Asked in PMC Practice Test 5Moderate

It consists of discrete molecules:

Correct answer: B. Dry ice

  • A. Sodium chloride
  • B. Dry ice
  • C. Copper
  • D. CaCl2

Explanation

'Discrete molecules' are units of a compound, the smallest possible i.e. molecules, atoms, etc., held together by chemical bonds; these are usually covalent. In the case of dry ice, the compound is CO2 and the molecules are basically CO2; one carbon and two oxygens.Dry ice is held together, in its solid form, by weaker forces such as Van Der Waals.

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About Ionic and Molecular Crystals

Ionic crystals contain oppositely charged ions held by electrostatic forces, whereas molecular crystals contain discrete molecules joined by weaker intermolecular forces. Their structures explain differences in melting point, hardness, volatility, solubility and electrical conductivity, especially between solid and molten states.

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