CBSE Class 7 Science Advanced Chemistry Chemistry in Daily Life Notes Set 01

Chemistry in Daily Life

Why We Study Chemistry

Chemistry studies what substances are made of and how they behave, both physically and chemically. It also looks at how substances serve us. Many useful chemicals are first made in laboratories and then in factories. Chemists also improve natural materials by changing their composition. This chapter surveys metals, non-metals, alloys, some important compounds and mixtures, and their uses.

Metals and Their Uses

Metals are used widely because of properties such as lustre (shine), tensile strength, conductivity, malleability (can be beaten into sheets) and ductility (can be drawn into wires).

Table: Properties and uses of some metals

Noble Gases and Their Uses

The six noble gases are helium, neon, argon, krypton, xenon and radon. They are inert, which means they do not react easily, and this is why they are put to special uses. Helium is the second lightest element, so it fills weather-observation balloons. Neon glows in advertising sign boards. Argon is filled in electric bulbs because it stops the tungsten filament from oxidising, and krypton and xenon slow down the sublimation (loss) of the hot tungsten. Radon is radioactive and is used to treat cancer.

Example 2. Describe the process of galvanization. How is it useful?
Answer:
Coating iron articles with zinc is called galvanization. Hot iron sheets are dipped in molten zinc and then passed between hot, heavy rollers. On cooling, a zinc layer remains on the iron. This layer protects the iron from rusting, so galvanized iron is used for roofing sheets, tanks and rainwater drain pipes.

Non-metals and Their Uses

Table: Properties and uses of some non-metals

Example 3. (a) What is the composition of pencil lead?
(b) How is the hardness of a pencil related to its composition?
(c) How are crucibles made from graphite?
(d) Why is graphite preferred to ordinary lubricating oils?
(e) What is the role of graphite in a nuclear reactor?
Answer:
(a) Pencil lead is graphite mixed with clay.
(b) The more clay it contains, the harder the pencil.
(c) Graphite is mixed with fine clay, moulded to the required size and shape and then sintered (strongly heated).
(d) Graphite does not break down at high temperature, so it can lubricate parts that become very hot by friction, where ordinary oils would char.
(e) It acts as a moderator: it slows down the neutrons released when uranium or thorium atoms split.

Example 4. An element X is non-malleable and non-ductile. X is widely used to protect another element Y from corrosion. Identify X and Y, name the process and compare the reactivity of X and Y.
Answer:
X is zinc and Y is iron. The process is galvanization. Zinc is more reactive than iron.

Example 5. Explain the principle behind the use of chlorine to sterilize drinking water.
Answer:
Chlorine dissolves readily in water and forms hypochlorous acid. This acid breaks down to release nascent (freshly formed, very active) oxygen, which kills germs and bacteria. That is why chlorine makes water safe to drink.

Some Common Alloys and Their Uses

To give a metal the properties needed for a job, the molten metal is mixed evenly with other metals, and sometimes with a little non-metal, and then cooled to a solid. This solid is an alloy. Each alloy has special properties because of its composition. In general, alloying makes a metal stronger, more resistant to corrosion and so longer lasting.

Table: Important alloys and their uses

Some Useful Compounds

In a compound, the elements lose their own properties and the compound has a new set of properties. Several compounds are important in daily life.

Sodium chloride (common salt). Sea water is the main source. When sea water evaporates, an impure solid called crude salt is left, because it contains sand and clay. The crude salt is dissolved in water, the insoluble impurities are removed, and the solution is recrystallized (the hot saturated solution is cooled so that pure crystals separate). A little sodium iodide or potassium iodide is mixed with the salt to give the common table salt we eat daily. Salt also preserves fish and meat, and it is the starting material for baking soda, washing soda, hydrochloric acid, caustic soda and chlorine.

Hydrochloric acid. It is made industrially by heating common salt with concentrated sulphuric acid, or by dissolving hydrogen chloride gas in water. It is used to make silver chloride for photographic paper, as a laboratory reagent, to clean metal surfaces in industry and as a bathroom cleaner at home.

Blue vitriol (hydrated copper sulphate). It is made by dissolving copper(II) sulphide, oxide or carbonate in dilute sulphuric acid. Its solution is sprayed on citrus trees and grape vines to kill insects.

Limestone and marble. Both are chemically calcium carbonate. Limestone, formed from the remains of sea shells, is a raw material for cement and glass, and heating it gives lime (calcium oxide), used in mortar and white washing. Marble is used for floors, statues and monuments such as the Taj Mahal.

Zincite (zinc oxide). Strongly heating zinc carbonate gives zinc oxide, which is used to extract zinc and to make paints.

Gypsum and plaster of Paris. Gypsum is a natural mineral, calcium sulphate dihydrate, \(\ce{CaSO4.2H2O}\), and the book says it is used to treat soil. Heating gypsum for a long time drives off part of its water of crystallization and gives plaster of Paris, \(\ce{CaSO4.1/2H2O}\). Mixed with the right amount of water, the plaster can be moulded and then sets hard. So it is used to hold broken bones in place, to cast statues and toys, and to make decorative items, modern blackboard chalk and ceiling panels.

