Question 1: Define a pure substance in the context of chemistry.

Answer: A pure substance consists of only one type of particle, either an element or a compound, with a fixed composition and properties.

Question 2: Explain why pure substances have specific melting and boiling points.

Answer: Because pure substances have a fixed and uniform composition, they melt and boil at specific temperatures where their particles overcome intermolecular forces uniformly.

Question 1: A sample of a pure compound contains 40.0 grams of carbon and 5.33 grams of hydrogen. Calculate the empirical formula of the compound.

Answer: CH4

Question 2: A pure sample of aluminium sulphate has a mass of 150 grams. The molar mass of aluminium sulphate is 342 g/mol. Calculate the number of moles in the sample.

Answer: 0.439 mol

Question 1: Describe a simple laboratory method to test the purity of a salt sample using melting point analysis.

Answer: Heat the salt gradually and record the temperature at which it melts; a pure salt will melt at a specific, sharp temperature, whereas impurities lower and broaden the melting range.

Question 2: Identify two safety precautions to observe when handling chemicals during purity tests.

Answer: Wear appropriate personal protective equipment (gloves, goggles) and work in a well-ventilated area to avoid inhaling fumes.

Question 1: A sample of a pure substance has a melting point of 150°C. When tested, a different sample melts over a range of 145°C to 150°C. What does this suggest about the second sample?

Answer: The second sample likely contains impurities, which cause the melting point to be lower and spread over a range instead of a sharp point.

Question 1: Explain how chromatography can be used to assess the purity of a substance. Include the principle behind the method in your answer. (6 marks)

Answer: Chromatography separates components of a mixture based on their different affinities for the stationary and mobile phases. A pure substance will produce a single, well-defined spot with a consistent Rf value, while impurities will result in multiple spots or irregular patterns, indicating contamination.

Question 1: Why is it important for pharmaceutical drugs to be of high purity, and what techniques are used to ensure this?

Answer: High purity in pharmaceuticals ensures safety and effectiveness. Techniques such as recrystallisation, chromatography, and spectroscopic analysis are used to confirm and improve purity.