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CBSE Class 11th Chemistry Notes for Chapter 1 Some Basic Concepts Of Chemistry

Matter exists in three physical states i.e. solid, liquid and gas.

  • Solids have definite shape and definite volume
  • Liquids have definite volume but not definite shape. They take the shape of the container in which they are placed.
  • Gases have neither definite volume nor definite shape. They completely occupy the container in which they are placed.

Matter can be classified into mixtures and pure substances.

  • Mixtures: A mixture contains two or more substances present in any ratio. Mixtures can be of two types:
    • Homogeneous mixture: The mixture in which components completely mix with each other and its composition is uniform throughout is called homogeneous.
    • Heterogeneous mixture: The mixture in which the composition is not uniform throughout and sometimes different components can be observed is called a heterogeneous mixture.
  • Pure substances: A pure substance has a fixed composition. Also the components cannot be separated by simple physical methods. Pure substances can be further classified into:
    • Elements: An element consists of only one type of particles. These particles may be atoms or molecules.
    • Compounds: When two or more atoms of different elements are combined, the molecule of a compound is formed.

Properties of matter can be classified into:

  • Physical properties: These are the properties which can be measured without changing the identity or the composition of the substance. E.g.: Colour, odour, melting point, etc.
  • Chemical properties: To observe chemical properties a chemical change is required. E.g.: acidity, basicity, combustibility, etc.

MASS: Mass of a substance is the amount of matter present in it.
WEIGHT: Weight of a substance is the force exerted by gravity on an object.
VOLUME: Volume is the amount of space that a substance occupies.
TEMPERATURE: Temperature is the measure of the degree of hotness or coldness of the substance.
Precision refers to the closeness of various measurements for the same quantity.
Accuracy is the agreement of a particular value to the true value of the result.
Significant figures are meaningful digits which are known with certainty. The rules to determine the number of significant figures are:

  • All non-zero digits are significant.
  • Zeros preceding the first non-zero digits are not significant.
  • Zeros between two non-zero digits are significant.
  • Zeros at the end or right of a number are significant provided they are on the right side of the decimal point.
  • Exact numbers have an infinite number of significant digits.

The rules for rounding off numbers are:

  • If the rightmost digit to be removed is more than 5, the preceding number is increased by one.
  • If the rightmost digit to be removed is less than 5, the preceding number is not changed.
  • If the rightmost digit to be removed is 5, then the preceding number is not changed if it is an even number but it is increased by one if it is an odd number.

Law of conservation of mass: It states that matter can neither be created nor destroyed.
Law of definite proportions: It states that a given compound always contains exactly the same proportion of elements by weight. It is also known as Law of definite composition.
Law of multiple proportions: It states that if two elements combine to form more than one compound, the masses of one element that combines with a fixed mass of the other element, are in the ratio of small whole numbers.
Law of gaseous volume: This law was given by Gay Lussac. It states that when gases combine or are produced in a chemical reaction they do so in a simple ratio by volume provided all gases are at the same temperature and pressure.
Avogadro’s Law: It states that equal volumes of gases at the same temperature and pressure should contain equal number of molecules.
Dalton’s Atomic Theory: This theory proposed the following:

  • Matter consists of indivisible atoms.
  • All atoms of a given element have identical properties including identical mass. Atoms of different elements differ in mass.
  • Compounds are formed when atoms of different elements combine in a fixed ratio.
  • Chemical actions involve reorganisation of atoms. These are neither created nor destroyed.

The atomic mass of an element is expressed relative to carbon-12 isotope of carbon which has an exact value of 12.
In a balanced chemical reaction, the coefficients indicate the molar ratios and the respective number of particles taking part in a particular reaction. The quantitative study of the reactants required or the products forms is called stoichiometry. 

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