Compounds of Silicon

Sand (silicon dioxide) is found naturally, and purified sand is a raw material for glass and cement. Sodium silicate dissolves in water and is used to preserve eggs. Silicone is a polymer made from silicon. It withstands heat, is non-sticky, repels water and does not conduct electricity, so it is used in non-stick pans, waterproof clothes, wax polishes and electrical insulators. Ferrosilicon is an alloy of iron and silicon (a very hard steel). Silicon carbide is the hardest compound and is used in grinding wheels.

Fertilizers and Their Types

Fertilizers are compounds, or mixtures of compounds, added to soil to supply nutrients that plants need. By source they are of two kinds.

1. Natural fertilizer: manure. Animal and human waste is mixed with straw and buried in pits for a few months, where it decomposes into nutrient-rich manure.
2. Artificial fertilizer: chemical compounds made in factories, containing one or more essential nutrients. They are grouped by the nutrient they supply:

Some Organic Compounds Used as Food

Living things need energy for their life processes, and animals get it from food. On the basis of chemical composition, food is grouped into carbohydrates, proteins and fats.

Table: Different food materials

Organic compounds made of only carbon and hydrogen are hydrocarbons. Burning them releases a lot of energy, so they are used as fuels. The simplest hydrocarbon is methane, \(\ce{CH4}\). It is the chief component of natural gas, which occurs in underground deposits. When farm waste and cow dung decompose without air, a gas mixture called biogas forms, and methane is its main part. Methane is an excellent fuel because it gives out a very large amount of energy for its mass. It burns to carbon dioxide and water vapour, with no harmful gas and no solid residue, and it is used in homes and as a vehicle fuel.

Some Common Solutions in Daily Life

The body needs enough fluid to carry digested food to the cells, where respiration releases energy. After hard work or an infection, fluid is lost, and it can be replaced by soda water, soft drinks, syrups or glucose solution. Glucose solution can also be given straight into the blood in an emergency.

Soda water and soft drinks. Soda water is made by forcing carbon dioxide into water at about 30 atmospheres pressure, so much more gas dissolves. If the gas is passed into flavoured sugar solution, the result is a soft drink.
Syrup. A very concentrated flavoured sugar solution. Diluted with water, it gives an energy-rich drink.
Glucose saline. A person with dysentery can lose a lot of water (dehydration). A glucose solution injected into the blood gives water and energy at once. Taken by mouth, glucose gives quick energy because it is the simplest carbohydrate.

Industrially Useful Materials

Mixtures such as lime mortar, cement and glass are made in industry through a series of processes. They meet everyday needs and are also used in advanced scientific and medical work.

1. Lime mortar. Calcined lime is mixed with sand and water to a thick slurry, used for joining bricks and for plastering.
2. Calcined lime. Limestone (\(\ce{CaCO3}\)) heated to about 1000 °C gives off carbon dioxide and leaves dry lime, called calcined lime.
3. Cement. Cement is a greenish-grey powder that sets to a hard mass when mixed with water and dried, and it is mainly used in construction. Early Egyptians built with lime or clay and gypsum, and Romans used volcanic ash and lime. In 1824 Joseph Aspdin, a mason of Leeds, England, found that strongly heating limestone and clay and then adding water gave a rock-like mass that looked like Portland stone, so the product was named Portland cement.

Manufacture of Cement

The raw materials are limestone and clay, and there are two methods.
Dry process: limestone and clay are taken in a fixed ratio and ground to a fine powder. The powder is heated to 1500 to 1600 °C, forming clinkers (hard nodules). The clinkers are ground again, and a little gypsum is added to give cement.
Wet process: water is added to the ground mixture to make a slurry, which is then strongly heated to form clinkers; the rest is as before.

Uses of Cement

Grouting: cement, or cement with sand and water, is put in the gaps between bricks or tiles to make the surface smooth and the structure stronger.
Cement mortar: cement and sand in the ratio 1 : 4 by weight, made into a slurry with water, for joining bricks and plastering.
Concrete: cement, gravel and sand in the ratio 1 : 2 : 3 by weight with water, used for casting roofs, pillars and beams.
Reinforced cement concrete (RCC): concrete strengthened with structural steel. It is very strong and long lasting, and is used for bridges, railway sleepers, prefabricated structures, columns, beams and roofs.

Example 6. What do you mean by grouting? Where is the technique used?
Answer:
Grouting is a building technique in which cement, or cement mixed with sand and water in a fixed proportion, is filled into the gaps between bricks, tiles and similar materials. It makes the surface smooth and strengthens the structure.

4. Brick. Red bricks are made from clay. The right amount of water is kneaded into the clay, which is pressed into brick shapes and baked at about 1000 °C to harden. Bricks are used to build walls.
5. Glass. Early humans used obsidian, a natural volcanic glass, for tools and spearheads. Today many kinds of glass are made by modern methods.

