crystalline solid.

Yellow Solid: This is incorrect because the pure form of benzenediazonium chloride is colorless, though impurities might sometimes give it a slight color.

Colourless Crystals: This is correct. The pure substance is colorless and crystalline.

White Powder: While this might describe the appearance of the ground-up crystalline form, the most accurate description for the pure substance is colorless crystals.

Conclusion
Benzenediazonium chloride is best described as colourless crystals due to its appearance in pure form and its typical preparation and storage conditions. This description accurately reflects the substance's physical state and color, distinguishing it from other less accurate options.