Characteristics of a Chemical Change

In our environment, we can see various types of changes taking place. We can categorise these changes into two types: physical and chemical changes.

Physical changes are those changes where new substance formation doesn’t take place. There is no alteration in the involved substance’s identity. This is a temporary change and is reversible in nature. A small amount of energy is gained or lost in such changes. Examples include melting ice, vaporisation, dissolving sugar in water, etc.

On the other hand, chemical changes are when there is the formation of new substances with new properties. A rearrangement of atoms occurs by breaking old bonds and then forming new bonds. These are permanent changes and are generally irreversible. 

Characteristics of a Chemical Change

We can tell whether a chemical reaction happened or not by observing if the following events took place or not:

• A gas is evolved 
• A precipitate is formed
• Colour change occurred
• Temperature change happened
• State of reaction changed
• Production of Light
• Volume Change
• Change in Smell or Taste

1.Evolution of a gas:  Some chemical reactions can be characterised by the release of a gas.

Example- When Zn granules are reacted with dilute sulphuric acid, bubbles of hydrogen gas form around the granules.

Zn + H2SO4 → ZnSO4 + H2

Similarly, when Mg is reacted with dilute hydrochloric acid, hydrogen gas is released.

Mg + 2HCl→MgCl2+ H

2. Formation of precipitate: If we add barium chloride to a dilute solution of sulphuric acid, a white precipitate of barium sulphate is seen, indicating that a chemical change has occurred.

BaCl2+ H2SO4→ BaSO4↓ + 2HCl

Similarly, when we pass hydrogen sulphide gas into a blue coloured solution of copper sulphate, black ppt of copper sulphide is formed.

CuSO4+ H2S → CuS↓ + H2SO4

3. Change in colour: When an iron nail is dipped in a blue solution of copper sulphate, a displacement reaction takes place and the solution turns green because of formation of iron sulphate.

Fe + CuSO4→ FeSO4+ Cu

Similarly, when a green coloured solution of copper carbonate is strongly heated, black copper oxide residue is formed, indicating chemical change.

CuCO3→ CuO + CO2

4. Change in temperature: Release or absorption of heat may cause increase or decrease in temperature in a chemical reaction.

When quicklime and water react to form slaked lime, rise in temperature takes place i.e., it is a heat producing or exothermic reaction.

CaO +H2O → Ca(OH)2+ heat

However, when barium hydroxide is added to ammonium chloride, then barium chloride, ammonia and water are formed. Heat is absorbed in this reaction because of which temperature decreases. Hence, it is an endothermic reaction.

Ba(OH)2+ 2NH4Cl → BaCl2+ 2NH3+ 2H2O

5. Change in state: Wax is solid. When it is burnt, water and carbon dioxide are formed, which are liquid and gas, respectively. This implies that the combustion reaction of candle wax can be characterised by changing from solid state to liquid and gas.

Another example is hydrogen and oxygen. Hydrogen and oxygen are gases; however, they react to form water molecules, which are in liquid state.

2H2(g)+ O2(g) → 2H2O(l)

Rusting

When the molecules of one or perhaps more compounds are reorganised, a chemical reaction occurs, resulting in one or even more new compounds. Rusting is an example of a chemical reaction. When iron is subjected to air, it develops a reddish-brown material called rust, and the process is known as rusting. Rusting is accelerated by the presence of moist air as well as seawater. We can use the following equation to illustrate rusting:

Iron (Fe) + Oxygen (O2, from the air) + Water (H2O) → Rust (Iron oxide, Fe2O3)

Rusting Prevention:

The process of preventing iron objects from being exposed to oxygen, water, or both, is known as the prevention of rust.

Mentioned below are a few ways to prevent rusting:

• Applying a layer of paint or lubricant is one of the most common ways to do it.
• Adding a metal coating to iron, such as chromium or zinc.
• Galvanization is the process of putting a coating of zinc on iron.
• The atmosphere’s ozone layer shields humans from the dangerous ultraviolet (UV) rays. Natural chemical processes involving UV light produce ozone.

Conclusion

The changes where formation of new substances possessing new properties take place are called chemical changes. Chemical composition of the substances changes by forming new bonds as well as breaking of old bonds. These changes are permanent and irreversible in nature, in contrast to physical changes, which are temporary and reversible in nature. Chemical changes can be characterised by either evolution of gas, formation of precipitate, colour change, temperature change because of release or absorption of heat as well as change in state of reaction.