Q. 294.4( 9 Votes )
State reasons for the following observations:
(a) The shining surface of some metals becomes dull when exposed to air for a long time.
(b) Zinc fails to evolve hydrogen gas on reacting with dilute nitric acid.
(c) metal sulphides occur mainly in rocks but metal halides occur mostly in sea and lake waters.
(a) Metals react with the things like moisture, oxygen, carbon dioxide to form a layer of substance like rust in iron which hides their shiny surface. This is what we call corrosion.
(b) Nitric acid is a strong oxidising agent and Zinc is not a very reactive metal according to the reactivity series. Hence the nitric acid oxidises the hydrogen gas which is produced may be in the first step if reaction between acid or metal had taken place. So the hydrogen gas does not evolve.
(c) Metal sulphides are insoluble in water and halides are soluble so they get washed away in spite of getting deposited over the crust.
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i. What is observed when a solution of potassium iodide is added to a solution of lead nitrate taken in a test tube?
ii. What type of reaction is this?
iii. Write a balanced chemical equation to represent the above reaction.
A brown substance ‘X’ on heating in air forms a substance ‘Y’. When hydrogen gas is passed over heated ‘Y’, it again changes back into ‘X’.
i. Name the substance ‘X’ and ‘Y’
ii. Name the type of chemical reactions occurring during both the changes.
iii. Write the chemical equations of the reactions.Xam idea - Science