A change in which one or more new substances are formed is called a chemical change. In other words, we can say "Change of substances into different substance is called a chemical change.

So what is a Chemical Change?

The type of change in which one substance reacts with another to undergo a change in its chemical composition, thus resulting in the formation of new substances. Such changes are called chemical changes. These changes are permanent and cannot be reversed.

Examples of chemical changes are:

● Burning of magnesium ribbon
● Burning of a piece of paper since it cannot give back the paper
● Rusting of iron
● Formation of iron sulfide on heating a mixture of iron and sulfur
● Digestion of food

These are some irreversible changes where the chemical properties of the original substance are lost upon undergoing change.

In addition to the new substances following may accompany a chemical change.

● Heat, light may be absorbed or given off.
● Sound may be produced
● Change in color or odor of the reactants may take place
● Precipitate may be formed.
● Gas bubbles may appear
● Change in smell may take place.

Let us take some examples of the chemical reactions from daily life:-

When a drop of phenolphthalein is added to a basic solution. The solution turns pink or if you cut a fresh slice of apple or brinjal or potato, the color of the slice changes after some time. The change in color is due to the formation of a new substance. In both the case there is a chemical reaction taking place.

Also, the Explosion of a firework produces heat, light, sound, and unpleasant gases. All types of fireworks are based on chemical reactions. Also, When fruits and vegetables rot, they produce a foul smell. Rotting of fruits and vegetables is also a chemical change.

As you might be knowing, Pure iron is rarely found in nature. If you put a piece of iron in the open for a day or two, its color changes from grey to brown. A thin film of brown substances is deposited on the iron piece. This brownish substance is called rust. It is important to understand that it is the chemical change that has taken place, which has resulted in the rusting of iron. Rust is not iron. Rust is a new substance formed which is different from iron.

In order to protect from rust, one has to protect the iron from rusting using different methods such as painting, greasing, or galvanization. Let us consider a few more examples where new substances are formed during a chemical change.

Burning of magnesium in air

Take a small piece of magnesium ribbon. Clean the piece with fine sandpaper to remove all the oxide coatings which might have been deposited on the ribbon. Now hold one end of the piece with a pair of tongs and keep the other end near a candle flame. You see the piece of magnesium ribbon burns with brilliant white light.

The burning of magnesium also produces powdery ash. Ash is different from magnesium. It is a new substance formed during the chemical change. This change can be represented by an equation.

Magnesium (Mg) + Oxygen present in the air(O2) = magnesium oxide (MgO)

Dissolve the ash produced by burning magnesium in a test tube filled with water. Shake the test tube gently to mix well both the ash and the water. Take one strip each of blue and red litmus paper. Put one or two drops of the mixture on each litmus paper. We see that the red litmus paper turns blue. The color of the red litmus paper does not change.

Therefore we can say that the mixture is basic in nature. It should be noted that on dissolving the ash in the water, again a new substance is formed. We can write this change in the form of an equation.

Magnesium oxide (MgO) + Water (H2O) = Magnesium Hydroxide [Ma (OH) 2].

We conclude that- when magnesium is burnt in the air a new substance magnesium oxide (MgO) is produced. When magnesium oxide (MgO) is dissolved in water, another new substance magnesium hydroxide is formed. In both cases a chemical change has taken place..Let us see another example, where a chemical change takes place

Putting an iron nail into a copper sulfate solution

Activity:- Prepare a copper sulfate solution by dissolving a teaspoonful of copper sulfate powder in about 100 c.c of water in a beaker. Add 2-3 drops of dilute sulphuric acid to the solution and stir it. You get a blue-colored copper sulfate solution. Take a small amount of this solution in a test tube as a sample.

Take an iron nail and clean it with fine sandpaper. Tie a thread to the nail and suspend it inside the copper sulfate solution such that it remains fully immersed in the solution. Wait for half an hour. Remove the nail from the solution. Do you notice changes in the colors of both the nail and solution? The nail becomes brown.

The color of the solution changes from blue to green due to the formation of iron sulfate. The brown film deposited on the nail is of copper metal. We see that two new substances copper and iron sulfate are formed due to the reaction between copper sulfate and iron.

We can write the reaction as:

Here again, a chemical change has taken place, and new substances are formed as a result.

Chemical Changes and their importance in our life

We have learned that in chemical reactions always one or more new substances are produced. In our daily life, a lot of useful materials like soaps, detergents, paints, varnishes, face creams, shampoos, plastics, synthetic fibers, insecticides, pesticides, fertilizers are produced by chemical reactions. Medicines are produced by a chain of chemical reactions.

Let us understand some more examples of chemical changes occurring around:-
A chemical change occurs when A match is lit, fuel burns, bread becomes toast, organic wastes are turned into compost, food gets spoiled, food is cooked, the egg is fried, milk turns sour, metals get tarnished and your body digest food. So in all these activities, a chemical change takes place. Everything around us is made of chemicals.

We are made of chemicals. Drugs are chemicals. Food is made from chemicals. Many of the changes we observe in the world around us are caused by chemical reactions. Hence it is rightly said that 'Chemistry Is Life'.

Let us now learn about a frequently occurring chemical reaction in nature

Rusting of Iron-

We have learned that if a piece of iron is left in open for some time, it gets rusted. The process of rusting can be represented by an equation. Iron (Fe) + Oxygen (O2 from the air) + Water (H2O) = rust (Iron oxide Fe2O3).

In order for Iron to become iron oxide, three things are required: iron, water, and oxygen. In the presence of moisture, iron readily reacts with oxygen to form iron oxide. It is very important to note that pure iron is only rarely found in nature. Rust eats away the metal making it weak and fragile.

How do we prevent rusting?

We can prevent iron objects from coming in contact with oxygen or water or both in many ways and thereby preventing it from rust.

By applying grease or oil on the surface of iron objects. Grease or oil forms a protective layer over iron.
By painting the metal with any durable acrylic paint or enamel.
By depositing a layer of metal like nickel or chromium or zinc on iron.

The process of covering the surface of the iron with a layer of more active metal zinc is called galvanization. GI (galvanized iron) pipes are used in our homes to carry water.

Read More: What is Physical Change? Definition and Examples | Crystallization