CBSE ADDA

Empirical formula  : The empirical formula is the simplest formula for a compound in which atoms of different elements are present in simple ratio. It shows the relative number of atoms of each element. For example CH 2 O is the empirical formula of Glucose C 6 H 12 O 6 Molecular formula   :  It is the formula in which   the actual number of atoms of  different element are present. For example, if the empirical formula of benzene is CH where as molecular formula  is C 6 H 6  , etc. An empirical formula is often calculated from elemental composition data. The weight percentage of each of the elements present in the compound is given by this elemental composition.  Let's determine the empirical formula for a compound with the following elemental composition:40.00% C, 6.66% H, 53.34% O. Element percentage Atomic mass Relative number of Atoms Dividing by least number Simple ratio C 40 12 40/12 = 3.33

  Ancient Greek philosophers – Democritus and Leucippus suggested that if we go on dividing matter, a stage will come when particles obtained cannot be divided further. Democritus called these indivisible particles atoms (meaning indivisible). How and why elements combine and what happens when they combine.  Antoine L. Lavoisier  laid the foundation of chemical sciences by establishing  two important laws of chemical combination. (a)  The Law of conservation of mass was stated by Antoine L. Lavoisier as” Mass can neither be created nor destroyed in a chemical reaction” Verification of “Law of Conservation of mass”  A solution of sodium chloride and silver nitrate are taken separately in the two limbs  of an 'H' shaped tube. The tube is sealed and weighed precisely. The two reactants   are made to react by inverting the tube.   The following reaction takes place.  AgNO3(aq) + NaCl (aq) à AgCl (s) + NaNO3 (aq)  The whole tube is kept undisturbed for some

                   Chapter 8 CELL — STRUCTURE AND FUNCTIONS for class 8 The basic structural and functional unit of an organism The basic structural and functional unit of an organism is called cell. Cells are like bricks that are assembled to make a building. Similarly, cells are assembled to make the body of every organism. This is why cell are called the basic structural and functional unit of an organism. Found on kidcourses.com Discovery of cell: Robert Hooke in 1665 observed slices of cork (a part of the bark of a tree) under a simple magnifying device. He Robert Hooke observed under a microscope there are many partitioned boxes or compartments like a honeycomb. Hooke coined the term ‘cell’ for each box. A hen’s egg can be seen easily. Is it a cell or a group of cells? It is a single cell. A white blood cell (WBC) in human blood is another example of a single cell. How do scientists observe and study the living cells? Scientists used microscopes w