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The periodic table is a tabular method of displaying the elements in such a way, that the elements having similar properties occur in the same vertical column or group”. Earlier attempts of the classification of elements: Dobereiner’s Triads, Newland’s law of octaves. Dobereiner’s Triads:  This classification is based on the atomic mass. According to this, when elements are arranged in order of increasing atomic masses, groups of three elements, having similar properties are obtained. The atomic mass of middle element of the triad being nearly equal to the average of the atomic masses of the other two elements. For Example Li (6.9), Na (23), K (39). Limitation: It fails to arrange all the known elements in the form of triads, even having similar properties. Newland’s Law of Octaves:  According to this ‘when elements are placed in order of increasing atomic masses, the physical and chemical properties of every 8th element are a repetition of the properties of the first elem...

Chemical Reaction:  The transformation of chemical substance into another chemical substance is known as Chemical Reaction.  For example: Rusting of iron, the setting of milk into curd, digestion of food, respiration, etc.   In a chemical reaction, a new substance is formed which is completely different in properties from the original substance, so in a chemical reaction, a chemical change takes place. Only a rearrangement of atoms takes place in a chemical reaction. Reactant The substances which take part in a chemical reaction are called reactants. • The new substances produced as a result of a chemical reaction are called products. Example:  The burning of magnesium in the air to form magnesium oxide is an example of a chemical reaction.                          2Mg(s) + O2(g) △→ 2MgO(s) Before burning in air, the magnesium ribbon is cleaned by rubbing with sandpaper. This is done to...

Work, Power and Energy Class 9 Notes Science Chapter 4th Work, Power and Energy Work:             When a force acts on an object and the object shows displacement, then it said to be a force has done work on the object.                               Work = Force x Displacement      Unit of workdone = Joule                          1 joule = Newton x metre Two conditions need to be satisfied for work to be done: (i) A force should act on object (a) The object must be displaced                              Define 1 joule work.    1 Joule work is said to be done when 1 Newton force is applied on an object and it shows the displacement by 1 meter. Energy:- ...

  Force:-   It is defined as the acceleration to move the object from one position to another.                                                Or  The push or pull on an object with mass that causes it to change its velocity. Force is an external agent capable of changing the state of rest or motion of a body.                              Force = Mass × Acceleration                                  F = m × a Where, the terms are denoted by the follow...

  1.   What is a redox reaction? 2.   What is corrosion? Explain its advantage and disadvantage. 3.   What is rancidity? How can we reduce the problem of rancidity? 4.   How is corrosion different from rusting? 5.   What is meant by endothermic and exothermic reactions? Give suitable example for each. 6.   Define different types of chemical reaction and give examples for each. 7.   Why is photosynthesis considered as an endothermic reaction? 8.   In electrolysis of water, why is the volume of gas collected over one electrode double that of the other electrode? 9.   What happens when water is added to solid calcium oxide taken in a container? Write a chemical formula for the same. 10.   Give one use of quick lime. 11.   Give three types of decomposition reaction. 12.   Name the compound used for testing CO 2  gas.

  1.   What is a redox reaction? 2.   What is corrosion? Explain its advantage and disadvantage. 3.   What is rancidity? How can we reduce the problem of rancidity? 4.   How is corrosion different from rusting? 5.   What is meant by endothermic and exothermic reactions? Give suitable example for each. 6.   Define different types of chemical reaction and give examples for each. 7.   Why is photosynthesis considered as an endothermic reaction? 8.   In electrolysis of water, why is the volume of gas collected over one electrode double that of the other electrode? 9.   What happens when water is added to solid calcium oxide taken in a container? Write a chemical formula for the same. 10.   Give one use of quick lime. 11.   Give three types of decomposition reaction. 12.   Name the compound used for testing CO 2  gas.