Question 1: Explain why halogens are useful in forensic investigations for detecting traces of biological samples or substances.

Answer: Halogens are useful because they can form distinctive compounds with biological samples or substances, allowing for their detection and identification due to their reactive and distinctive colour changes or spectroscopic signatures.

Question 2: Describe the general trend in reactivity of halogens as you move down Group 7.

Answer: Reactivity decreases as you move down Group 7 because the atomic size increases, making it harder for the halogen atoms to attract electrons and react.

Question 1: Define the term 'halogen' and list three elements that belong to Group 7.

Answer: Halogens are non-metal elements in Group 7 of the periodic table; examples include fluorine, chlorine, bromine.

Question 2: Explain how halogens react with alkali metals and what type of compounds are formed.

Answer: Halogens react with alkali metals to form ionic halide salts, such as sodium chloride, through a reaction where the halogen gains an electron and the metal loses one.

Question 1: A forensic scientist reacts a suspected sample containing halide ions with silver nitrate solution. If 0.025 mol of silver nitrate is used to precipitate the halide, calculate the number of moles of halide ions in the sample.

Answer: 0.025 mol

Question 2: If the molar mass of silver chloride (AgCl) is 143.32 g/mol, and the precipitate weighs 10 g, determine the amount of chloride ions present in the sample.

Answer: Approximately 0.0697 mol

Question 1: Design a simple forensic test to identify the presence of bromide ions in a sample. Include the reagents used, the expected observation, and safety precautions.

Answer: Add dilute nitric acid to remove impurities, then add silver nitrate solution; a cream precipitate indicates bromide ions. Safety precautions include wearing gloves, eye protection, and working in a well-ventilated area to handle chemicals safely.

Question 1: A forensic sample produces a yellow precipitate when tested with silver nitrate solution. Which halogen ion is most likely present? Explain your reasoning.

Answer: The most likely halogen ion is iodide because silver iodide forms a yellow precipitate, distinguishing it from other halides.

Question 2: In a forensic investigation, a sample was found to contain a halogen that reacts less vigorously with halide ions in solution. Which halogen is this, and why?

Answer: The halogen is astatine, as it is the least reactive and reacts less vigorously compared to fluorine, chlorine, bromine, and iodine.

Question 1: Describe how the unique properties of halogens, particularly their ability to form various halide ions, are utilised in forensic science to detect and identify trace evidence. Include specific examples of how these properties are exploited in forensic tests and the importance of safety precautions.

Answer: Halogens can form characteristic halide ions that react with specific reagents, such as silver nitrate, to produce distinct precipitates (e.g., chloride = white, bromide = cream, iodide = yellow). These reactions allow forensic scientists to identify trace amounts of halogen-containing compounds in samples like biological fluids or environmental residues. The reactivity and colour change of these precipitates facilitate quick identification. Safety precautions, including wearing protective gear and working in ventilated areas, are essential due to the toxicity of halogen reagents and precipitates.