To carry out the following reactions and classify them as Physical or Chemical changes.
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.
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:
There are also some chemical changes which are reversible in nature.
Let us discuss about some simple chemical reactions.
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.
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.
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.
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.
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.