A student investigates an unfamiliar white solid, X
CHEMISTRY · Multi-part · CBSE · Chemistry · Acids, Bases and Salts · Acids, Bases and Salts
Problem
A student investigates an unfamiliar white solid, X. Three separate tests are performed.
Test I – A few crystals of X are gently heated in a dry test tube. The solid gradually disappears, forming a colorless, odorless gas that turns cobalt(II) chloride test paper from blue to pink. A white solid residue remains.
Test II – The original solid X is dissolved in water at 25 °C. The temperature of the solution decreases by 9 °C.
Test III – The solution from Test II is left in an open dish. Over several days, colorless crystals grow that are visually identical to the original solid X.
Assume the gas produced in Test I is water vapor.
(1) Using the information from the three tests, propose a chemical formula for solid X that is consistent with all observations.
(2) Determine whether each of the following should be classified as a physical change or a chemical change, and justify your choice with particulate-level evidence.
(i) The disappearance of solid X and formation of water vapor during Test I.
(ii) The temperature change observed during dissolution in Test II.
(iii) The growth of crystals during Test III.
(3) Based on your classifications in part (b), evaluate whether the overall sequence of Tests I–III constitutes a net chemical transformation of the substance originally present. Support your answer with evidence from the experiment.