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What is a Decomposition Reaction with Light?
Grade Level:
Class 10
AI/ML, Physics, Biotechnology, Space Technology, Chemistry, Engineering, Medicine
Definition
What is it?
A decomposition reaction with light, also known as photolytic decomposition, is a chemical reaction where a single compound breaks down into two or more simpler substances when exposed to light energy. The light provides the necessary energy to break the chemical bonds within the compound.
Simple Example
Quick Example
Imagine you leave a packet of potato chips out in the sun for a very long time. The oil in the chips might start to smell different or taste bad because the sunlight has enough energy to break down some of its compounds. Similarly, if you leave a silver ornament in the sun, it might slowly turn dark due to light breaking down a thin layer on its surface.
Worked Example
Step-by-Step
Let's look at the decomposition of silver chloride (AgCl) using light.
Step 1: We start with a compound called silver chloride (AgCl), which is white in colour.
---Step 2: Place a small amount of silver chloride in a watch glass or petri dish.
---Step 3: Expose this silver chloride to direct sunlight or a strong light source for some time.
---Step 4: Observe the change. You will notice the white silver chloride slowly turning greyish.
---Step 5: This colour change shows that silver chloride has decomposed. The light energy broke the bonds in AgCl.
---Step 6: Silver chloride breaks down into silver (Ag), which is grey, and chlorine gas (Cl2), which escapes into the air.
---Answer: The chemical equation for this reaction is: 2AgCl(s) --(Light)--> 2Ag(s) + Cl2(g).
Why It Matters
Understanding decomposition with light is crucial in fields like photography, where it was used in traditional film development, and in solar energy, where scientists explore materials that efficiently break down using sunlight. It's also vital in medicine for packaging light-sensitive drugs and in environmental science for studying how pollutants break down in sunlight.
Common Mistakes
MISTAKE: Thinking all decomposition reactions need light. | CORRECTION: Decomposition reactions can be caused by heat (thermal decomposition) or electricity (electrolytic decomposition) too. Light is just one specific energy source.
MISTAKE: Confusing decomposition with combination reactions. | CORRECTION: In decomposition, one compound breaks into many. In combination, many substances combine to form one compound. They are opposite types of reactions.
MISTAKE: Believing the product of decomposition is always a gas and a solid. | CORRECTION: The products can be any state of matter (solid, liquid, or gas) depending on the specific compound decomposing. For example, hydrogen peroxide can decompose into water (liquid) and oxygen (gas).
Practice Questions
Try It Yourself
QUESTION: What happens when silver bromide (AgBr) is exposed to sunlight? Write the balanced chemical equation. | ANSWER: Silver bromide decomposes into silver metal and bromine gas. Equation: 2AgBr(s) --(Light)--> 2Ag(s) + Br2(g)
QUESTION: A compound 'X' is white in colour. When exposed to sunlight, it turns grey and releases a yellowish-green gas. Identify 'X' and the gas released. | ANSWER: 'X' is Silver Chloride (AgCl). The yellowish-green gas is Chlorine (Cl2).
QUESTION: Why are some medicines stored in dark-coloured bottles? Relate your answer to decomposition reactions. | ANSWER: Some medicines contain compounds that are sensitive to light. Storing them in dark-coloured bottles prevents light from reaching them, thus stopping photolytic decomposition reactions that could break down the medicine and reduce its effectiveness or even make it harmful.
MCQ
Quick Quiz
Which of the following is an example of a photolytic decomposition reaction?
Heating calcium carbonate to form calcium oxide and carbon dioxide.
Passing electricity through water to get hydrogen and oxygen.
Exposing silver chloride to sunlight to form silver and chlorine.
Combining hydrogen and oxygen to form water.
The Correct Answer Is:
C
Option C describes a reaction where light energy (sunlight) causes a compound (silver chloride) to break down, which is the definition of photolytic decomposition. Options A and B are thermal and electrolytic decomposition, respectively. Option D is a combination reaction.
Real World Connection
In the Real World
You might have seen old black and white photographs. The magic behind them was photolytic decomposition! Early photographic films used silver halides (like silver bromide). When light hit the film, it caused these compounds to decompose, creating tiny silver particles that formed the image. This technology laid the foundation for modern cameras and imaging.
Key Vocabulary
Key Terms
Decomposition Reaction: A reaction where one compound breaks into simpler substances. | Photolytic: Caused or promoted by light. | Reactant: The starting substance(s) in a chemical reaction. | Product: The substance(s) formed as a result of a chemical reaction. | Silver Halides: Compounds of silver with halogens (like chlorine, bromine, iodine).
What's Next
What to Learn Next
Great job understanding decomposition with light! Next, explore 'Thermal Decomposition' and 'Electrolytic Decomposition'. These concepts will show you how different forms of energy can break down compounds, building on your knowledge of how light energy works.
