A student took HCl in a conical flask and placed it on a white tile with a cross mark. When seen through the flask the mark was visible. On adding some sodium thiosulphate in the flask the cross mark disappeared when seen through the flask. The student observed some reaction as the products formed were different than the reactants. Both the reactants were transparent.i) Name the type of reaction seen in the set up.
a) Redox
b) Precipitation
c) Displacement
d) Double displacement

ii)Why did the cross mark disappear?
a) The insoluble precipitate that is formed in the flask does not allow the light to pass through it to see the cross mark.
b) The resultant solution is dark coloured which does not allow the light to pass through it.
c) The resultant solution has high viscosity due to which it refracts the light to other angle.
d) The solution so formed has ability to absorb black colour hence black coloured cross is not seen.

iii)Give the chemical formula of the gas formed in the flask.
a) SO2
b) SO3
c) O2
d) Cl2i

v) Name the resultant soluble substance formed in the flask.
a) Sodium sulphide
b) Sodium sulphate
c) Sodium sulphite
d) Sulphur

v) Which of the following reaction is of similar type as observed in the given set up.
a) Na2SO4 + BaCl2 → BaSO4 + 2NaCl
b) Fe + CuSO4 → FeSO4 + Cu
c) 2FeSO4 + Heat → Fe2O3 + SO2 + SO3
d) CaO + H2O → Ca(OH)2