Physical and Chemical Changes Notes PDF

CHAPTER -6 PHYSICAL AND CHEMICAL CHANGES ANSWER THE FOLLOWING 1)Why are water pipes galvanized? Ans: Water pipes are made up of iron. Rusting of iron occurs when iron interacts with air and water. Therefore, to prevent rusting of iron, water pipes are galvanized, i.e. iron pip...

CHAPTER -6 PHYSICAL AND CHEMICAL CHANGES ANSWER THE FOLLOWING 1)Why are water pipes galvanized? Ans: Water pipes are made up of iron. Rusting of iron occurs when iron interacts with air and water. Therefore, to prevent rusting of iron, water pipes are galvanized, i.e. iron pipes are coated with metals such as zinc or chromium to prevent iron from interacting with air. 2)When water is frozen, it forms ice that has different properties. Why then do we say that freezing is physical change? Ans: In a physical change, only the physical properties of a substance change and no new substances are formed. Freezing of water is a physical change because only the physical state of water is changed; i.e. water is converted into ice. Hence, freezing is a physical change. 3)Distinguish between endothermic reactions and exothermic reactions. Ans: Endothermic Reaction Exothermic Reaction These reactions are These reactions are accompanied with accompanied with absorptionof heat. evolutionof heat. Melting of solids, fusion, Combustion of petrol evaporation and andoxidation of vaporisation ammonia are are examples examples of 4)What is crystallization? Ans: Crystallization is a process in which pure solid is separated from a solutionin the form of its crystals. For example, common salt obtained from sea water contains solubleimpurities such as magnesium chloride. To purify common salt, crystallization process is applied.  5)What is a supersaturated solution? Ans: A solution in which more solute is dissolved by increasing the temperature of a saturated solution is called supersaturated solution. When a supersaturated solution is cooled to room temperature, it crystallizes easily. 6)Rust is a mixture and does not have a chemical formula. True or false? Ans: False. Rust is a compound. Its chemical formula is Fe2O3.xH2O. 7)Explain why rusting of iron objects is faster in coastal areas than deserts ? Ans:Two conditions are required for rust formation: oxygen and moisture content in the atmosphere.The climate of desert regions are hot and dry, whereas the climate of coastal areas are humid. Hence, rusting is faster in coastal areas than in deserts. 8)Why lime water turns milky on passing CO2 gas into it? Ans: When carbon dioxide gas is passed through or over limewater, it turns milky due to the formation of calcium carbonate. 9)When a soda water bottle is opened, a gas is released. Does a chemical reaction occur? Explain Ans: No, the chemical reaction does not occur. When a soda bottle is opened, carbon dioxide is released. The carbon dioxide is already dissolved inside the soda bottle. On removing the cap, the pressure inside is released and carbon dioxide escapes. 10)Setting of curd is a chemical change. Explain. Ans: Setting of a curd is a chemical change because curd cannot be converted into milk again. Moreover, a new substance, curd is formed which is completely different in taste and properties from the milk. 11)Why should we not put stainless steel spoons in pickle jars? Ans: Steel reacts with acids present in the pickle and forms rust. Hence, we should never put stainless steel spoons in pickle jars.  12)LPG (liquefied petroleum gas) in a gas cylinder is in the form of a liquid. Which physical and chemical changes occur when it comes out of the cylinder and burns? Ans: LPG in a cylinder exists as a liquid. When it comes out of the cylinder, it becomes a gas, which is a physical change. Burning of LPG is a chemical change. 13).Why atom is said to be electrically neutral? The number of electrons in an atom is always equal to the number of protons. So an atom as a whole is electrically neutral. 14).Write a short note on periodic table? The periodic table is a system for arranging the chemical elements in the order of atomic numbers.The vertical column of the periodic table are called groups and the horizontal rows in the periodic table are known as periods. 15)Draw and explain the structure of atom? The atom is the basic building block for all matter in the universe. Atoms are extremely small and are made up of a few even smaller particles. The basic particles that make up an atom are electrons, protons, and neutrons.At the center of the atom is the nucleus. The nucleus is made up of the protons and neutrons. The electrons spin in orbits around the outside of the nucleus in a path called orbit. Protons are positively charged particles and electrons are negatively charged particles.Neutrons are neutral in nature. 16)What is an ion? An ion is defined as an atom or molecule that has gained or lost one or more of its  electron giving it a net positive or negative electrical charge. Positively charged ions are cations and negatively charged ions are anions. 17)What is ozone?How its protect us from ultraviolet radiation? Ozone (O3)is a highly reactive gas composed of three oxygen atoms.Ozone absorbs ultraviolet radiations and breaks down to form oxygen 18)Differentiate between elements and compounds.Give examples? Elements are pure substances which Compound are substances which are formed by two or are composed of only one type of more different types of elements that are united chemically atom. in fixed proportions. Eg-Carbon (C )Nitrogen(N) Eg-Carbondioxide(CO2) Oxygen(O2)