Reaction of Zinc with Dilute Sulphuric Acid (Chemical Reactions) Video Lecture | Science Class 9

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.