Theory - To carry out the following Reactions and Classify them as Physical or Chemical changes. | Science Class 9 PDF Download

Our Objective

To carry out the following reactions and classify them as Physical or Chemical changes.

• Burning of magnesium in air
• Sodium sulphate with barium chloride in the form of their solutions in water
• Iron nail with copper sulphate solution in water
• Zinc with dilute sulphuric acid
• Heating of copper sulphate

The Theory

What is a chemical change?

In a chemical change, chemical reaction takes place and the substances undergo a change in their state.  During chemical reactions, one substance reacts with another to form a new substance. The chemical composition of the new substance is different from that of the reacting species.  Due to a chemical change, the chemical properties of matter also change. That means the product is entirely different from either of the reactants. Some chemical reactions may be either exothermic or endothermic in nature.

What are exothermic and endothermic reactions?

In endothermic reactions, a substance absorbs energy in the form of heat and undergoes a chemical reaction. An example is the heat energy absorbed during a chemical change involved in cooking, like baking a cake.

In exothermic reactions, heat energy is released. Explosion of fireworks is an example of exothermic chemical change. The explosion releases energy in the form of light.

Do you know what a photochemical reaction is?

Some chemical reactions do not take place in the dark, but take place only in the presence of sunlight or other radiations. Such reactions are called photochemical reactions. The most commonly known example is photosynthesis, in which plants convert carbon dioxide and water into sugars using energy from sunlight  and produce oxygen as a side product.

A chemical change changes the identity or property of a substance and may or may not be reversed.

Examples of Some Irreversible Chemical changes:

• Combustion (burning) of fuels:In combustion reaction, the fuel burns with an oxidizing agent such as oxygen or fluorine to form new products.
• Neutralization reaction:Neutralization reaction is a type of chemical reaction in which an acid and a base react to form salt and water. For example, sodium hydroxide reacts with hydrochloric acid to form sodium chloride and water. This reaction is highly exothermic, which means it releases heat in the form of energy.
• Rusting of iron:Rust is a chemical substance formed by the chemical action of moist air on iron. It is basically an oxidation reaction. The formula of rust is Fe₂O₃.xH₂O.
• Ripening of fruit and cooking of eggs are also chemical changes.

There are also some chemical changes which are reversible in nature.

Examples:

• The thermal decomposition of ammonium chloride is a reversible chemical change. On heating above 340°C, it decomposes to form two gases such as ammonia and hydrogen chloride. On cooling, the reaction is reversed and solid ammonium chloride is formed. 

• The synthesis of ammonia from hydrogen and oxygen is a reversible reaction

• The synthesis of hydrogen chloride from hydrogen and chlorine is also a reversible reaction.

Let us discuss about some simple chemical reactions.

Burning of Magnesium in Air

Magnesium is an alkaline earth metal with symbol Mg. It is a silvery white metal. Magnesium is a highly inflammable metal and it is easy to ignite its powdered form or thin strips. Magnesium burns in air by taking oxygen from air to form magnesium oxide and is basic in nature. 

Reaction between Sodium Sulphate and Barium Chloride

When barium chloride combines with sodium sulphate in the form of their aqueous solutions, a white precipitate of barium sulphate is formed which is insoluble in water. The reaction also creates sodium chloride, which remains dissolved in water and so cannot be seen. The precipitate of barium sulphate is insoluble in dil.HCl.

Here, the products formed are entirely different from the reactants in chemical composition and chemical properties. So this reaction is a chemical change.

Reaction of Iron Nails with Copper Sulphate Solution in Water

When an iron nail is dipped in copper sulphate solution, a brown coating of copper is formed on the surface of iron and the colour of copper sulphate solution changes from blue to light green. This reaction shows that iron is more reactive than copper as it displaces copper from its solution and iron passes into solution as Fe²⁺ ions and ferrous sulphate solution is formed.

This reaction is a single displacement reaction.

Reaction of Zinc with Dilute Sulphuric Acid

Zinc is more reactive than hydrogen and it displaces hydrogen from dilute acids. Zinc reacts with dilute sulphuric acid to form zinc sulphate and hydrogen gas is evolved. This is a single displacement reaction of a non-metal by a metal.

The products ZnSO₄ and H₂ (g) are entirely different in chemical composition and chemical properties from the reactants Zn and H₂SO₄. So, this reaction is a chemical change.

Heating of Copper Sulphate

Copper sulphate pentahydrate contain 5 moles of water per one mole of copper (II) sulphate. Its formula is written as CuSO₄.5H₂O. It can be converted into anhydrous copper sulphate when heated strongly. The blue colour of copper sulphate is due to water of hydration. When heated, it loses two water molecules at ~63°C followed by two more at ~109°C and the final water molecule at ~200°C and turns to white coloured anhydrous copper sulphate.

The above process can be reversed. When water is added to white coloured anhydrous copper sulphate, its colour changes to blue, indicating that the blue coloured copper sulphate pentahydrate is regenerated.  So, it is a reversible chemical change.

On further heating at ~650°C, the anhydrous copper sulphate decomposes to form copper oxide (CuO) and sulphur trioxide (SO₃) and the reaction become irreversible.