Physical and Chemical Changes
Physical Change – Physical change is a change in which only some of the physical properties of a substance undergoing change are altered temporarily but the actual composition and the mass of the substances remains the same, and no new substances are produced. The original substances can be obtained back by simple physical means.
For example –
- Evaporation of water is a physical change
- The dissolution of common salt into water
Chemical change – Chemical change refers to a transformation of the chemicals under change into new substances with entirely different characteristics (mass and composition) from the original substances. Even if the source of the transformation is removed, the original substances cannot be recovered again.
Characteristics of a Chemical Reaction –
- Evolution of gas
- Change in colour
- Change in state
- Change in temperature
Chemical Equations
The method of representing a chemical reaction with the help of symbols and formulae of the substances involved in it is known as a chemical equation.
There are two ways of representation of a chemical equation –
- a) In terms of words (called word equation)
Zinc + Dilute sulphuric acid → Zinc sulphate + Hydrogen
- b) In terms of symbols and formulae (called chemical equation)
Zn + H2SO4 → ZnSO4+ H2
Balanced Chemical Equation
A balanced reaction is one that has the same amount of atoms of each element on the reactants and product sides of the chemical equation.
The equation must always satisfy the Law of Conservation of Mass by being in balance. (total mass of all the products is equal to the total mass of all the reactants).
Step involved in the balancing of a chemical equation: [Balancing done by Hit and Trial Method]-
- Write reactants on the left side and products on the right side of the equation.
- In case of two or more reactants or products use plus (+)
- The Reactants and the Products are separated by an Arrow ( → ) sign, which points towards the products.
- First, write a word equation for the chemical equation.
- Now, replace the compound name with a chemical formula.
- Start balancing the reaction.
- Start balancing with the compound that contains the maximum number of atoms − It may be Reactant or the To equalize the number of atoms, do not alter the formulae of compounds or elements. Then, balance the rest or other remaining atoms in the same fashion.
For example – Reaction of Iron with steam.
- Step – I : Firstly write the Word Equation for the Chemical Reaction
Iron+Steam → Iron Oxide + Hydrogen
- Step- II : Replace names of the compounds with Symbols and Formulae of Reactants and Compounds.
Fe + H2O → Fe3O4 + H2
- Step- III : Balancing the Reaction
Fe + H2O → Fe3O4 + H2
Do not change anything inside the boxes while balancing the equation
List the number of atoms of different elements present in the unbalanced equation.
| Elements | Number of atoms in Reactions ( LHS ) | Number of atoms in Products( RHS ) |
| Fe | 1 | 3 |
| H | 2 | 2 |
| O | 1 | 4 |
- (c) We should always choose a compound with a maximum number of atoms in it, either on reactants side or products side.
Fe + H2O → Fe3O4 + H2
Here we select Fe3O4 which contains 4 oxygen atoms on the right hand side.
As we cannot make H2O to H2O4 in order to increase oxygen atoms, so we make it to 4H2O.
Fe + 4H2O → Fe3O4 + H2
Balance other atoms now like Fe and H.
Fe + 4H2O → Fe3O4 + H2
- Step IV:
Check the equation whether it is balanced or not by counting the number of atoms on L.H.S and R.H.S
3Fe + 4H2O → Fe3O4 + 4H2
This is the balanced equation.
- Step V: Indicate the physical state of all the reactants and products by writing symbols in brackets as subscripts on the right corner below the formula.
- (d) Atoms of H are 8 on L.H.S and 2 ON R.H.S, so multiply H2 on R.H.S with 4, so we get
Fe + 4H2O → Fe3O4 + 4H2
- (e) Fe + 4H2O → Fe3O4 + 4H2
Pick the last element Fe from the above partly balanced equation. Fe are 3 on R.H.S and 1 on L.H.S, so multiply 3 on L.H.S
3Fe + 4H2O → Fe3O4 + 4H2
What are Chemical Reactions and Equations?
- A chemical equation is a symbolic representation that describes a chemical reaction in terms of symbol and formulae.
- A chemical reaction is the transformation of chemical substance called reactants into another chemical substance called products. In a chemical reaction, only rearrangement of atoms takes place.
- The substances which take part in a chemical reaction are called reactants. The reactants are written on the left hand side. The new substances produced as a result of chemical reaction are called products. The products are written on the right hand side.
Some of the symbols used in chemical equations are :
| Symbols used in Chemical Equations | |
| Solids (s) | Liquids (l) |
| Gases (g) | Aqueous solutions (aq) |
| Solids (s) | Precipitate formed in the reaction (↓) |
| Direction of reaction (→) | Used to separate multiple reactants or products (+) |
| Formula written above the arrow is used as a catalyst in the reaction (Pt) | |
| Triangle indicates that the reaction is being heated (△) | |
| Replaces the yield sign for reversible reactions that reach equilibrium (⇌) | |
Steps to balance a chemical equation :
- (a) Write word equation.
- (b) Then write skeletal chemical equation.
- (c) Enclose the formula in the boxes.
- (d) List the number of atoms of different elements present in the unbalanced equation.
- (e) Start balancing with the compound that contains the maximum number of atoms.
- (f) Start balancing other atoms.
- (g) Check the correctness of the balanced equation.
Types of chemical equations
Characteristics of a chemical equation: (Flow chart)
Effects of oxidation in everyday life
| Keywords and Definitions: | |
| Chemical Reaction: The transformation of chemical substance into another chemical substance is known as chemical reaction. | Antioxidants: These are the substance that inhibits oxidation or inhibits reactions promoted by oxygen. |
| Reactant: A chemical substance that takes part in a chemical reaction and undergoes change during a reaction. | Catalyst: A substance that alters the rate of a chemical reaction without undergoing any change in itself during the reaction. |
| Product: A new chemical substance formed as a result of a chemical reaction. | Precipitate: The solid formed as a result of a precipitation reaction. |
| Reactivity series: A new series of metals ranked in order of decreasing reactivity to displace hydrogen gas from water and acid. | Corrosion: An irreversible damage or destruction of material in which metals are gradually eaten up by the action of air, moisture due to a chemical or electrochemical reaction on the surface of metal. |
| Oxidation: The reaction in which addition of oxygen or removal of hydrogen takes place. | Rancidity: It is the process of complete or incomplete oxidation or hydrolysis of fats and oils when exposed to air, light or moisture or by bacterial action resulting in unpleasant taste and odour. |
| Reduction: The chemical reaction which involves addition of hydrogen or removal of oxygen. | Rusting: The slow conversion of iron into hydrated ferric oxide, in the presence of moisture and air |
| Oxidising agent: A substance which helps in oxidation but itself gets reduced. | Rust: A reddish or yellowish-brown flaking coating of iron oxide that is formed on iron or steel by oxidation, especially in the presence of moisture. |
| Reducing agent: A substance which helps in reduction but itself gets oxidised. | |
| Redox reaction: A chemical reaction in which both oxidation and reduction takes place simultaneously | |
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