Chapter 2

Physical and Chemical Changes

The following Topics and Sub-Topics are covered in this chapter and are available on MSVgo:

Introduction

When a substance changes its shape, size, colour or chemical composition then there are certain kinds of physical and chemical changes. You perform many activities in your day-to-day living where changes of substances are involved. These changes are physical and chemical changes of one or more than one substance. Think of this example when your mother asks you to add sugar to your milk. You are dissolving a solid substance in a liquid.

Therefore, you just performed a physical change and chemical change of milk and sugar into a new substance. Another example is the stretching of a rubber band, where you perform physical change only. Whereas, settling of curd and then its getting soured is an example of chemical change.

You will understand what exactly physical and chemical changes are all about in the next section.

A physical change takes place when there are changes in the physical properties of a substance. It is a temporary change. These physical properties include the shape, colour, size, and state of a substance (Solid or Liquid or Gas). Also, whenever a physical change takes place there is no emission of energy in the form of heat, light or temperature.

An example of shape and size, physical change is a flattening of a metal rod. Time to understand what chemical change is all about.

When there are one or more new substances formed than that kind of change comes under a Chemical Change. In most cases, there is an involvement of a chemical reaction that forms a completely new product. So, you can say that the change is permanent in case of chemical changes.

Let’s take an example of the rusting of iron ore or burning of wood. In both cases, different chemical reactions are involved. The iron when it gets oxidised (reaction with oxygen in the air), then starts degrading its physical properties and turns into a different colour that is not reversible.

You now know what physical and chemical changes are all about. Hereunder, you will learn the comparison between Physical and Chemical Changes. In the below tabular format, we have listed the differences between physical and chemical changes.

Sr.Physical Change Chemical Change
1. All the components of a substance are in their original chemical composition. All molecules are only rearranged to form change in size and shape.Whenever a substance undergoes a chemical change, its entire molecular composition is changed. The state of matter changes.
2.Physical changes are always temporary changes.Chemical changes are always permanent changes.
3.There are always changes seen in the physical properties of a substance such as size and shape.The substance changes both physical and chemical properties.
4.No energy absorption takes place.In this, either absorption or evolution of energies takes place.
5.Examples are water freezing, water boiling, wax melting, etc.Examples are food digestion, iron ore rusting, coal burning, etc.
6.There is no production of energy. There is always a production of energies such as light, heat, sound, etc.
7.There is no formation of a new substance or a by-product.There is a formation of a new substance/s.
8.The substance is easily reversible and can be recovered in its original form.There is no recovery as the changes are irreversible in a chemical change.
9.There is no production of smell in a physical reaction.There might be the production of smell as chemical reactions are involved.

Let us take a few examples of changes in states of matter and energy to understand the concept more clearly.

  1. Burning Of Magnesium Ribbon – When this chemical reaction takes place the oxides of magnesium are formed, so you can see that there is a change in states of matter and energy is also released in the form of heat where heat is absorbed in this case.
  2. Reaction Between Baking Soda And Vinegar – When this chemical reaction takes place, there are bubbles formed and you can also easily hear the hissing sound. Again, release of energy can be seen.
  3. Crystallisation Process – The process of separating the salts from their solution is known as crystallisation. With this process, the impurities are separated with the help of a physical change. So, here again, you notice that there are changes in states of matter and energy.


Now, let’s move to another important topic classification of physical and chemical change.

You have to identify a few of the key features to understand physical and chemical changes.
You need to classify the matter by physical change and chemical change. Let us first discuss the classification of Physical Change.

Physical Change Classification

Here, fixed means that whenever it is placed in a container, its shape remains intact. 

Whereas indefinite means that it takes the shape of the container itself.

  • State= Solid
    It has a fixed shape and volume. Also, there is no compression and flow.
  • State=Liquid
    It has an indefinite shape but a fixed volume. Also, there is no compression but has a flow.
  • State=Gas
    It has an indefinite shape and volume. Also, there is always compression and flow.

Chemical Change Classification 

When a chemical reaction is involved there are various types of reactions that take 

place, based on that we can classify the chemical changes further to more extent.

    1. Synthesis Process –  Example: Burning of Magnesium ribbon where oxide is formed.
    2. Decomposition Process – Example: Rotting of Banana where it changes colour, odour and taste.
    3. Combustion Process – Burning of coal where there is always a release of water and carbon dioxide.
    4. Single Replacement Reaction – When one of the main components gets displaced.
    5. Double Replacement Reaction – When there are more than two chemical substances formed in a single reaction.

If you observe, you will find that whenever a physical change or a chemical change happens, there are changes in states of matter and energy. The states of the matter may or may not necessarily change in a physical change but in a chemical change, it has to occur as chemical reactions are involved.

Q.1 Is burning a physical or a chemical change?
Solution – Burning is a chemical change.
Q.2 What are reversible and irreversible changes? Give an example of each.

Solution –  A reversible change means that the substance can be reversed to its previous state by putting in some conditions. Example – Wax melting

Irreversible changes are types of changes where a substance cannot be converted into a previous state of matter. Example – Souring of Milk.

Q.3 State your observation for the case when few Ice cubes are put in a glass jar.

Solution –  The ice cube turns into water. It changes its state from solid to liquid. It is a physical change. 

Q.4 Respiration of Human Beings involves what type of change?

Solution – It is a chemical change.

Q.5 Sublimation of Iodine is an example of __________________?

Solution – Sublimation of Iodine is an example of Physical change.

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