Samacheer Class 8 Science - Changes Around Us

A chemical reaction is a process in which one or more substances, called reactants, undergo a transformation to form one or more new substances, called products. During a chemical reaction, the chemical bonds within the reactant molecules are broken, and new chemical bonds are formed to create the product molecules. This process can be represented by the equation: Reactant(s) → Product(s). Chemical reactions involve the rearrangement of atoms and the formation of new combinations of elements, resulting in substances with different chemical properties and characteristics from the original reactants. Chemical reactions are fundamental to all natural processes and are essential for life, including digestion, respiration, photosynthesis, and combustion.

When an iron nail is placed in a copper sulphate solution, a displacement reaction occurs. Iron is more reactive than copper and displaces copper from the copper sulphate solution. As a result, ferrous sulphate is formed and copper metal is deposited on the surface of the iron nail. The blue colour of the copper sulphate solution gradually changes to green due to the formation of ferrous sulphate, which is green in colour. The iron nail becomes coated with a reddish layer of copper metal as the reaction progresses. This is an example of a single displacement reaction where a more reactive metal displaces a less reactive metal from its salt solution. The reaction demonstrates the reactivity series of metals and shows that iron is more reactive than copper.

Tarnishing is the process by which shiny metal surfaces and articles lose their bright, shining appearance due to chemical reactions occurring on their surface when exposed to air, moisture, or other atmospheric conditions. Silver articles become black on exposure to atmospheric air because silver reacts with sulphur compounds present in the air to form silver sulphide, which is black in colour. Similarly, brass vessels, which contain copper as one of their constituents, develop a greenish layer on prolonged exposure to air. This greenish layer is formed due to a chemical reaction between copper and moist air, resulting in the formation of basic copper carbonate and copper hydroxide. Tarnishing is a slow chemical change that affects the appearance of metal articles but does not necessarily affect their functionality. The tarnished layer can often be removed by polishing or chemical treatment to restore the original shine of the metal.

During the electrolysis of brine, which is a concentrated solution of sodium chloride, several important products are formed. Chlorine gas is produced at the anode, hydrogen gas is produced at the cathode, and sodium hydroxide is formed in the solution. The electrolysis of brine is represented by the chemical reaction where sodium chloride is broken down by electrical energy into these three products. This is a very important industrial process used to produce chlorine on a large scale. Chlorine has numerous applications in industries, including the manufacture of bleaching powder, disinfectants, and various chemicals. Hydrogen gas produced during this process is also useful as a fuel and in various chemical processes. Sodium hydroxide produced is widely used in industries for various purposes. The electrolysis of brine demonstrates how electrical energy can drive a non-spontaneous chemical reaction to produce useful industrial chemicals.

A catalyst is a substance that alters the speed of a chemical reaction without being permanently consumed or changed in the process. The primary role of a catalyst is to provide an alternative reaction pathway that requires lower activation energy, thereby allowing the reaction to proceed faster. Catalysts can increase the rate of both forward and reverse reactions equally, thus not affecting the equilibrium position of a reversible reaction. In the Haber's process for the manufacture of ammonia, metallic iron is used as a catalyst to speed up the reaction between nitrogen and hydrogen gases. Catalysts are essential in many industrial processes because they improve efficiency and reduce the time and energy required for reactions to occur. In biological systems, enzymes serve as biological catalysts that regulate the speed of metabolic reactions. A single catalyst molecule can be used repeatedly for many reaction cycles, making catalysts economically important in industrial applications.

Photosynthesis is a chemical reaction because it involves the breaking and formation of chemical bonds to produce new substances. In photosynthesis, light energy from the sun is used by plants to convert carbon dioxide and water into glucose (starch) and oxygen. This process involves chemical reactions between carbon dioxide and water molecules, which are driven by light energy. The term photosynthesis is derived from two words: photo, meaning light, and synthesis, meaning production. The chemical reactions that occur in photosynthesis and are driven by light energy are called photochemical reactions. During photosynthesis, the chemical bonds in carbon dioxide and water are broken, and new chemical bonds are formed in glucose and oxygen. This rearrangement of atoms and formation of new substances with different properties from the reactants confirms that photosynthesis is indeed a chemical reaction. The products formed (glucose and oxygen) have completely different properties from the reactants (carbon dioxide and water), which is a key characteristic of chemical reactions.

