Alloys, Polymers, Fertilisers and Everyday Chemistry Notes

Chemistry is all around us, in the steel of a bridge, the plastic of a bottle, the fertiliser in a field and the soap in the bathroom. For competitive exams such as SSC, railways, NDA, banking and State services, the questions on alloys, polymers, fertilisers, soaps and common chemical names are asked every year and are very scoring because they are fact-based. This note explains these topics in simple language with tables, examples and practice questions. For the basic ideas of chemistry used here, you can first read our notes on acids, bases and metals and carbon compounds and fuels.

Alloys

An alloy is a mixture of two or more metals, or of a metal and a non-metal, made by melting them together and cooling. Alloys are made to improve properties such as strength, hardness, resistance to rust and appearance. Pure gold, for example, is too soft for ornaments, so it is mixed with copper or silver.

Note that brass is copper and zinc, but bronze is copper and tin. This pair is the most asked. Also remember that the gold of jewellery is described in carats: 24 carat gold is pure, while 22 carat gold has about 91.6 per cent gold.

Polymers and Plastics

A polymer is a very large molecule made by joining many small molecules, called monomers, in a long chain. Polymers are of two types: natural (cellulose, starch, protein, natural rubber) and synthetic (nylon, polythene, PVC).

Plastics are of two kinds. Thermoplastics become soft on heating and can be moulded again, such as polythene and PVC. Thermosetting plastics are moulded only once and do not soften on re-heating, such as Bakelite and melamine. Plastics are not biodegradable, so they create a pollution problem, and this is why the government has banned many single-use plastic items. The 3 R principle of reduce, reuse and recycle is the answer.

Fertilisers, Pesticides and Soil Nutrients

Plants need nutrients, and the most important are nitrogen (N), phosphorus (P) and potassium (K), which are called the NPK nutrients. A fertiliser is a chemical substance added to the soil to supply these nutrients.

Biofertilisers such as Rhizobium, Azotobacter, blue-green algae (cyanobacteria) and mycorrhiza add nutrients naturally. Overuse of chemical fertilisers can pollute water and cause eutrophication, in which the excess of nutrients leads to heavy algae growth that uses up the oxygen of a lake. Pesticides kill the pests: insecticides kill insects, fungicides kill fungi, and herbicides kill weeds. DDT is a well-known insecticide that was banned in agriculture in India because it stays in the food chain.

Soaps and Detergents

Soap is the sodium or potassium salt of long-chain fatty acids, and it is made by the reaction of fat or oil with an alkali like sodium hydroxide. This process is called saponification. A soap molecule has a water-loving head and an oil-loving tail, and it removes dirt by forming small groups called micelles. Soap does not work well in hard water because it forms a white scum with calcium and magnesium salts. Detergents are synthetic cleaning agents that work in both soft and hard water. Some detergents are not biodegradable and harm water bodies.

Common Names of Chemical Substances

Glass, Cement and Ceramics

Glass is made by heating sand (silica) with sodium carbonate and limestone at a high temperature. Its colour changes with the added substance: cobalt gives blue glass, copper gives green or red, and chromium gives green. Borosilicate glass (Pyrex) does not crack easily on heating, so it is used in laboratory vessels and cooking dishes. Glass is called a supercooled liquid, an amorphous solid with no regular crystal structure. Cement is made from limestone and clay, and gypsum is added at the end to slow down its setting. Portland cement was invented by Joseph Aspdin. Ceramics, such as pottery, bricks and tiles, are made from clay by heating, and they are hard but brittle. Porcelain is a fine ceramic used for cups and electrical insulators.

Natural and Synthetic Fibres

Fibres are the thin threads from which cloth is made. Natural fibres come from plants and animals: cotton, jute and flax come from plants, and wool and silk come from animals. Synthetic fibres, such as nylon, polyester, rayon and acrylic, are made in factories from chemicals. Rayon is called artificial silk, and it is made from wood pulp, so it is semi-synthetic. Synthetic fibres are strong, dry quickly and do not shrink, but they melt on heating and can catch fire dangerously, so cotton clothes are better in a kitchen. Nylon was the first fully synthetic fibre.

Important Facts to Remember

• The most abundant metal in the earth's crust is aluminium, and the most abundant element is oxygen.
• Mercury is the only metal that is liquid at room temperature, and bromine is the only liquid non-metal.
• Rusting of iron needs both air (oxygen) and moisture, and it is prevented by painting, galvanisation (coating with zinc) and making stainless steel.
• Diamond and graphite are both pure carbon, but diamond is the hardest natural substance and graphite is a good conductor of electricity.
• The hardness of a metal can be increased by making an alloy, but the electrical conductivity usually decreases.
• Gold, platinum and silver are called noble metals because they do not react easily.

Common Mistakes

Students often write that bronze is made of copper and zinc, but that is brass. German silver is another trap, because it has no silver at all. In fertilisers, candidates think that all of them supply only nitrogen, while there are three nutrients, N, P and K. Some write that soap is a synthetic cleaning agent, but soap is made from natural fats and oils, and the synthetic cleaning agent is the detergent. Another error is to say that plaster of Paris is calcium sulphate and not hemihydrate; check the formula with one half molecule of water. Finally, do not confuse the baking soda (NaHCO3) with washing soda (Na2CO3).

Tips for the Exam

1. Learn the pair brass (copper + zinc) and bronze (copper + tin) first.
2. Remember that the highest nitrogen fertiliser is urea.
3. Connect each plastic with one product: Teflon with the non-stick pan, PVC with the pipe, Bakelite with the switch.
4. Write the formula along with the common name while revising.

Practice Questions With Answers

• Q1. Brass is an alloy of which metals? Answer: Copper and zinc.
• Q2. Which fertiliser has the highest nitrogen content? Answer: Urea.
• Q3. What is the process of making soap called? Answer: Saponification.
• Q4. What is the chemical name of baking soda? Answer: Sodium bicarbonate.
• Q5. Which plastic is used in non-stick cookware? Answer: Teflon.

Quick Revision

• Alloy = metal + metal or non-metal; steel is iron + carbon.
• NPK are the main plant nutrients; urea is rich in nitrogen.
• Soap is made by saponification; detergents work in hard water also.

Frequently Asked Questions (FAQ)

What is an alloy?

An alloy is a mixture of two or more metals, or of a metal and a non-metal, made to improve properties.

What is the difference between brass and bronze?

Brass is copper and zinc, while bronze is copper and tin.

What is steel made of?

Steel is an alloy of iron and a small amount of carbon.

What is a polymer?

A polymer is a large molecule made by joining many small molecules called monomers.

What is the difference between thermoplastic and thermosetting plastic?

Thermoplastics soften on heating and can be moulded again, while thermosetting plastics cannot be softened again.

Which fertiliser has the highest nitrogen content?

Urea has the highest nitrogen content, about 46 per cent.

What do NPK stand for in fertilisers?

They stand for nitrogen, phosphorus and potassium.

What is saponification?

It is the reaction of fat or oil with an alkali to make soap.

Why does soap not work well in hard water?

Because it forms a white scum with the calcium and magnesium salts of hard water.

What is the chemical name of baking soda?

Sodium bicarbonate, with the formula NaHCO3.

What is the formula of plaster of Paris?

It is CaSO4.1/2H2O, calcium sulphate hemihydrate.

What is eutrophication?

It is the heavy growth of algae in a water body due to excess nutrients, which reduces the oxygen in the water.