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Reactions. 1.

posted Jan 29, 2015, 7:08 PM by Ranmini Perera   [ updated Aug 20, 2019, 11:27 AM by Upali Salpadoru ]
In chemistry a reaction is a process
by which matter changes.

We often change matter.

W?hat is matter?

Anything or any substance that has
a mass can be considered as matter.  

Non matter includes different forms of energy such as heat, light,  and electricity.

What matter does the picture show?

 There is a boy , he is wearing a cotton shirt.
 He is holding a metal spoon. The spoon contains some kind of food.   He is inside a room. The room is filled with air.
 Matter has mass and consists of Atoms and Molecules.

  In chemistry we study three kinds of changes. They can also be described as 'reactions'.
   They are 1. Physical changes, Chemical changes and Nuclear changes.

 I.    Physical changes
 After a physical change the characteristic properties of the original substance, such as the boiling point, freezing point, the ability to react with other substances, under identical conditions, will remain the same.  

1.Crushing or grinding.  . 2. Electrical conduction.  3. Magnetising.        

4. Changes of state.( Melting,vaporation etc,.)


Actions like cutting, grinding, or squeezing,  change only the shape of the individual parts or  portions.  The substance remains the same.

 Please note:-
It is not correct to say that the physical changes can easily be reversed. 
For example ground coffee cannot be brought back to coffee beans.
    For further facts click:- Equations.3. Equations.

  2.  Chemical changes.
 Here the molecules break up or bond in a different way.


When an electric current is passed through acidic water, we get the two gases Oxygen and Hydrogen.

More examples.

1.Rusting of iron.     2. Burning.

3. Fermentation of sugar to alcohol.  4 Boiling an egg.

 5. Charging or Discharging a battery.

These changes are usually described, employing equations.  

 3. Nuclear changes

1.Radium giving heat and light.       88Ra226 – 2He486Rn222  

     2. Sun giving out solar wind and radiation.

     3.Disintegration of radio active materials at nuclear power plants.

     4. Atomic explosions  

     5. Using of radioactive materials for medical ailments.

  Is there a difference between Mixing and Dissolving.

     Dissolving is a special type of mixing. 

     Dissolving should produce a solution.

  What is a solution?

    When you add salt powder to water the solid part disappears in the liquid.

                                   SALT      +          WATER                 SALT WATER. 

   That is...................   Solute   +        Solvent     →         Solution.

   Dissolving produces a homogeneous product.

                                                                       Dissolving and Mixing.



Dissolving gives a solution.

Properties of a solution.

A solution is a Homogeneous mixture.

The solid part cannot be separated by filtering or sedimentation.

A solution is clear but may have a colour.

Light can pass through a solution without scattering.

A solid Mixing in liquids.

The product is a a Heterogeneous mixture.

In a heterogeneous product the ingredients can be seen through a microscope.

The liquid is foggy.

Solid part in a liquid can be separated by filtering or sedimentation.


Sugar + Water =Syrup

Rubber + Petrol = patch solution.

(Iodine   + Alcohol    =  Tincture.


Green leaves+onion+salt +lime   = salad.

Coffee + Milk = Cappuccino.


 Chemical Changes can be represented by equations.
These changes are usually described, employing equations.  

Some of the information  given by them in this chapter are explained by this diagram.

Classifying Chemical Reactions.
Chemical changes are vivid and varied. There are many ways of classifying them. 
Here is a simple way to do that using a basic knowledge of chemistry.

A:-Chemical combinations (synthesis).     

A letter denotes a particular chemical substance.  
Type                     A  +   B  =  AB
Experiment :-       Ignite a piece of Magnesium wire.
What to expect:-  It burns producing a brilliant light resulting in a white powder.
  When ignited the metal combines with Oxygen in the atmosphere producing a new compound called Magnesium oxide. This reaction is exothermic (releases heat energy) which makes the surrounding molecules glow.

 This is an example of burning or combustion.
Word equation:-    Magnesium  + Oxygen =   Magnesium oxide.
Using symbols:-      2Mg(s)       +  O2 (g)     =    2 MgO (s)

Other examples:-
                              CaO(s) + CO2 (g)  =  CaCO3(s)     

        Thereaction is slow at low temperature but heating will  reverse the reaction.

                              S (s) + O2 (g)  =  SO2 (g)             (Combustion)
                             C2H4 (g)  + 3 O2 (g)   =   2 CO2 (g) +  2 H2O (l)   (Combustion)

 B:-   Chemical decomposition.
Type:-     AB  =  A   +    B
Experiment: Electrolysis of water.
Method:-  Using a 9 V battery pass an electric current as shown here.

6 Electrolysis of wateer.

Word equation:-      Water   =   Hydrogen   +   Oxygen
Using symbols:-        2H2O(l)  =     2H2 (g)    +      O2 (g)
A few other example.

                                2 KMnO4 (s) = (Heat)    K2MnO4 (s)  +  2 O2 (g)
                                     2  KClO3 (s)  =    2 KCl (s)  + 3  O2 (g)    

                   (A catalyst can speedup)

C:- Decomposition yet reversible.
Experiment:-   heating ammonium chloride. 

