Practical work 1. Experimental problems on “Halogens”
An experimental problem asks you to find the composition of an unknown substance by chemical reactions, or to prepare a given substance. First a plan is made: which ion or group are we detecting, which reagent gives a clear result, and what will we observe (colour of a precipitate, gas bubbles, colour change). The chloride ion gives a white precipitate with AgNO₃, bromide a pale yellow one, iodide a yellow one; carbonate gives CO₂ bubbles with acid: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑. For H⁺ in an acid the evidence is litmus turning red or hydrogen released with Zn. Zinc chloride can be made in several ways: Zn + 2HCl, ZnO + 2HCl, Zn(OH)₂ + 2HCl, ZnCO₃ + 2HCl, Zn + CuCl₂. The practical work is done only under the teacher’s supervision: wear goggles and gloves, take small amounts of solution, never smell or taste substances, wash hands at the end and pour waste into the vessel the teacher shows. In this lesson you prepare the experiment as a plan, a table and equations.
“Lab notebook”: make a table for 6 problems: columns — aim, reagent, expected observation, equation, safety note. For each write “What I will not smell or taste, and what protection I wear”. The teacher demonstrates, you observe and record.