On adding dilute hydrochloric acid to copper oxide powder,the solution formed is blue-green. Predict the new compound formed which imparts a blue-green colour to the solution.

Electrolysis of water is a decomposition reaction. The mole ratio of hydrogen and oxygen gases liberated during electrolysis of water is

You could have noted that when copper powder is heated in a china dish, the surface of copper powder becomes coated with black coloured substance.
$(i)$ Why has this black coloured substance formed?
$(ii)$ What is this black substance?
$(iii)$ Write the chemical equation of the reaction that takes place.

Barium chloride on reacting with ammonium sulphate forms barium sulphate and ammonium chloride. Which of the following correctly represents the type of the reaction involved?
$(i)$ Displacement reaction
$(ii)$ Precipitation reaction
$(iii)$ Combination reaction
$(iv)$ Double displacement reaction

Balance the following chemical equations and identify the type of chemical reaction.
$(a)$ $TiCl_4(l) + Mg(s) \to Ti(s) + MgCl_2(s)$
$(b)$ $CaO(s) + SiO_2(s) \to CaSiO_3(s)$
$(c)$ $H_2O_2(l) \xrightarrow{UV} H_2O(l) + O_2(g)$

Write two observations for the following chemical reaction:
$(a)$ Potassium iodide solution is added to lead nitrate solution.