What happens
White silver chloride slowly turns grey in sunlight. Light breaks it down into silver and chlorine gas.
2AgCl(s) sunlight→ 2Ag(s) + Cl2(g)
NCERT activity 1.8. Only a tiny amount of chlorine forms, so you do not see it.
The grey colour is finely divided silver. Very small particles of a metal look grey or black, not shiny.
Decomposition caused by light
A decomposition reaction breaks one compound into simpler substances. The energy can come from heat, electricity or light. When light supplies the energy, it is called photodecomposition or a photochemical reaction.
| Energy source | Example |
|---|
| Heat | Ferrous sulphate and lead nitrate (activities 1.5 and 1.6). Limestone to quicklime. |
| Electricity | Electrolysis of water (activity 1.7). |
| Light | Silver chloride and silver bromide (activity 1.8). |
The reaction absorbs energy from light, so it is endothermic.
Why the controls matter
- A dish kept in a dark cupboard stays white. Air and temperature are the same, so light must cause the change.
- Black paper over half a dish keeps that half white. Only the part the light reaches turns grey.
- Brighter light or a longer time gives a darker grey. More light energy means more silver chloride breaks down.
Photography
2AgBr(s) sunlight→ 2Ag(s) + Br2(g)
Silver bromide behaves in the same way. In black-and-white film, the parts hit by light form silver and darken. That is how the paper mask in this lab makes a simple picture.
Storage
Silver chloride, silver bromide and silver nitrate are kept in dark brown bottles. Brown glass blocks most light, so the compounds do not decompose on the shelf.
Making silver chloride
AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)
A white precipitate forms. This is a double displacement reaction, used to prepare the solid. It is not part of activity 1.8.
Safety
- Silver nitrate is corrosive and stains skin black. Wear gloves and goggles.
- Do not breathe in chlorine. Work in a ventilated room.
- Dispose of silver residues as your teacher says. Do not pour them down the sink.
Write it like the mark scheme
- Give the colour change: white to grey.
- Name the products: silver and chlorine.
- Name the type of reaction: decomposition caused by light, a photochemical reaction.
- Balance the equation and write “sunlight” over the arrow.
Syllabus links
CBSE class 10 science, chapter 1, activity 1.8 and the section on decomposition reactions. The same chemistry appears in IGCSE 0620 under types of reaction and photochemical reactions.