Question

Krishna and Gargi went for a summer camp in a remote village. There they visited many ancient temples. One day they noticed that a lady is cleaning old vessels that became dull in color. They had a discussion with the lady and Gargi noticed that the lady used tamarind solutions to clean
the tarnished copper vessels. while moving to store room for old utensils they noticed many rusted iron vessels and utensils dumped here and there. Then they had a mutual discussion over a related article, they read in a science magazine.
Corrosion resistant coatings protect metal components against degradation due to moisture, salt spray, oxidation or exposure to a variety of environmental or industrial chemicals. Anti-corrosion coatings allow for added protection of metal surfaces and acts as barrier to inhibit the contact between chemical compounds or corrosive materials. In addition to corrosion prevention many of the coatings also provide a bonus of abrasion resistance, non –stick performance and chemical
protection.

1.Gargi noticed green patches over the tarnished copper vessels that are kept in store room. What are these green patches?
A)Mixture of copper hydroxide and copper carbonate.
b)Mixture of copper sulphate and copper carbonate.
c)Copper hydroxide
d)None of the above.

2.It is advisable to use sour substances in cleaning tarnished vessels. What is your answer. Explain your answer?

3.In one of the store rooms of the temple, Krishna noticed many aluminum vessels kept aside that are not polished or painted. Why aluminum objects do not require painting or polishing even though it is very reactive?

4.Bath taps, car parts, kitchen gas burners, bicycle handle bars etc... remain shiny, rust free and scratch resistant for long time. Which method of metal protection is adopted here?

Answer 1

Answer:

1. A

2. tarnish on metals is the metal salt with oxygen, carbondioxide

and other gases. on applying sour substance(acidic substance) the metal atom separates from the compound and the tarnish layer gets removed by neutralisation as metal compounds are basic in nature.

3. Aluminum forms a protective oxide which prevents furthe corrosion.

4. electroplating by chromium

Answer 2

The questions to the answers are written below:

1. The green patches noticed by Gargi over the tarnished copper vessels that are kept in store room are A)Mixture of copper hydroxide and copper carbonate.

When metals are exposed to the atmosphere, oxides of the metal are formed as oxygen reacts with metal in presence of moisture.

Rusting of copper can be known from the following equation:

$2Cu+H_{2}O+CO_{2}+O_{2} \rightarrow Cu(OH)_{2}+CuCO_{3}$

2. It is advisable to use sour substances in cleaning tarnished vessels because the rust in the tarnished vessels is alkaline. Sour substances are mostly acidic in nature and thus, a neutralisation reaction will take place. The carbonates will react with acid forming a salt, which in turn, cleans the vessels.

3. Aluminum objects do not require painting or polishing even though it is very reactive as the upper layer of aluminium rusts and forms aluminium oxide. This prevents further rusting of aluminium as the contact with the air of the inner metal is cut off.

4. Copper electroplating method of metal protection is adopted in bath taps, car parts, kitchen gas burners, bicycle handle bars etc so that they remain shiny, rust free and scratch resistant for long time.