5. Physical and Chemical Changes

Definition of chemical change:

A change in which one or more new substances are formed is called as a chemical change. 

Properties of chemical change:

1. It is also called a chemical reaction. 
2. It forms a new substance or substances.
3. The new substance has properties that a different from the original substances.
4. These changes are irreversible (means the original substance cannot be obtained again).

During a chemical change when a new substance is formed following changes may also occur:

1. Heat, light or any other energy may be released or absorbed.
2. Sound maybe produced.
3. There may be a change of smell or colour.
4. A gas maybe formed.

Some of the most common examples of chemical changes in our daily life:

1. Burning of wood:

• When wood burns, it reacts with oxygen.
• It produces new substances like ash & carbon dioxide.
• Heat is released. 

2. Curdling of milk:

• A new substance, curd is formed.
• We cannot get milk back from curd.

3. Rusting:

• When iron reacts with oxygen & moisture, it forms a new substance, rust (iron oxide).
• This deteriorates the metal.
• It also changes the colour of metallic iron (gray) to reddish-brown.

4. Digestion: 

• The food we eat undergoes chemical changes in our bodies.
• Breaking down into simpler substances that can be absorbed.

5. Vinegar & baking soda:

• When the two react, there is a hissing sound & bubbles of gas can be seen.
• This is carbon dioxide gas.

6. Explosion of a firework:

• When a firework explodes, it produces heat, light., sound & unpleasant polluting gases.

Photosynthesis, cooking food, ripening of fruits, fermentation of grapes, medicine, detergents, etc. are all chemical changes.

All new substances around us are due to chemical changes.

To Remember:

• Chemical Change: is a chemical reaction to form new substances.
• It is a irreversible change.

Experiments that show chemical changes are:

1. Burning of Magnesium:

• Take a thin strip of magnesium (Mg) ribbon.
• Bring the tip near candle flame.
• It burns brightly due to oxygen (O₂).
• It leaves a powdery ash i.e. Magnesium oxide (MgO) after burning completely.

Equation:

2Mg + O₂  → 2MgO

Magnesium (Mg) + oxygen (O₂) Magnesium oxide (MgO)

2. Magnesium hydroxide:

• Dissolve the above Ash (i.e. magnesium oxide) in water.
• At forms a new substance (i.e. magnesium hydroxide) that is a base.

Equation:

Magnesium oxide (MgO) + water (H₂O) → Magnesium hydroxide [Mg(OH)₂]

3. Copper sulphate & iron:

• Take teaspoon of copper sulphate in half cup water.
• Add few drops of dilute sulphuric acid to it.
• The solution turns blue in colour.
• Drop a iron nail in this solution.
• After an hour, you will see that the solution has turned green due to formation of new substance, iron sulphate.
• And the iron nail has a reddish brown deposit (precipitate) on it, which is copper.
• This is also known as single displacement reaction.

Equation:

Copper Sulphate [CuSO₄] (blue solution) + Iron (Fe) → Iron Sulphate [FeSO₄](green solution) + Copper (Cu) (brown deposit)

4. Vinegar & baking soda:

• Take a teaspoon of vinegar in a test tube.
• Add a pinch of baking soda to it.
• You will hear a hissing sound & see gas bubbles coming out.
• This gas is carbon dioxide gas.

Equation:

Vinegar (acetic acid) + baking soda → CO₂ ( carbon dioxide) + other substances

5. Carbon dioxide & lime water:

• Pass the above carbon dioxide gas into another test tube having lime water (Calcium hydroxide).
• It turns lime water milky as it forms calcium carbonate.
• This is a standard test of to confirm formation of carbon dioxide.

Equation:

Calcium Hydroxide [Ca(OH)₂] + Carbon dioxide (CO₂)→ Calcium carbonate [CaCO₃] + Water (H₂O)