Chemical reactions & Equations best class 10

Chemical reactions

1. What is chemical reactions?

Chemical reactions

  • Chemical reactions are the processes in which new substance with new properties are form which involve chemical changes.
  • It involves breaking of old chemical bonds which exist between the ate of substance which act to form new chemical bonds.
old bonds Chemical reactions
New bonds Chemical reactions

During a chemical reactions, atoms of one elements do not change in those of another elements.

Ex.- Hydrogen react with oxygen to form water. -> 2H2+O2 ⟶ 2H2O

2. what is Reactants?

The substance which takes part in Chemical reactions are called reactants

3. What is Products?

The new substance produced as a result of Chemical reactions.

Ex. C(solid) +O2(gass) ⟶ CO2(g) ~Here reactants are trans into products.

The products thus formed have propenties entieny differents from those of the reactants in Chemical reactions.


Magnesium ribbon experiment –

  1. Before experiment please note that magnesium is silvery while It is available in the form of magnesium ribbion or wire
  2. When magnesium ribbon is heated, it burns in air with dazzling white flame to form white powder called magnesium oxido.
  3. actually on heating magnesium combines with oxygen present in air air 2 form magnesium oxide


Magnesiam + Oxygen ⟶  Heat  –> Magnesium oxide (As ribbon) (fromair) (white powder)

In these magnesium and oxygen are reactant, but only one product MgO2 (magnesium oxide)

  1. The magnesium nibbon which we use usuallyhas a coating of magnesium oxide, on its surface which is formed by slow action of oxygen of air on it.

Que – why the magnesium ribbon is cleaned by rubbing with sand paper be fore buming in air? ( 1 mark)

→. This is done to remove the protective layer of magnesium oxide from the surface of magnesium nibbon before buming in air.

  • During burning the dazzling while light given out during the burning of mognesium ribbon is harmfal to eyes.
  • so, the magnesium ribbon should be bumed by keeping it far as possible from the eyes.


  1. Take about 2 cm long magnesium ribbon and clean it by nubbing its surface with sand paper.
  2. Hold mg nibbon with a pair of tongs at one end, and heats its others end over a bumer.
  3. The magnesium ribbon start buming with dazzling white flame.
  4. Hold the buming magnesium ribbon over a watch glass so that the magnesium oxide powder calect in the watch glass.
  (B) magnesium ribbon burns in airs to form magnesium oxide. 1. Souring of milk Chemical reactions
(A) magnesium ribton being heated over a burner Chemical reactions

Characteristics of chemical Reactions-(2m)

Imponant characten’stics of chemical mx are-

  1. Evolution of gas.
  2. Formation of a precipitate
  3. change in colour
  4. change in temperature
  5. change in state

1. Evolution of Gas –Chemical reactions

some chemical reactions are characteristics of some substance that react and evolve gas with some product.

  • Example – When zinc granules react with Dilute sulphurio acid, then the bubbles of hydrogen gas produce.


  1. Take some zinc granule in conicas flask, and add zn over dilute sulphuric acid. or dilute Hol.
  2. We will see the bubbles of hydrogen gas formed around zn granules. 
  3. By touching flask, we can find it is somewhat hot, here change in temperature also occurs in chemical reaction.


-Example- 2 – when magnesium react with dilute acid (H2SO4 or HCl), then hydrogen gas evolved.

 – Mg+HCl⟶MgCl+H2 (evolve) 

Chemical reactions

Example – 3 when sodium carbonate and dilule Hydrochloric acid is characteri by evolution of CO2 gas

What is Precipitate?

  • Precipitate is a ‘solid state’ which seperates out the from the solution during a chemical reaction.
  • precipitate usually seen under testube at bottom pait.
  • It can form by passing a gas in aquears solution of a substance (like passing cabon dioxide gas into lime water)
  • Formation of insoluble solid precipitate is called Precipitation. 2. Formation of Precipitate-

Example – When potassium iodide solution is added to solution of lead nitrale, then yellaw precipitate of lead io dide is formed.

Experiment – we can camy out this chemical reaction as follows –

  1. Take some lead nitrate solution in a test tube
  2. Add potassium iodide solution to it.
  3. A yellow precipitate of lead iodide is formed at unce.
  4. A change in colour (colourless to yellow) also takes in this chemica) reactions.

 – Pb(NO3)2(aq) + 2Kl(aq) ⟶ PbI2+2KN3(aq)

Example2 – When sulphuric acid and Barium chloride solution is formed by a white precipitate of Banium sulphate.

  • H2SO4+BaCl2⟶BaSO4+2HCl

3. Change in colour –

Example – when citric acid reacts with potassium permaganate solution, then the purple colour of potassium permanganate solution, disappears (it becomes colourless).

