CBSE ADDA

CBSE ADDA _ Quick recap  Chemicals Equations  1. Chemical reactions- Chemical changes or chemical reactions are the changes in which one or more new substances are formed. 2. Chemical Equations – Representation of a chemical reaction in terms of symbols and formulae of the reactants and products is known as chemical equation. 3. Balanced Chemical equation – The chemical equation in which the no. of atoms of different elements is same on both sides of the arrow is called balanced chemical equation. 4. The chemical reactions can be classified into different types such as (a) Combination reactions- The reactions in which two or more substances combine to form a new substance are called combination reactions. For example, 2Mg(s) + O2 (g) → 2 MgO (s) (b) Decomposition reactions – The reactions in which a single compound breaks up into two or more simpler substances are called decomposition reactions. For example, 2Pb (NO3)2 (s) → 2PbO(s) + 4NO2 (g) + O2 (g) The decompos

 Solved Questions Set-01 Q.1. Why does silicon is classified as Metalloid? Ans: Silicon is gray color solid at room temperature with very high melting   point and   boiling point that lose or gain 4 electrons  [  3s 2 , 3p 2 ] having both metallic and non metallic properties so it is classified as Metalloid eg. Sio 2 Q.2.Why inert gases have zero valencies? Ans: It is because Inert gases have 2 or 8 electrons in valence shell. Q.3. How is valency of an element determined? Ans: The valency of an element measures its ability to combine with other elements. The valency is determined by knowing place in periodic table.. All the elements in group VIII have eight electrons in their outer shells, and thus have a valency of zero (highly stable). Elements in group I just have one valence electron in their outer shells and thus have a valency of one. Q.4. why could no fixed position be given to hydrogen in Mendeleev’s Periodic table? Ans: Hydrogen placed in the

1.Q. Why carbon form  covalent bond   by sharing electron? Ans: Carbon has four electrons in its outermost shell and needs to gain or lose four electrons to attain noble gas configuration.  (i) it could gain four electrons forming C 4- anion. But it would be difficult for the nucleus with six protons to hold on to ten electrons, that is, four extra electrons. (ii) It could lose four electrons forming C 4+ cation. But it would require a large amount of energy to remove four electrons leaving behind a carbon cation with six protons in its nucleus holding on to just two electrons. Hence, it is difficult for the carbon to lose or gain four electrons therefore  carbon formcovalent bond by sharing electron 2.Q. Why do covalent compounds are in gaseous or liquid state at normal temperature and pressure. Ans: due to Weak Vanderwaal’s forces present between the molecules of covalent compounds.   3.Q. Why do covalent compounds have less melting and boiling points? Ans: As ene