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Chemical change refers to a change that occurs and results in the formation of one or more substances and are irreversible, unlike physical change. Usually, these changes are expressed through chemical equations which aid in understanding how elements are reacting with each other. On the other hand, Physical change is the change that is related to shape, size, color, and state of matter. A matter is any substance that has a mass and occupies space. Most of the physical change is reversible as change does not alter the molecular composition, which is altered in a chemical change.
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Key Takeaways: Chemical Changes, Physical Changes, Matter, Energy, Irreversible and Reversible
Chemical Changes
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Chemical changes are the changes in which a new substance is created or formed and this change is irreversible. As they are irreversible they are permanent. In this change molecular composition is completely altered.
In this type of change, mass is changed or modified because mass is either removed or added. During a chemical change in the energy either the energy is released or gets absorbed. If energy is released then it is called an exothermic and if the energy gets absorbed then it is called endothermic reactions.
Example of Chemical Change
Rusting of iron
There are many objects like shovels, iron rods, etc. which when kept in open air, will result in a reddish-brown layer, which means it gets oxidized by the oxygen in the atmosphere.
Fe + O → Fe2O3
This is the chemical reaction that takes place during rusting of iron.
So in this process, a new reddish-brown substance is formed and it is an irreversible process. However, we can prevent rusting of iron by applying a layer or coat of grease or paint. There is another process that involves depositing the layer of metal (the layer of zinc and chromium can be applied). This process is known as Galvanization.
Ozone layer
A protective layer that protects us from the harmful UV rays from the sun. In this layer, some chemical reactions are responsible to convert 02 to 03. This O3(Ozone) plays a crucial role to protect us from the harmful effects of UV rays.
Burning of Magnesium
If you burn a magnesium ribbon then it would be burned with a bright white light. In this process, magnesium reacts with oxygen to form Magnesium oxide. Therefore a new substance is formed and the ribbon would be turned into ashes and cannot return to its original state hence it is an irreversible process.
Mg + O → MgO (Magnesium Oxide)
Further, if in these ashes water would be added then it would be responsible for the formation of magnesium oxide which is basic.
MgO + Water(H2O) →[ Mg(OH)2] Magnesium Hydroxide.
The substance is acidic or basic and the nature of a substance can be identified by litmus paper. The litmus paper turns to a particular color which helps to identify the nature of the substance.
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Physical Changes
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During the physical change, the arrangement of the molecules is changed but no new substance is formed. Along with that no change in the energy has taken place. As the energy needed to make a change in the physical state is equal to reverse that change hence no change in energy takes place.
The changes are reversible and temporary as well as mass remains the same during the whole process of the physical change.
Examples of Physical Changes
Now, let’s understand the physical changes and types of changes that occur by some examples.
Change in the water of the state
If you take water in a bowl and put it in your freezer for a day, then after a day the water will get converted into ice. It means that water that was in a liquid state changed its state from liquid to <a data-cke-saved-href=" solid-definition-properties-classification-examples-chemistry-articleid-1809="">solid. So, a change of state occurs. Similarly, if you take out that bowl and keep it at room temperature then the solid state of water will get converted into liquid. Therefore, it implies that physical change is reversible also.
Similarly, if you boil the water then the steam will upwell from the boiling water. However, if you put the lid above the boiling water to obstruct the way of steam then some droplets will get accumulated on the lid, this droplet is again water in a liquid state.
Inflation and deflation of the balloon
If you inflate a balloon by filling some air into it the shape of the balloon will get changed as it will expand. This is related to the change in the shape of a matter hence it is an example of physical change. If you deflate the balloon by letting the air come out from the balloon then it will again regain its previous shape which shows that it is also reversible.
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Difference between Chemical and Physical Changes
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| Physical Change | Chemical change |
|---|---|
| The arrangement of the molecules is changed however mass remains the same during the whole change in the physical change. | The molecular composition is altered hence mass is either added or removed from a substance. |
| No new substance is formed. | One or more substances are created. |
| The changes are reversible and temporary | This change is irreversible. |
| These changes are temporary. | As they are irreversible they are permanent. |
| No change in the energy takes place as the energy needed to make a change in the physical state is equal to reverse that change hence no change in energy takes place. | During a chemical change, either the energy is released or gets absorbed. If energy is released then it is called exothermic or if the energy gets absorbed then it is called endothermic |
Things to Remember
- Physical changes are changes that have constant mass, unaltered molecular composition, and a new substance is not created hence reversible. It does not include energy change.
- Chemical changes are the changes in which molecular composition is altered, change in energy, a new substance is created. Mass is either absorbed or removed in this change.
- Rusting of iron, burning of coal, burning of magnesium, cooking any food, and digestion of food are examples of chemical change.
- Inflating balloons, crushing a bottle, boiling water, breaking a glass, cutting papers are examples of physical change.
- Rusting of iron can be prevented by applying a layer of chromium or zinc.
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Sample Questions
Ques. What is Chemical Change? What are some examples of chemical changes? (5 marks)
Ans. Chemical changes are changes in which a new substance is formed. It includes a change in the molecular composition of the substance. The energy change takes place which can be either endothermic or exothermic. It is irreversible.
Rusting of iron, burning of coal, burning of magnesium, cooking any food, and digestion of food are examples of chemical change.
Ques. What is an endothermic or exothermic reaction?(5 marks)
Ans. Endothermic reactions are the reactions in which energy is getting absorbed while on the other hand exothermic reactions are responsible for releasing energy.
Ques. What is Physical change? What are some examples of physical changes? (5 marks)
Ans. Physical changes are the change that is related to size, shape, color, and state of the matter. In this kind of change, mass remains constant while the arrangement of the is changed. It is reversible.
Inflating balloons, crushing a bottle, boiling water, breaking a glass, cutting papers are examples of physical change.
Ques. What Is the matter? What are the three states of water? Name the process which is responsible for changing the state of water? (5 marks)
Ans. A matter is a substance that has its mass and occupies space. Matter can be found in three states: liquid, solid, and gas.
The three states are solid, liquid and gas.
The process which is responsible for changing the state of water is evaporation, condensation, and freezing of water.
Ques. How does rusting of iron take place? How can we prevent rusting of iron?(5 marks)
Ans. It occurs when the iron is kept open in the air due to which oxidation of iron takes place because of moisture and air in the atmospheric air.
Rusting of iron can be prevented by paint or it can also be prevented by applying a coat of chromium or zinc.
Ques. Give the differences between chemical and physical changes. (5 marks)
Ans. The difference between the chemical and physical changes are:
| Physical Change | Chemical change |
|---|---|
| The arrangement of the molecules is changed however mass remains the same during the whole change in the physical change. | The molecular composition is altered hence mass is either added or removed from a substance. |
| No new substance is formed. | One or more substances are created. |
| The changes are reversible and temporary | This change is irreversible. |
| These changes are temporary. | As they are irreversible they are permanent. |
Ques. Give a few characteristics of chemical changes. (5 marks)
Ans. Few characteristics are:
- The chemical changes are irreversible and permanent.
- When chemical changes take place there is a change in the mass, either added or removed.
- There is a change in the energy, either produced or absorbed during a chemical change.
- During a chemical change there is a change in the molecular structure and a new product is formed.
- While a chemical change takes place, old bonds are broken and new bonds are formed.
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