  • Purple ⟶Δ colourless  3KmnO4+C6H8O7⟶3HMnO4+C6H5 K3O7 

Experiment – we can camy out this reaction as follows-

  1. Take some dilute potassium permanganate solution in test tube. It has purple colour.
  2. Add some lemonjuice with helpof dropper and shake it.
  3. The purple colour of potassium permanganale solution goes on fading and ultimately it becomes colourless. 

Example 2 – when sulphur dioxide gas is passed through acidified potassium dichromoate solution by change in colour from orange to green.

SO2+K2Cr2O7+3H2SO4⟶K2SO4+Cr2(SO4)3 +3H2O

  • Change in Temperature–Exothemic reaction.

Example – when quicklime react with water, then slakedlime is formed and lot of heat energy is produced by a change in temperature. It is a exothermic reaction which means heat producing reaction.


  1. Take little of quicklime in hard glass beaker and add water to it slowly.
  2.   Touch the beaker carefully, the beaker feels to be quite hot its temp is high.
  • Calcium oxide + water ⟶ calcium hydroxide
(b.) Jhe beaker becomes hot It temp. rises Chemical reactions
(a) quicktime reacts with water to form slacked line release a lot of heat Chemical reactions

Example 2 – The carbon bums in the air to form canbondioxide which also releases a large amaint of heat.

(carbon)C+O2 (oxygen) ⟶  CO2 (carbondioxide )

  1. Endothermic reaction. The reaction which absorbs the heat are called as Endothemic reaction:

Example- The reaction between barium hydroxide and ammonium chloride to form Banium chlonide, ammonia and water by change in temperature.

Ba(OH)2+2NH4Cl⟶ΔBaCl2+2NH4OH↓(absrb) heat

4. change in state-

Example – When wax is bumed, then water and carbondioxide are formed 

now, wax is solid, water is a liquid whereas oarbondioxido is a gas.

  • This means that during the combustion reaction of wax, the physical state changes from solid to liquid and gas.
  • The combustion of candle wax is characlerised by chanoe in state.


  • The method of representing a chemical reaction with help of symbol and formula of the substance invalve in it known as chemical Equations.
  • for example- zinc metal react with dilute sulphunio acd to form zinc oulphate and hydregen gas. this mx can be writlen as-
  • zinc + sulphuric acid ⟶ Zinc sulphate + Hydrogen
  • shis is known as the word equation.
  • we can change the word equation in to symbol and formula of various substance.
  • Putting the symbol and formulae of all substance in above word equation


  • Here, zine and sulphurio acid are reactant. and reactant are always written on the left hand side in an equation with plus( (+) sign between them.

-Here, zinc sulphate and hydrogen are products, and it is always written on the right hand side in an equation with plus sign between then .

  • The arrow sign (⟶) pointing towards the right hand side is put between the reactant and products.
  • Arrow also indicate that the substance written on the left hand side are combining to give substances written on the right hand side in the equation.

chemical equation is a shorthand method of re presenting a chemical mx.

balanced and unbalanced chemical equations:-

1. A balanced chemical equation has an equal number of atoms of different elements in the reactant and products.

Example – Zn+H2SO4⟶ZnSO4+H2

  • In other words, a balanced equation has equal number of atoms of diff. elements on both the side must be equal

Now, Let us count the number of atoms of all the elements in the reactant and product sepretely.

.In reactantIn product
No. of. zn. atoms 11
No. of. H. atoms22
No. of. S. atoms11
No. of. O. atoms44
by self

Since, above there is an equal number of atoms of different elements in reactant and products is equal, so above equation is balanced equation.

  1. A unbalanced chemical equation has unequal number of atoms of one or more elements in the reactant and product.
  • Example – Hydrogen reacts with oxygen to fom water, this point will written as –


  • Let us count the number of hydrogen atoms and oxygen atoms in the reactant and product.
. In reactantIn product
No. of. H. atoms 2 2
No. of. O. atoms 2 1
by self
  • In these equation thus, the no. of hydrogen atoms are equal, but the oxygen atoms are not equal on both side of reactant and product. so it is called un equal or unbalanced chemical equation.
  • Reactant = Product (always)

NoteThe chemical equation should be balanced because it satisfy the law of conservation of mass in a reaction

Some rules about the balancing the equation –

We should never change the formula of an element or a compound to balance the equation. 

2. We cOD only multiply a symbol or formula by Figures like 2,3 and 4 

3. It will be good to note here that here the elements which exist as diamotic molecules are oxygen 02, hydrogen H2, nitrogen N2, Fluorine F2, chlonine cl2, bromine Br2 and iodine I2.

