Reaction of Zinc with Dilute Sulphuric Acid Everything that has mass and volume is called matter. All matter, regardless of state, undergoes physical and chemical changes. A physical change occurs when a substance changes its state, but does not change its chemical composition. For example, tearing a paper, melting of cheese, and cutting a piece of wood into smaller pieces are physical changes. A chemical change occurs when a substance changes into something new. For example, formation of a precipitate, evolution of a gas, and corrosion are chemical changes. In this experiment, our aim is to carry out the reaction of zinc with dilute sulphuric acid and classify it as a physical or chemical change. Materials Required: Boiling tube, dilute sulphuric acid, NaOH, zinc dust, spatula, dropper, Procedure: Take a boiling tube and add a few drops of dilute sulphuric acid using a dropper and place it in a test tube rack. Then add some zinc dust into the boiling tube using a spatula. In this reaction, zinc reacts with dilute sulphuric acid to form an aqueous solution of zinc sulphate and hydrogen gas evolves. i.e. Zn + H2SO4 -> ZnSO4 + H2 Here the zinc sulphate solution formed in the reaction is entirely different in chemical composition and chemical properties from the reactants, zinc and dilute sulphuric acid. Hence it is a chemical change. Now take a red litmus paper and hold it over the mouth of the boiling tube. There is no change in the colour of the red litmus paper. Repeat this same step with blue litmus paper.
In this case also, no change is observed. From this, we can infer that hydrogen gas evolved is neither acidic nor basic in nature. We can do two tests to confirm the presence of zinc sulphate solution in this reaction. In the first test, add a few drops of NaOH solution into the boiling tube containing zinc sulphate using a dropper. On adding the NaOH, a white gelatinous precipitate of zinc hydroxide is formed. i.e. ZnSO4 + 2 NaOH -> Zn(OH) 2 + Na2SO4 Again add excess of NaOH into this white precipitate and shake the boiling tube well. We can see that the precipitate dissolves on adding excess of NaOH. In the second test, take a clean boiling tube and clamp it in a stand and add a small amount of ferrous sulphide into this boiling tube using a spatula. Close the mouth of the flask with a double bored cork and insert a thistle funnel and a bent tube into the two different holes in the rubber cork. Then take a boiling tube containing zinc sulphate solution and clamp it in another stand. Now add a few drops of dilute HCl to the boiling tube containing ferrous sulphide using the thistle funnel. We can see that hydrogen sulphide gas evolves in the boiling tube containing ferrous sulphide. i.e. FeS + 2 HCl -> FeCl2 + H2S This hydrogen sulphide gas passes through the bent tube and reacts with zinc sulphate solution to form a white precipitate of zinc sulphide. i.e. ZnSO4 + H2S -> ZnS +H2SO4 Precautions: Use the chemicals judiciously. Keep the mouth of the boiling tube away from your face and also from other classmates.
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Source: amritacreate
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