Manufacture of Glass

The main raw materials are sand (silica), limestone and feldspar. Broken glass, called cullet, is added because it starts the melting at a lower temperature. Red lead, aluminium oxide, manganese oxide, chromium oxide and similar substances are added to give special properties. The mixture is melted in a glass tank furnace or pot furnace. The molten glass is then shaped by one of three techniques.

1. Blowing: for laboratory glassware and bottles.
2. Pressing (optical glass): molten glass is pressed into a round shape from which optical glasses are made.
3. Float (sheet) glass: molten glass is poured on a bath of molten tin, where it spreads to an even thickness and is drawn off as sheets.

Finally all the shaped glass is cooled in a controlled way in a long tunnel-type chamber. This slow cooling is called annealing.

General Properties of Glass

Glass is a supercooled liquid. As molten glass cools slowly, its viscosity rises enormously below its freezing range, and it can no longer flow, so it looks like a solid. Glass is transparent, strongly resistant to chemicals, a poor conductor of heat and electricity, and easy to mould.

Table: Types of glass

Polymers and Plastics

The word plastic comes from plasticity, the ability of a material to be moulded into shapes. Plastics are made of polymers, long chains formed by joining many identical small molecules. One small molecule of the chain is a monomer. For example, the polymer polyethylene is made from the monomer ethylene, \(\ce{C2H4}\). Plastics are light, strong, good looking and poor conductors of heat and electricity.

On the basis of their behaviour on heating, plastics are of two kinds.

Soaps and Detergents

Soaps are sodium salts of fatty acids. Fatty acids come from oils and fats such as olive, cottonseed, rapeseed and coconut oil. Treating vegetable oil with sodium hydroxide solution gives soap, with glycerine as a useful by-product.

In words: vegetable oil + sodium hydroxide gives soap + glycerine.

Table: Kinds of soap

Detergents

Soap forms a curdy precipitate (scum) with hard water and does not lather well, so it works poorly there. Detergents are sodium salts of alkyl hydrogen sulphates, such as sodium lauryl sulphate, and they work well even in hard water, so their cleaning action is better than that of soap.

Cleansing Action of Soaps and Detergents

Both dissolve in water and act as surfactants. They surround oil and grease spots with a thin film and lift them away together with the dirt. Because this removal of dirt involves forming an emulsion, it is called emulsification, and the cleaning substances are emulsifiers.

Example 7. A sample of hard water is taken in two containers, A and B. A detergent is added to A and a soap to B. The water in A cleans better than the water in B. Justify.
Answer:
Soap loses its effectiveness in hard water. Hardness is due to mineral salts, mostly of calcium and magnesium. These salts react with soap to give an insoluble precipitate called scum, and the lather is reduced. A detergent forms soluble compounds with calcium and magnesium, so there is no scum. Hence the water in A cleans better than the water in B.

Example 8. Write a short note on the cleansing action of a detergent.
Answer:
A detergent dissolves in water and acts as a surfactant. It forms a thin film around oil and grease spots on clothes and helps remove them along with dirt. This is emulsification, and the detergent is the emulsifier.

Paints

A paint is a colouring material dissolved or dispersed in a suspension of basic lead carbonate in linseed oil. Plastic emulsion paint is made by dissolving coloured plastics in amyl acetate, and it lasts much longer than the basic lead carbonate paints.

Example 9. How are paints and plastic emulsions prepared?
Answer:
Paints are colouring materials dissolved in a suspension of basic lead carbonate in linseed oil. Plastic emulsion is a paint made by dissolving coloured plastics in amyl acetate. It is more durable than basic lead carbonate paint.

Medicines

In the past, people treated diseases with parts or extracts of medicinal plants such as tulsi, vasaka and neem. Scientists analysed these substances, made similar molecules artificially, and also developed new, more effective molecules that give quicker relief and cure serious diseases. Such molecules are medicines, for example penicillin, tetracycline and sulphadiazine. Quinine, obtained from the bark of the cinchona plant, is used against malaria.

Example 10. “Sand and clay, which are plentiful in the earth's crust, play a big part in the comfort of modern life.” Comment on this statement.
Answer:
Sand is an important part of all mortars (lime mortar and cement mortar), plain cement concrete and RCC, which build the infrastructure of modern life. Clay is the basis of ceramics, bricks, other building materials, sanitary ware and household articles. Their useful properties make both materials important to everyday comfort.

Points to Remember

1. Aluminium is the third most abundant element in the earth's crust, and hydrogen is the most abundant element in the universe.
2. Helium is the second lightest element and is inert, so it is used in weather balloons.
3. Carbohydrates give instant energy, while fats are the store of energy.
4. Methane is the simplest hydrocarbon and the main part of natural gas and biogas.
5. Polymers are large molecules formed by joining many small molecules.
6. Cement is a mixture of silicates and aluminates, and it sets because these undergo hydration (reaction with water).
7. Glass is mainly sodium silicate and calcium silicate; other ingredients give it special properties.
8. Glass is a supercooled liquid that looks solid because its viscosity rises as it cools.
9. Detergents beat soaps because they form no insoluble precipitate with hard water.