White washing, in the context of Class 10 chemistry, refers to the process of applying a solution of slaked lime onto walls and roofs, primarily for aesthetic and protective purposes, which then reacts chemically with the environment to form a durable, shining layer.
Understanding White Washing
White washing is a traditional and economical method used to coat surfaces, most commonly interior and exterior walls of buildings. Beyond its cosmetic appeal, it involves an interesting chemical reaction that provides a hard, reflective finish.
The process typically involves:
- Preparation of Slaked Lime: Calcium oxide (commonly known as quicklime or lime) is dissolved in water. This highly exothermic reaction produces calcium hydroxide, also known as slaked lime.
- Application: The resulting solution of slaked lime is then applied evenly onto the desired surfaces, such as walls and roofs.
- Carbonation: After application, the slaked lime on the surface slowly reacts with carbon dioxide present in the air. This gradual reaction leads to the formation of calcium carbonate.
- Formation of Shining Layer: The calcium carbonate forms a thin, hard, and shiny layer on the surface, which gives the characteristic lustrous finish observed after white washing. This layer is also quite durable.
The Chemical Process of White Washing
The chemistry behind white washing is a key concept in Class 10 science, illustrating acid-base reactions and the formation of carbonates.
Step 1: Slaking of Lime (Formation of Slaked Lime)
When quicklime (calcium oxide) is mixed with water, it undergoes a vigorous reaction to produce slaked lime (calcium hydroxide). This process is known as slaking.
- Reactants: Calcium Oxide (Quicklime) and Water
- Product: Calcium Hydroxide (Slaked Lime)
- Equation:
$$CaO_{(s)} + H2O{(l)} \rightarrow Ca(OH)_{2(aq)}$$
Step 2: Carbonation (Formation of Calcium Carbonate)
Once the slaked lime solution is applied to the walls, it slowly reacts with carbon dioxide from the atmosphere over a period of a few days. This reaction forms calcium carbonate, which is the same chemical compound found in limestone and marble.
- Reactants: Calcium Hydroxide (Slaked Lime) and Carbon Dioxide
- Products: Calcium Carbonate and Water
- Equation:
$$Ca(OH){2(aq)} + CO{2(g)} \rightarrow CaCO_{3(s)} + H2O{(l)}$$
The formation of calcium carbonate is responsible for the hard, glossy, and shining appearance that develops on the walls a few days after white washing is complete.
Practical Insights and Benefits
- Aesthetics: Provides a bright, clean, and shiny finish to surfaces.
- Economy: It is a relatively inexpensive method for coating walls compared to paints.
- Durability: The calcium carbonate layer formed is quite durable and weather-resistant.
- Antiseptic Properties: Historically, lime has been valued for its mild antiseptic properties, which can deter mold and mildew growth.
In summary, white washing, from a Class 10 perspective, is not just a painting technique but a practical application of fundamental chemical reactions that transform simple raw materials into a protective and aesthetically pleasing surface coating.