Yeast plays a crucial role in making cakes by acting as a biological catalyst in the dough. Yeast is a living microorganism that breaks down fermentable sugars present in the cake batter through a process called fermentation. During this fermentation process, yeast produces carbon dioxide gas as a byproduct of the breakdown of sugars. The carbon dioxide gas generated forms numerous small bubbles throughout the dough, which causes the cake to rise and become light and fluffy. This rising action, known as leavening, gives cakes their characteristic spongy texture. The yeast also produces other compounds during fermentation that contribute to the flavour and aroma of the baked cake. The amount of yeast used and the temperature of the dough affect the rate of fermentation and the extent to which the cake rises. Without yeast or other leavening agents, cakes would be dense and heavy. Therefore, yeast is an essential ingredient in baking that acts as a biological catalyst to improve the quality and texture of cakes.

The burning of fossil fuels is a major contributor to global warming. When fossil fuels such as coal, petroleum, and natural gas are burned for energy, they undergo combustion reactions that release large amounts of carbon dioxide into the atmosphere. Carbon dioxide is a greenhouse gas that traps heat in the Earth's atmosphere by preventing infrared radiation from escaping into space. As more and more carbon dioxide accumulates in the atmosphere due to continuous burning of fossil fuels, the greenhouse effect intensifies, leading to an increase in the average temperature of the Earth. This rise in global temperature causes various environmental problems including melting of polar ice caps, rising sea levels, and changes in weather patterns. The combustion of fossil fuels also releases other greenhouse gases such as methane and nitrous oxide, which further contribute to global warming. Human activities, particularly the burning of fossil fuels for electricity generation, transportation, and industrial processes, have significantly increased the concentration of greenhouse gases in the atmosphere. Therefore, reducing the consumption of fossil fuels and transitioning to renewable energy sources are essential steps to mitigate global warming and protect the environment.

Acid rain occurs due to the emission of certain pollutants from vehicles and industrial processes into the atmosphere. The primary pollutants responsible for acid rain are sulphur dioxide and nitrogen oxides. When vehicles burn fossil fuels, they release nitrogen oxides as byproducts of combustion. Industrial processes also emit large quantities of sulphur dioxide, particularly from power plants and factories that burn coal and oil. These gaseous pollutants rise into the atmosphere and react with water molecules present in clouds and rain droplets. Sulphur dioxide reacts with water to form sulphuric acid, while nitrogen oxides react with water to form nitric acid. These acids dissolve in rainwater, making it acidic in nature. When this acidic rain falls on the Earth's surface, it can damage ecosystems, corrode buildings and monuments, harm aquatic life, and affect soil quality. Acid rain also damages vegetation and can leach harmful metals from soil into groundwater. Reducing emissions of sulphur dioxide and nitrogen oxides through pollution control measures and the use of cleaner fuels are important steps to prevent acid rain and its harmful effects on the environment.

No, rusting is not good for iron materials and is considered a harmful process. Rust is a form of iron oxide that forms when iron combines with oxygen present in the air in the presence of moisture. Rusting is a slow chemical change that occurs on the surface of iron and its alloys when exposed to air and water. The rust that forms is a flaky, powdery substance that is much weaker than the original iron metal. As rusting progresses, the rust layer replaces the strong iron with this weak, crumbly powder, causing the iron material to lose its strength and structural integrity. This weakening of iron materials can lead to failure of structures and equipment made from iron. Rusting also affects the appearance of iron articles, making them look unsightly and corroded. The process of rusting is irreversible and continuous if the iron remains exposed to air and moisture. To prevent rusting, iron materials are often coated with paint, oil, or other protective layers that prevent contact between iron and oxygen. Galvanization, which involves coating iron with a layer of zinc, is another effective method to prevent rusting. Therefore, rusting is definitely not desirable for iron materials and various preventive measures must be taken to protect iron from rusting.

