CBSE Class 10 Science 2019 Outside Delhi Set 1

Question : 22 of 27

Marks: +1, -0

A teacher provided acetic acid, water, lemon juice, aqueous solution of sodium hydrogen carbonate and sodium hydroxide to students in the school laboratory to determine the

{\pH}

values of these substances using

{\pH}

papers. One of the students reported the

{\pH}

values of the given substances as

3,12,4,8

and 14 respectively. Which one of these values is not correct ? Write its correct value stating the reason.
OR
What would a student report nearly after 30 minutes of placing duly cleaned strips of aluminium, copper, iron and zinc in freshly prepared iron sulphate solution taken in four beakers?

Solution:

The value of

{\pH}

for water is not correct. The correct value of

{\pH}

of water is 7 because it has almost equal concentration of

{\H}^{+}

and

{\OH}^{-}

due to which it is neutral.
OR
Aluminium displaces the iron from iron sulphate and the colour of two solution changes from green to colourless.
No change takes place when copper strip is dipped in iron sulphate solution.
No change will be observed when iron strips are dipped in iron sulphate solution.
The colour of the solution changes from green to colourless when zinc is added to iron sulphate solution.

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