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Class IX

Property of Sound Waves that Sound needs Material Medium to Travel

The sound waves need material medium to travel because they travel by the vibrations of particles of the medium. The material medium is gas, liquid and solid.  Sound waves are called mechanical waves as they need a material medium for their travel.  An activity described below tells that sound can not travel through vacuum. The …

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Work, Energy, Power – Question and solution

A body of mass 20 kg is projected vertically upwards with a velocity of 20m/s. Find – (a) its potential energy at the starting point         (b) its kinetic energy at the starting point         (c) its potential energy at the highest point          (d) its kinetic energy at the highest point. Answer – (a) its potential energy …

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Force and Laws of Motion – Balanced and Unbalanced Force, Inertia, Types of Inertia   

When the sum of forces becomes zero, the forces balance each-other and the forces are called balanced forces. Activity – We take a box and tie two strings to it in opposite direction. One string is at end A and other string is at end B. When we pull string at A, the box moves …

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FORCE AND LAWS OF MOTION / THE LAWS OF MOTION – SUMMARY

Summary – If the resultant of all the forces acting on a body is zero, the forces are called balanced forces. If the resultant of all the forces acting on a body is not zero, the forces are called unbalanced forces. An unbalanced force acting on a body gives it an acceleration. Force is an …

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WHAT IS FORMULA UNIT MASS OR FORMULA MASS?

The compounds which are formed by ions are called ionic compounds. In general, cations are made by metals and anions are made by non-metals. So, it can be understood as compounds made by metals and non-metals are ionic compounds. Examples – Al2(SO4)3, CaCl2, K2SO4, KNO3, CaCO3, NaCl, MgCl2 etc. Read more – Law of Conservation of …

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How do you Write Chemical Formula for an Ionic Compound?

Rules for writing a chemical formula  Write down the symbols of the ions (cation and anion) involved. Write the symbol of cation first (L.H.S) followed by the symbol of anion (R.H.S.). Write the valency of each ion. Interchange the valencies of the ions and write as the subscript. Write them close together and ignore ‘1’ …

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RULES FOR WRITING A CHEMICAL FORMULA OF SIMPLE COMPOUNDS

Chemical formula is a symbolic representation of a molecule’s composition. It indicates – (a) the elements that are present in the compound and (b) the ratio of these elements in the compound. A chemical formula is also called molecular formula. The symbols in the molecular formula tell us about the elements and the subscript tells …

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Problems / Numericals based on Mole Concept (Atomic Mass/ Molar Mass/ Avogadro number)

Formulas in brief –               Mass = Molar mass x Number of moles  Number of moles = Given number of particles / Avogadro number      Number of particles = (Given mass / molar mass) x Avogadro number          (from 1 and 2)  If one carbon atom has a mass of 12 atomic mass units and one magnesium atom has …

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Mole Concept – Introduction, Importance and Formulae

Introduction  The structure of matter can be one of the following – atoms → molecules → bulk compound or element atoms → bulk element atoms → ions → bulk ionic compound It clearly shows that the masses that are measured in the laboratory are masses corresponding to large numbers of tiny atoms or molecules. Logically …

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List of Polyatomic Ions and Charges

 Simple ions – The ions which are formed from single atom carrying a positive or negative charge are called simple ions or mono-atomic ions. Example – Na+, Cl–, O-2, Al+3, N-3(nitride), Mg+2, Ca+2, Cu+2, Fe+2etc. Polyatomic ions -Polyatomic ions or compound ions are molecules formed from groups of atoms bonded together that carry a positive …

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IONS – Anions and Cations

Atoms are neutral; they contain the same number of protons (positive particles) and electrons (negative particles). When the numbers of these subatomic particles are not equal, the atom is electrically charged and is called an ion i.e. ions are electrically charged atoms with extra electrons or missing electrons. It means ions are formed when atoms lose or gain …

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