Class 10th Science Chapter 1 Chemical Reactions and Equations NCERT Solutions

CBSE board class 10 Science Chapter 1 Chemical Reactions and Equations textbook solutions
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Important Points to Remember about Class 10 Chemical Reactions and Equations
  1. A chemical reaction is a process in which some substances undergo bond breaking and are transformed into new substances by formation of new bonds. The substances taking part in chemical reaction are called reactants, whereas the substances formed as a result of a chemical reaction by formation of new bonds are called products. 
  2. In chemical reactions old bonds are broken and new ones are formed.  Reaction often requires energy.
  3. Energy can be given in any form such as heat, light, electricity, or mechanical reactions in mechanical energy by shaking them to create sufficient contact between the molecules of the reagent, the atoms can be rearranged in a many ways.
  4. There are many examples of chemical reactions in daily life, such as digestion of food, making yogurt of milk, ripening of fruits, fermentation of grapes for making wine, fermentation of rice flour and lentils for making Idli or Dhokla and making food.
  5. If the number of atoms of the elements in the reactants in this equation is same as the number of atoms of those elements in the products. Such an equation is called a ‘balanced equation’. 
  6. If the number of atoms of each element is not the same on the two sides of an equation, it is called an ‘unbalanced equation’.
  7. When two or more reactants combine in a reaction to form a single product, it is called a combination reaction. 
  8. The chemical reaction in which two or more products are formed from a single reactant is called ‘‘Decomposition reaction”.
  9. The reaction in which the place of the ion of a less reactive element in a compound is taken by another more reactive element by formation of its own ions, is called displacement reaction.
  10. The reaction in which the ions in the reactants are exchanged to form a precipitate are called double displacement reactions.

Different types of chemical reactions
The simple rearrangement of atoms in a reaction is as follows -

 On heating NH4CNO (ammonium cyanite) → NH2CONH2 (urea)

In this reaction, the molecular formulas of both ammonium cyanate and urea are the same, but rearrange the bonds.  These two compounds are said to be isomorphic to each other.

Wöhler, a German chemist, discovered this reaction and showed that organic chemicals can also be made from other sources and that they are not necessarily made only from biomaterials.

When a molecule breaks down into smaller parts, dissociation occurs.  H2 and O2 are produced when electric current flows in water.  This electrolysis reaction is an example of a dissociation reaction (electro means electrolyte and lysis means breakdown).

 2H2O ——– When electric current flows → 2H2 + O2

Note that the dissociation reaction is the opposite of the combination reaction.  Hydrogen gas can be formed by simply burning H2 and O2 by burning it in air.  The second example of combination reaction comes to us when we whitewash the walls with lime.  When lime is poured into water, extinguished lime is formed.

 In this reaction when Calcium hydroxide reacts slowly with carbon dioxide present in the air to form a thin layer of calcium carbonate on the walls.

 CaO + H2O → Ca (OH) 2

 Lime + Water → Quenched Lime or Calcium Hydroxide

 Ca (OH) 2 + CO2 → CaCO3 + H2O

Calcium Hydroxide + Carbon Dioxide → Calcium + Water Carbonate


 Chemical Formulas and Equations
We write formulas and chemical equations with the help of molecules, just as expressions and equations are written in algebra.  In fact, formulas and equations have a definite meaning.  Think what the formula H2O explains.

This means that a molecule of water has two atoms of hydrogen and one atom of oxygen. It shows that it has a valency of oxygen, whereas the valency of hydrogen is one. This makes it clear that the molecular mass of water would be 18.  , Which is the sum of the mass of one atom of oxygen (16) and two atoms (2) of hydrogen. This indicates that compound H2O is not electrically charged.  It is not an ion but a neutral molecule.

The chemical formula of a substance determines which elements are present in it and in what proportion. The formula O2 states that the reaction occurs with oxygen molecules.

The letter g, written next to O2, indicates that oxygen is in gaseous state.  The letter l is written for fluid and s for solid state. This equation is not written as H2 + O2 → H2O + O, why?

 Elemental oxygen, which consists of two oxygen atoms, reacts with elemental hydrogen H2.  Each molecule of water has one atom of oxygen.  But it has been observed by experiments that free oxygen is not obtained in this chemical reaction.  Therefore we balance the equation such that the number of atoms of the elements present on the left side of the equation is equal to the number of atoms to the right of the equation.

 If we did not balance the equation, we would either be generating atoms or losing them.  This is why H2 is shown as 2H2.  2 molecules of hydrogen react with one molecule of oxygen to produce two molecules of water.  Thus the equation is balanced against all atoms.

Equations 2H + O → H2O and H2 + O → H2O are also balanced equations.  But these equations are not correct, because in general, oxygen and hydrogen are not in atomic form but in molecule form. Salvation not only balances the number of reagents and atoms of the products but also their masses.  2H2 means 4 atoms of hydrogen with a total mass of 4 (this mass is taken into the unit of atomic mass).  O2 means 2 × 16 = 32 a.m.u atomic mass unit, atomic mass unit).  The total mass on the left side of the equation is 4 + 32 = 36 a.m.u.  Heat is produced in the chemical reaction of hydrogen and oxygen to form water.  This emitted energy is usually expressed differently.  The free (excreted) heat of each mole (18 g) of liquid H2O is about 286 kJ.  This reaction actually produces 572 kJ of heat, as two moles of H2O are formed.


 Balance equations

Q .How the formulas and equations are obtained.  We can accurately display chemical reactions.  What are the requirements for this task?
Answer : The correct molecule formulas of all reagents and products. The state of the reaction - is the energy given in the reaction or is the energy released from the reaction? Determine whether the equation is balanced or that the total number of atoms on both sides remains equal.