Physical and chemical changes are very important in our lives. Every day we see many changes in our surroundings. These changes may occur one or more substances. For example, you are making a sugar solution by dissolving some sugar in water.
Making a sugar solution is a kind of change. Sometimes milk undergoes sour. Souring of milk represents a change. Similarly, the setting of curd from milk is a kind of change.
In this tutorial, we will understand the types of changes occurring in any matter, substance, or chemical compound.
Physical and Chemical Changes
There are two types of changes occurs in any matter, substance or chemical compound. They are:
- Physical change
- Chemical change
What is Physical change or process?
A physical change is a kind of change in which a substance changes its physical state or properties but keeps its chemical identity or properties. In other words, physical change is a change which takes place without a change in actual composition and in the chemical nature of the substance.
A change in the physical properties such as colour, shape, size, odour, solubility etc., and change in the state of a substance (i.e. from solid to liquid or liquid to gas) are examples of physical change.
When physical changes occur in a substance, then no new substance forms. The substance will remain in its original state. During a physical change, the actual composition, mass, and the chemical nature of the substance remain the same.
That is, a physical change doesn’t affect the actual composition, mass, and chemical properties of any substance or matter. It affects only physical properties temporarily. During the physical change, molecules rearrange themselves without affecting the actual composition and mass.
The inter-convention of states of any matter is a physical change. For example, a change of water in ice is a physical change because water keeps all of its chemical properties, whether it is in the form of ice or water or steam.
Chemically, liquid water, ice or steam all are the same, but display different physical properties. Ice melts to form water and water converts again into ice when placing it in a freezer. When 20 g of ice melts, 20 g of water is got.
Familiar Examples of Physical Change
(a) Transformation of water into ice and vice versa is a physical change because of the change of temperature. For example, the melting of ice into water on heating is an example of physical change.
(b) Cutting, bending, dissolving, evaporation of water, condensation, freezing, sublimation, distillation, boiling and melting are some familiar examples of physical changes.
(c) When we draw a piece of metal to make wire changes only in its physical features, the metal is still the metal itself.
(d) When we break a piece of chalk into smaller pieces, is a physical change.
(e) Passing of electric current through metallic conductor is also an example of physical change. When we pass an electric current through the filament, then the filament becomes hot and emits light. On cooling, the filament remains unchanged or the same.
(f) Dissolving a solute like a salt or sugar in a solvent like water is a kind of physical change.
(g) Making of a magnet from an iron piece is a kind of physical change.
(h) Effect of heat on zinc oxide is another good example of a physical change. Generally, when most compound heats, there is a change in their chemical composition that results in chemical change. But, when zinc oxide heats, there is only change in color and it becomes yellow. When zinc oxide cold, it becomes white. In both changes, zinc oxide remains the same.
(i) Tearing a paper or breaking of glass is also an good example of physical change.
What is Chemical Change or Chemical Reaction?
A chemical change is a type of change in which the chemical properties and composition of substance change and a new substance form. It is also commonly called chemical reaction. In chemical changes, one substance transforms into one or more other substances.
In other words, one substance reacts with another substance and forms one or more new substances. The substance that transforms is called reactants and new substances formed are called products.
The chemical properties and composition of new substances are entirely different from that of original substance. For example, we are very familiar with rusting of iron. When you leave a piece of iron in the open for sometime in the moisture and atmospheric, it gains a film of brownish substance. This substance is nothing but rust and the process is called rusting.
Another familiar example of chemical reaction is a burning of a block of wood producing smoke and ash.
Other familiar examples of chemical changes are burning of coal, souring of milk, digestion of food in our body, etc.
Most of the chemical changes are irreversible, meaning that the obtained substance cannot be changed back to its original substance. For example, when you burn a paper, it liberates heat and light. A completely new substance called ash forms after burning. You can not retrieve back the original paper from this ash.
These changes occur because of the absorption or evolution of light and heat. It means that there is an effective gain or loss of energy.
When a chemical change takes place, bonds between the molecules break and the mass or composition of the substance change. In other words, chemical reaction leads to the breaking and making of bonds.
Some Examples of Chemical Changes
(1) When a piece of iron dips into a solution of copper sulphate, iron is coated with copper.
- Fe + CuSO4 = FeSO4 + Cu
In this change, a completely new substances form. This change occurs because iron has been placed above copper in the metal reactivity series and therefore, it displaces copper from copper sulphate.
(2) When a piece of magnesium metal is heated, it burns and converts white ash on cooling. The white ash is nothing but magnesium oxide, which is totally different from the original magnesium metal.
- 2Mg + O2 = 2MgO
(3) When some sulphur is heated on a spoon, it first melts and then begins to burn with a bluish flame. It gradually decreases in quantity and finally disappears from the spoon as gaseous sulphur dioxide. You can identify with its pungent odour.
- S + O2 = SO2
(4) Combining iron and sulphur, forming iron(Ⅱ) sulphide is a kind of chemical change.
(5) Burning of coal releases heat and light. It is an example of chemical change.
(6) Rusting of iron in the presence of moisture and atmosphere is a kind of chemical change.
(7) When sulphur dioxide is passed through potassium dichromate, the colour of potassium dichromate solution changes from orange to green. This is a kind of chemical change.