Method:-Heat some ammonium chloride in an evaporating dish . Place a funnel as shown                                                                          

What to expect:-
                       A white substance condenses in the cooler parts of the funnel.
Explanation:- The white substance is ammonium chloride. Due to heat the salt decomposes as follows:-
Word equation:- 
                                    Ammonium chloride =  Ammonia  +  Hydrogen chloride
 On cooling they combine again.
                                     Ammonia  +  Hydrogen chloride =  Ammonium chloride
Reversible reactions:  
                                   NH4Cl (s)           ↔     NH3 (g)           +  HCl (g)   

                               N2O4        2    NO2   (High temperature shifts to right)            
                                 H2O(l) +  CO2 (g)    ↔        H2CO3 (l) 
(Low temperature and high pressure shifts to right)    
( Yhis really is a border line case, between physical and chemical)

D:-  Displacement
Type:-                               AB  +   C    =  AC  +   B
Experiment:-                    Heat a mixture of lead oxide and magnesium, powder in a crucible..

          WARNING:-  Wear tinted safety glass and stand back.
What to expect:-             There will be a vigorous reaction producing a brilliant light.

E:- Double displacement
Type:-                              AB  +  CD  →  AC  +  CD
Add a few drops of Barium chloride to a solution of sodium sulphate.
What to expect:-  A white precipitate.
Mixing produces an insoluble substance.
Barium chloride + Sodium sulphate →  Barium sulphate + Sodium chloride.

          BaCl2 (aq) +   Na2SO4 (aq)    → BaSO4 (s)  ↧    + 2 NaCl (s)

Some other ways of categorising chemical reactions.

1.     Acid + Metal reactions.     

           Zn(s) + 2 HCl(aq) → ZnCl2 (aq) + H2 (g)

2.     Acid + alkali reactions     

           NaOH (aq)   + HCl (aq)  →  NaCl (aq)  +  H2O (l)

3.     Acid + salt reaction.

           2 HCl (aq) + CaCO3 (s) →  CaCl2 (aq)  + H2CO3

           H2CO3 (aq)   →   H2O (l)    + CO2(g)

4.     Salt + Metal reactions

              Fe  +  CuSO4(aq)   → FeSO4(aq)   + Cu (s)

5.     Salt +  Salt reactions

              2 NaCl(aq) + AgNO3(aq)  → AgCl↧ + Na2NO3(aq)  

6.     Exothermic reactions

   C6H12O6  +    6  O2=   6 CO2  + 6 H2+ Energy   (Respiration reaction)

7.     Endothermic reactions

    6 CO2  + 6 H20  + light =   C6H12O6  +    6  O2          (Photosynthesis)

8.     Oxidation and reduction reactions

                       3  C (s) +  2 Fe2O3(s)    =  4  Fe(s)   +  3 CO2 (g)

Oxidation originally meant combining with oxygen or removal of hydrogen. Now it has been extended to other reactions.

A good way to classify them is by examining the transfer of electrons. Donating of electrons is Oxidation while gaining of electrons is Reduction.

How to remember.                     Think of an OIL RIG.

 OIL     RIG  Oxidation Is Lending       and    Reduction is Gaining.   of   e -

Fig. 7. Reactions in a simple cell producing a current.

Anodic Reaction   ......              Oxidation

 Zn =  Zn++ 2 electrons.          

Zinc ions go into solution.              

Electrlyte  dil. Hydrochloric acid ionises as …...

                           HCl    ↔  H+  +  Cl-

Cathodic reaction   .....           Reduction

                           2H+  + 2 electrons  =  H2 (g) 

From question 1. to 6.     
    What type of a reaction  is illustrated by the picture shown? 
    A. Physical change.    B. Chemical change.      C. Nuclear change. 
    D. No change.

 1. Using a mixer.         2. Burning magnesium.    3. Magnetizing iron.

 4. Pounding yam.                     5. Sun producing heat.        6. Mixing baking powder and vinegar.

 From Q.7 to Q.10.

 7.  Lylie added some soil into a test tube and stirred well. She observed that the resulting mixture to be cloudy while some settled down as a deposit. 
 What could be her best inference,referring to diagrams 1 and 2.
A. Soil is soluble in water   B. Soil is not soluble in water. C. A chemical change has taken place.
D. A physical change has occurred.

8.According to result shown in diag. 3, what is the best inference can you get?
A. Soil is insoluble.  B. Soil is insoluble or partly soluble. C. Soil is soluble. D. Only water goes through a filter paper.

 9.  A drop of the filtrate was placed on a clear glass slide and slightly heated. There was a white residue left on the glass . What inference can you take now?
A. Soil is insoluble.  B. Soil is insoluble or partly soluble. C. Soil has a soluble substance. D. Filtrate is heterogeneous.

Find the incorrect  statement regarding  this current generating cell.
A.  Zinc oxidizes.  B  Carbon oxidizes.  C. Zinc ionizes. D. Zinc dissolves.

   ANSWERS.   1- A,  2- B ,  3. -A,   4. - A,  5.- C,   6.- B,  7, - D,  8. - B  9.- C,  10.- B.

     Highlight to get the answers.