4. All other element are usually considered monoatomic in equation writing and reprensled by their symbols.

Balancing of chemical Equation-
  • the process of making the number of different types of atoms equal on both the side of the equation called balancing of equation.
  • The simple equations are balanced by hit and trial method.
Example of the balancing the equations –
  • Hydrogen bums in oxygen to form water. The reaction can be writlen in an equation.

  – H2+O2⟶H2O

  • count the number of hydrogen and oxygen atoms in reactant and product.
.In reactantIn product
No. of. H. atoms 4 4
No. of. O. atoms 2 2
  • The no. of H2 atoms is equal on both side, but the number of oxygen atoms are unequal.
  • There are 2 oxygen atoms on left side and 1 Oxygen atoms on right side to have 2 oxygen atoms on the right side, we multiply H2O by 2 and write 2H2O so that:

  – H2+O2⟶2H2O

  • Though the number of oxygen atoms has become equal, but the no. of hydrogen atoms has become unequal.
  • There are 2 atoms hydrogen atums on the left side but 4 hydrogen atoms on the right side., we multiply it H2 by 2 and write 2H2 :

  – 2H2+O2⟶2H2O  –This chemical equations contains equal no. of atoms of hydrogen and oxygen on both the sides. So this is balanced equations. — By these way we can balanced the equations

To make Equation More Informative –
  • The chemical equations can be more in formative – by three ways –
  • By indicating the “physical change or state” of the reactant and product.
  • By indicating the “heat change” taking place in the reaction.
  • By indicating the “conditions” under which the reaction takes place.
To indicate the physical change or state of the reactant and product:-

The physical state for reactant and product of chemical reaction –

  • solid eliquid – aqueous solution – gas
  • solid state is indicated as – symbol (s).
  • liquid state is indicated as – symbol (l)
  • Aqueous state or solution is indicaled as – symbol (aq)
  • Gaseous state is indicated as – aymbol(g).
  • The physical state of the reactant and product are shown by putting the above “state symbol” just after their symbol or formulae in aneq.

FOR Example:-

zinc react with dilule sulphuric acid to form zinasulphate solution and hydrogen gas.


Here, -zinc metal is solid, so we write zn(s)

  • Dilute sulphuric acid is aqueous solution, so we can write H2SO4(aq )
  • zinc sulphale is also an aqueaus solution, 80 we can write ZnSO4 – Hydrogen is gas, oo we write hydrogen.
  • The above equation writien as-
    •  – Zn(s)+H2SO4(��)⟶ZnSO4(��)+H2 (g) 
  • This equation is more informative because it tell us the physical state of the varials substance involved in it.
  • In same case 2 – Insoluble product called precipitate is formed by the mx between solutions of reactants, and its is indicated by symbol “ppt:”
  • When calcium hydrooxide eolution (lime water) react with carbon-dioxide gas, a white precipitate of calcium carbonale is formed along with
    •  Ca(OH2(aq)+CO2(9)⟶CaCO3( s)+H2O(Q)

5. To indicate the Heat change in an Equation –

Jhere are two types of reaction on the basis of heat changes involve.

1. Exothermic Reactions- 2. Endothermic recations.

1. Exothermic Reactions-

those reactions in which heat is evolved called as Exothermic reactions. 


carbon bums in oxygen to form carbondioxide, a lot of heat is produced in this reaction.

C(s)+O2( g)⟶CO2( g)+ Heat  

The burning of carbon in oxygen is an exothermic reaction because heat is erolved in this reaction.

An exothermic reaction is indicaled by writing + Heat or + Heat energy or just “tenergy on the products side.


Que – why Buming of natural gas is an exothermio reaction? (2M)

-Natural gas is mainly methane (CH4), when natural gas bums in the oxygen of air, it forms carbondioxide and water vapour. a large amount of heat is produced.

  • Heat is mainly produce so called Exothemic reaction.
  • All the combustion reaction are exothermio reaction.
    • CH4(�)+2O2(9)⟶CO2(�)+2H2O(�)+ Heatenergy

Example 3

Que – why respiration is an exothermic neaction? Explain-

  • Itis exothermio reaction because energy is produced during this process.
  • Duning digestion rood is broken down into simpler substances.
  • She carbohydrate which we get from the foods like chappati, bread rice and potato we eat broke down into simple form called gluccse – This glucose then undergoes slow combustion by combining with oxygen in cells of our body to produce energy in a process called Respiration. these energy maintain our body heat.
  • C6H12O6+6O2(9)⟶6CO2(g)+6H2O(L)+ Energy
  • It is also an combustion reaction.,the%20product%20formation%20from%20reactants.

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