No, not all fruits and vegetables undergo browning. Enzymic browning can be observed in certain fruits such as apricots, pears, bananas, grapes and avocados, and in vegetables such as aubergines, potatoes, and lettuce. This browning occurs due to the presence of an enzyme called polyphenol oxidase, also known as tyrosinase. When these fruits or vegetables are cut or bruised, this enzyme comes into contact with oxygen in the air. The enzyme then catalyzes a biochemical reaction that converts the plant's phenolic compounds into brown pigments known as melanin. This results in the fruit or vegetable turning brown in color. However, fruits and vegetables that lack this enzyme or have very low levels of it do not undergo enzymic browning, even when exposed to air. Therefore, browning is selective and depends on the presence and activity of the polyphenol oxidase enzyme in the particular fruit or vegetable.

1. Spoilage of food and vegetables:
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 taste and odour, deterioration and loss of nutrients.
Example 1 : Rotten eggs develop a bad smell due to formation of hydrogen sulphide gas.
Example 2 : Decaying of vegetables and fruits due to microbes.
2. Rancidity of fishes and meat:
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.
3. Apples and fruits turn brown when cut:
Apples and some fruits turn brown due to chemical reaction with oxygen in air. This chemical reaction is called browning.
The cells of apples, fruits and other vegetables contain an enzyme called polyphenol oxidase or tyrosinase, when come in contact with oxygen catalyses a biochemical reaction of plants, phenolic compounds to brown pigments known as melanins.

Conditions required for a chemical reaction to take place:
Chemical reactions can be done through:
Physical contact
Solution of reactants
Electricity
Heat
Light
Catalyst
1. Chemical Reactions Based On Solution Of Reactants:
When we mix two substances (Reactants) in solution form, the chemical reaction takes place to form new substances (Products).
Example:
Take small amount of solid silver nitrate and sodium chloride in a test tube. The reactants in solid state have no reactions.
Dissolve the same reactants in water in separate test tubes.
Mix both the solutions. 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.
2. Chemical Reaction Based On Electricity:
When electricity is passed through water containing small amounts of sulphuric acid, hydrogen and oxygen gases are liberated.
Similarly, 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.
From the above two reactions, we infer that some chemical reactions proceed only by the passage of electricity.
Hence, such reactions are called as electrochemical reaction or electrolysis.
3. Chemical Reactions Based On Light:
Sunlight is important not only for us but also for plants as welll. 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.
Example:
The sunlight uses the chemical reaction between carbon dioxide and water, which finally ends up in the production of starch (photo means light and synthesis means production). These chemical reactions is used by light are called as photochemical reactions.

Yes, Ramesh is absolutely right in his suggestion. The older iron rods that appear brownish have undergone rusting, which is a chemical change that occurs when iron reacts with oxygen and moisture in the air. Rusting severely damages the structural integrity and quality of iron rods, making them weak and unsuitable for construction purposes. Using rusted rods in building construction would compromise the safety and durability of the structure, potentially leading to collapse or structural failure over time. Although the older rods are cheaper, they would not provide the strength and reliability needed for a safe building. Ramesh's suggestion demonstrates several important values including responsible behavior, as he is thinking about long-term consequences rather than short-term savings. He shows correct decision-making by prioritizing safety and quality over cost. His advice also reflects caring for his friend's welfare and awareness about the dangers of using inferior materials in construction. These values highlight the importance of making informed choices based on quality and safety rather than being swayed by lower prices.

Yes, Rajasekar is absolutely right in his attitude and his refusal to move to that location is justified. Rajasekar understands that living near a chemical industry would expose his family to significant air pollution and other environmental hazards. Chemical industries release various toxic gases, particulates, and pollutants into the atmosphere as byproducts of their manufacturing processes. Prolonged exposure to such pollution can cause serious health problems including respiratory diseases, allergies, asthma, skin disorders, and other chronic illnesses. Rajasekar is more concerned about his family's health and well-being than saving money on rent, which is a wise and responsible approach. He recognizes that no amount of financial savings is worth compromising the health of his family members. The values demonstrated by Rajasekar include being environmentally sensitive and aware of the dangers posed by industrial pollution. He shows caring and concern for his family's welfare by prioritizing their health over economic benefits. His decision-making is correct because it reflects an understanding that long-term health costs far outweigh short-term rental savings. These values emphasize the importance of environmental consciousness and making decisions that protect the well-being of loved ones.