NCERT Summary - Elements
Category : 8th Class
Elements
Elements
Types of Elements
(a) Metals
(b) Non-Metals
(c) Metalloids
COMPOUNDS
Types of Compounds
(a) Organic compounds
(b) Inorganic compounds
MIXTURES
Types of Mixture
(a) Homogeneous
(b) Heterogeneous
(a) A mixture is said to be homogeneous if its composition is uniform throughout.
(b) A mixure is said to be heterogeneous if its composition is not uniform.
Separation of Mixtures
(a) Sublimation: In this process, a solid substance passes directly into its vapours on application of heat, and when vapours are cooled, they give back the original substance, e.g., iodine, napthalene, benzoic acid, etc.
(b) Sedimentation and decantation: This method is used when one component is liquid and the other is insoluble solid.
(c) Crystallisation: This method is based on the difference in solubility of the various compounds in a solvent, e.g., mixture of \[KN{{O}_{3}}\] and \[NaCl\] can be separated by this process.
(d) Filtration: It is used for quick and complete removal of solid suspended particles from a liquid (or gas) by passing the suspension through a filter.
(e) Evaporation: In this method the solution is heated so that the solvent vapourises to give the solute (solid substance), e.g., salt can be obtained from salt solution.
(f) Distillation: A mixture of two substances, only one of which is volatile, can be separated by this process, e.g., \[NaCI\]can be separated from water by distillation.
(g) Fractional distillation: This process is used if both the components of a mixture are volatile. It is based on the difference of boiling points, e.g., the various fractions of crude petroleum can be separated by this process.
(h) Steam distillation: It is used to separate a liquid (should be immiscible with water) from a mixture by heating with steam, e.g., lemon oil, toluene, nitrobenzene, etc.
(i) Mechanical separation: Two immiscible liquids can be separated by using a separately funnel, e.g., oil and water.
(j) Magnetic separation: Two solids one of which is magnetic substance can be separated by this method.
(k) Atmolysis: In this process a mixture of gases can be separated based on their rates of diffusion, e.g., Isotopes of uranium \[({{U}_{235}}\,and\,{{U}_{238}})\] are separated by converting them into gaseous \[U{{F}_{6}}\].
(I) Chromatography: This is the most versatile separation method which can be applied to solid, liquid or gas. In this method the components of a mixture are adsorbed on a suitable adsorbent at different rates and thus get separated, e.g. to separate coloured materials from dyes, green vegetables, ink, etc.
Atom
Atomicity
MOLECULE
Atomic Weight (or Atomic Mass)
\[\therefore \] Atomic wt. of an element
= Average wt. of one atom of the element\[\times \]12 wt. of one atom of \[{{C}_{12}}\]
Atomic Mass of Some Common Elements
Elements |
Atomic Mass |
Element |
Atomic Mass |
Hydrogen |
1 |
Sulphur |
32 |
Helium |
4 |
Potassium |
39 |
Carbon |
12 |
Iron |
56 |
Nitrogen |
14 |
Copper |
64 |
Oxygen |
16 |
Zink |
65 |
Sodium |
23 |
Uranium |
238 |
Molecular Weight
\[\therefore \] Molecular weight of an element or compound
= Average weight of one atom of the element or compound \[\times \] 12 wt. of one atom of \[{{C}_{12}}\]
Mole (Or Mol)
\[1\,mole\,=\,6.022\times {{10}^{23}}amu,\]
Here, \[6.022\times {{10}^{23}}\] is known as Avogadro’s number
= 1 gram = \[6.022\times {{10}^{23}}\] amu.
EQUIVALENT WEIGHT
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