Changes Around Us Online Test 8th Science Lesson 5 Questions in English

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Adithya, a standard VIII student once visited Qutub Minar, Delhi and wondered about the 1500 years old rust resistant iron-pillar. He was thinking about why the iron pillar has not rusted for more than 1500 years.

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As you studied earlier in standard VII changes like folding a paper, drying wet clothes, bending of iron rod are some examples for physical changes

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On the other hand, changes like burning of paper, digestion of food, turning of milk into curd and decaying of vegetables are some of the examples for chemical changes.

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We can you define a chemical change. A chemical change is a permanent, irreversible change and produces a new substance.

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Chemical changes are otherwise called as chemical reactions, because one or more substances(Reactants) undergo a reaction to form one or more new substances (Products).

Reactant(s) – Product(s)

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In a society people live in different conditions not under same conditions. Likewise, all chemical reaction will not occur at all conditions. For every chemical reaction to take place, certain specific condition is required.

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Chemical reactions can be done through;

 Physical contact
 Solution of reactants
 Electricity
 Heat
 Light
 Catalyst

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Students, these changes are due to chemical reactions by contact in physical state. Combination of reactants in their naturally occurring states (solids, liquids, gases) is referred as physical contact.

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When dry wood comes into contact with fire, it burns with the help of oxygen to form carbon dioxide, which is given out as smoke.

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When a matchstick is rubbed on the sides of a matchbox, a chemical reaction takes place to form heat, light and smoke.

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When quick lime (calcium oxide) comes in contact with water, it forms slaked lime (calcium hydroxide).

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As your mother does, when milk is mixed with coffee decoction the colour of milk and decoction changes due to chemical reaction. Your mother adds enough sugar to make it tasty.

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The head of a matchstick contains potassium chlorate and antimony tri sulphide. The sides of the matchbox contain red phosphorous.

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Take small amount of solid silver nitrate and sodium chloride in a test tube. No, the reactants in solid state have no reactions.

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Silver nitrate solution reacts with sodium chloride solution to form a white precipitate of silver chloride and sodium nitrate solution. From the above reaction, we infer that some chemical reactions proceed only in solution form not in solid form

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As you know, water is made of hydrogen and oxygen molecules. When electricity is passed through water containing small amounts of sulphuric acid, hydrogen and oxygen gases are liberated.

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A concentrated solution of sodium chloride called BRINE is electrolysed to produce chlorine and hydrogen gases along with sodium hydroxide. This is a very important reaction to produce chlorine industrially.

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We infer that some chemical reactions proceed only by the passage of electricity. Hence, such reactions are called as electrochemical reaction or electrolysis.

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The term electrolysisl was introduced by Michael Faraday in the 19th century. Electrolysis is a combination of electron + lysis. Electron is related to electricity and lysis means decomposition.

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As you know food is very important for our survival and also many other living beings. Have you closely watched your mother cooks food for you? She boils rice, cooks vegetables, and prepares kuzhambu and rasam etc by heating them over stove. When enough heating is given some chemical reactions take place to convert the raw food (uncooked) items into cooked ones

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Take small amount of lead nitrate in a dry test tube and heat it gently over a flame. Observe the changes closely. You will hear cracking sound and an evolution of reddish-brown coloured gas(nitrogen dioxide).

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In industries limestone rocks are heated to get quicklime (calcium oxide). Hence, some of the chemical reactions can be achieved by the supply of heat only. These reactions are called thermo chemical reactions or thermolysis.

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Limestone is the raw material for quicklime, slaked lime, cement and mortar. Chemical reactions accompanying evolution of heat are called exothermic reactions whereas reactions involving absorption of heat are called endothermic reactions.

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All the human activities will be affected and there will be no food for us to survive. Sunlight is important not only for us but also for plants as well. As you know photosynthesis is a process in which light energy from the sun is used by the plants to prepare starch from carbon dioxide and water.

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The sunlight in uses the chemical reactions between caron dioxide and water, which finally ends up in the production of starch (photo means light and synthesis means production). These chemical reactions in used by light are called as photochemical reactions.

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The ultraviolet rays from the sun break Ozone (O3) molecules in the stratosphere into oxygen and atomic oxygen. This atomic oxygen again combines with molecular oxygen to form Ozone.

