Question

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) Cl2

iv) 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​

Answer 1

Answer:

please notice that at Last correct answer is Na2S2O3+2HCl---------2NaCl+S+SO2+H2O

.From Dhanraj Dahariya 10 A

Answer 2

Answer:

i) A correct answer is option (b). The type of reaction seen in the set-up is a precipitation reaction.

ii) The correct option is (a). The cross mark disappears because the insoluble precipitate that is formed in the flask does not allow the light to pass through it to see the cross mark.

iii) The correct option is (a). The chemical formula of the gas formed is SO₂.

iv) The correct option is (d). The resultant insoluble substance formed in the flask is sulphur.

v) The reaction which is similar to the type as observed in the given set-up is an option (a) Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl

Explanation:

i) When HCl reacts with Sodium thiosulphate, following reaction takes place- Na₂S₂O₃ + 2HCl → 2NaCl + H₂O + SO₂↑+S↓(ppt)

When HCl reacts with sodium thiosulphate, there is the formation of a yellow-coloured precipitate. Hence, the reaction was considered a Precipitation type of reaction.

ii) The tile was initially marked with a cross mark however when the reaction occurs between HCl and Sodium thiosulphate, yellow-coloured precipitate gets formed which makes it difficult to see the cross mark present on the tile since the insoluble precipitate formation prevents the cross mark from being visible to light passing through it.

iii) From the following reaction, Na₂S₂O₃ + 2HCl → 2NaCl + H₂O + SO₂↑+S↓(ppt) it can be understood that the gas formed in the flask is Sodium dioxide which is chemically formulated as- SO₂.

iv) The resultant insoluble substance formed in the flask is sulphur. It appears as yellow coloured cloudy precipitate that remains insoluble.

v) The reaction- Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl is also a precipitation reaction since BaSO₄ is formed as a precipitate.

Thus, i) The type of reaction seen in the set-up is a precipitation reaction.

ii) The cross mark disappears because the insoluble precipitate that is formed in the flask does not allow the light to pass through it to see the cross mark.

iii) The chemical formula of the gas formed is SO₂.

iv) The resultant insoluble substance formed in the flask is sulphur.

v) The reaction which is similar to the type as observed in the given set-up is option Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl

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