Types of Chemical Changes with Examples
There are various types of chemical changes or chemical reactions. Some of the important ones are as:
(a) Combination: When a direct reaction between elements occurs, a compound is produced and is called direct or synthesis. For example:
- H2 + Cl2 = 2HCL
- C + O2 = CO2
- 2Mg + O2 = 2MgO
(b) Decomposition: When a compound decomposes or break down into two or more simple constituents, the change is called decomposition. Heat energy is required for the decomposition. Such decomposition is called thermal decomposition. For example:
- 2HgO = 2Hg + O2
- 2KCIO3 = 2KCI + 3O2
- CaCO3 = CaO+ CO2
- 2NaHCO3 = Na2CO3 + H2O+ CO2
(c) Substitution: When one element enters into a compound by the replacement of the other elements, the change or reaction is called substitution reaction. For example:
- CuSO4 + Zn = Cu + ZnSO4
- C2H6 + Cl2 = C2H5CI + HCI
- 2KI + Cl2 = 2KCI + I2
- Zn + 2HCI = ZnCl2 + H2
(d) Addition: In addition change or reaction, two or more substances combines to form a bigger chemical substance without elimination. For example:
- KI+I2 = KI3
- C2H4 + HBr = C2H5Br
(e) Internal rearrangement: When various atoms present in a molecule rearrange, a new compound forms. In this rearrangement, nothing is added or nothing is eliminated from a chemical substance. For example, when ammonium cyanate is heated, a new substance urea is formed.
- NH4CNO = NH2CONH2
The chemical change or reaction is called isomerisation when one isomer converts into another.
(f) Polymerization: When two or more molecules of a substance combine, they form a giant molecule. For example, three acetylene molecules combine, they form a benzene.
- 3C2H2 = C6H6
Features of Physical and Chemical Changes
There are so many important features of physical and chemical changes. They are as:
(1) When a physical change takes place in any substance, no new substance forms. The substance will remain unchanged, i.e. in its original state. There is only a temporary change of state or physical properties.
(2) When chemical change occurs in the substance, a new kind of substance forms. It means that the substance loses its original state.
(3) A physical change is temporary and can possibly be reversed by any physical means. Whereas, a chemical change is complete and permanent. We cannot reverse it.
(4) Physical change takes place faster and sometimes instantaneously. On the other hand, most chemical changes take a longer time.
(5) During the physical change, there is no transformation occurring in the original molecular composition of substance. But with chemical change, molecular structure transforms, thus we get a new substance or matter.
(6) A chemical change may also cause a physical change, but a physical change alone cannot lead to a chemical change.
(7) Physical reaction can or cannot be initiated, but chemical reactions start only after they are initiated.
(8) In physical reaction, energy changes in small as compared to a chemical reaction.
(9) Chemical changes occur in molecular level while physical changes are concerned with energy and states of a substance.
Difference between Physical and Chemical Changes
There are many differences between physical, and chemical changes that are listed in the below table.
| Physical Change | Chemical Change |
|---|---|
| 1. When a physical change occurs in a substance, its composition and mass remains unchanged despite its molecules being rearranged. | 1. When a chemical change occurs in a substance, its composition and mass changed and forms a new substance. |
| 2. Physical change undergoing a substance is a temporary change. | 2. Chemical change undergoing a substance is a permanent change. |
| 3. No new substance is formed during the physical change. | 3. One or more new substance is formed during the chemical change. |
| 4. It affects only physical properties such as shape, size, colour, etc. | 4. It affects both physical and chemical properties of the substance including its composition and mass. |
| 5. During a physical change, there is a very little or no absorption of energy. | 5. During a chemical change, absorption and evolution of energy occurs. |
| 6. Generally, there is no production of energy in the physical change. | 6. During the chemical reaction, there is usually the production of energy which can be in the form of light, sound, heat, etc. |
| 7. Most of the changes are easily reversible, i.e. we cannot recover the original substance. | 7. Most of the changes are irreversible, i.e. we can recover the original substance. |
| 8. Some familiar examples of physical change are the melting of ice, melting of wax, freezing of water, boiling of water, etc. | 8. Some familiar examples of chemical change are digestion of food, burning of coal, rusting of iron, etc. |
FAQs on Physical and Chemical Changes
Q. What is a physical change?
A process in which a substance experiences a changes in its physical properties but keeps its chemical properties is called a physical change. Physical properties are shape, size, colour, and state of a substance. A physical change is generally reversible process. In such a change, there is no new substance formed.
Q. What is a chemical change?
A change in which a substance reacts with another substance and forms one or more new substances is called chemical change. It is also called chemical reaction. It is very important in our lives. All new substances are formed because of a chemical change.
Q. What are the two important features of physical and chemical changes?
(1) When the physical change takes place in a substance, no new substance forms. The substance remains as in the original state.
When the chemical change takes place in the substance, one or more different kind of substances form. It means that it loses its original state.
(2) A physical change can possibly be reversed; a chemical change is irreversible. It is permanent.
Q. What are two examples of physical changes that you observe in nature?
Examples of physical changes are plucking of flowers, cutting of trees.
Q. What are the two examples of chemical changes that you observe in nature?
Examples of chemical changes are burning of wood, rusting of iron.
In this tutorial, you have known about physical and chemical changes with different kind of examples. Hope that you will have understood the basic points of physical and chemical changes.
Thanks for reading!!!