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Photochemistry is the branch of chemistry that deals with chemical reactions involving light. If there is no light, all the human activities will be affected and there will be no food for us to survive.

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Have you ever questioned about why idly batter prepared by your mother turns into sour taste after few hours? The answer for your question is fermentation.

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Fermentation is a chemical reaction in which a substance is decomposed with the help of yeast or bacteria to give simpler products.

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In the case of yeasts, the enzymes released by the yeast makes the reaction faster. Like this, in industries some chemical substances are used to alter the speed of a chemical reaction. These substances are called catalysts. Inhibitors slows down the reaction.

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Metallic iron is used as a catalyst in the manufacture of ammonia using Haber process. This ammonia is the basic material for the production of urea, an important fertilizer in agriculture.

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In Vanaspati ghee (dalda) preparation finely divided nickel is used as a catalyst. Thus, speed of the certain reactions is influenced by the catalysts and such reactions are called catalytic reactions.

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Alcoholic beverages like beer, wine etc are produced by fermentation process in industries. The beer making industries are called BREWERIES.

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We know that every chemical reaction requires a specific condition to occur. When chemical reactions take place there will be production of heat, light, sound, pressure etc. Let us discuss these effects elaborately.

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Food spoilage may be defined as any change that causes food unfit for human consumption. The chemical reactions catalyzed by the enzymes result in the degradation of food quality such as development of bad tastes and odour, deterioration and loss of nutrients.

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Rotten eggs develop a bad smell due to formation of hydrogen sulphide gas. Decaying of vegetables and fruits due to microbes.

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Fishes and meat containing high levels of polyunsaturated fatty acids that undergo oxidation causes bad odour when exposed to air or light. This process is called Rancidity.

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Apples and some fruits turn brown due to chemical reaction with oxygen in air. Th is chemical reaction is called browning.

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The cells of apples, fruits and other vegetables contain an enzyme called polyphenol oxidase or tyrosinase that when in contact with oxygen catalyses a biochemical reaction of plants’ phenolic compounds to brown pigments known as melanin.

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Our environment provides air to breathe, water to drink and the land to produce food. Due to human activities like industries, increasing number of automobiles etc our environment is badly aff ected now-a-days. So, there is an unwanted change in physical, chemical and biological properties of the environment. This is termed as pollution. Th e substances which cause these changes are called pollutants.

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Brass vessels which contain copper as one of constituents develop a greenish layer on exposure to air for a long time. This is due to a chemical reaction between copper and moist air to form basic copper carbonate and copper hydroxide.

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Shiny metal surfaces and other articles lose their shining appearance due to chemical reactions on the surface. Iron metal comes into contact with water and oxygen, it undergoes a chemical reaction called RUSTING.

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Chemical reactions also produce heat energy. Such reactions are called EXOTHERMIC REACTIONS. For example, when you add water to quicklime (Calcium oxide), lot of heat is released to produce slaked lime (Calcium hydroxide).

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when a piece of magnesium ribbon is burnt in a flame, bright light is produced with heat. Even the fireworks during festival times produce different coloured lights which are all due to chemical reactions. Similarly, when we ignite methane gas, it produces heat and light. So, we can say that light is produced during the chemical reactions.

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We produce sound when we speak. When you hit metals like iron, copper etc sound is heard. Some chemical reactions do produce sound when they take place. So, sound will be produced in certain chemical reactions.

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When a metal like zinc or magnesium reacts with diluted acids hydrogen gas is produced. Since hydrogen gas is highly flammable it reacts with oxygen in air to produce POP sound

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Some chemical reactions produce gases which build up the pressure when the reaction takes place in a closed container. If the pressure level goes beyond the limit, we get the explosion. Explosives, fireworks work on this basis. When they are ignited, they explode due to pressure generated by gases from the chemical reactions. Thus, you hear a huge sound.

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You can see air bubbles coming out of soda water. Similarly, gas evolution may take place as a result of chemical reactions. For example, when dilute hydrochloric acid is added to a solution of sodium carbonate or sodium bicarbonate carbon dioxide gas is evolved.

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when you place a iron nail in a solution of copper sulphate, the blue colour of copper sulphate
slowly changes into green due to chemical reaction between iron copper sulphate solution.

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when you burn a piece of camphor, smoke comes out as result of chemical reaction between solid camphor and oxygen. Here, there is a change of state from solid to gas.