Nath Konkona (Utilities Service Investigator)

It's because silver chloride is very sensitive to light. If you leave it in sunlight, it starts to break down. The light provides the energy for a reaction. It decomposes into silver metal and chlorine gas. This is why you'll see it turn a greyish colour. So, we have to store it in dark bottles. The dark glass blocks the light and stops this from happening. It helps keep the chemical from getting spoilt.