Understanding Why Silver Articles Turn Black

Have you ever noticed how silver jewellery or utensils lose their shine over time and turn black? This common phenomenon is known as tarnishing. Tarnishing is a type of corrosion that happens when silver reacts with certain substances present in the environment, especially in the air.

The Reaction with Sulphur

The main reason silver articles turn black is their reaction with sulphur or, more commonly, compounds containing sulphur like hydrogen sulfide gas ($\text{H}_2\text{S}$). Hydrogen sulfide is a gas found in small amounts in the air, often released from sources like pollution, decaying organic matter, and even some foods.

When silver ($\text{Ag}$) comes into contact with hydrogen sulfide ($\text{H}_2\text{S}$), a chemical reaction takes place. This reaction forms a new compound called silver sulfide ($\text{Ag}_2\text{S}$).

The Chemical Equation for Silver Tarnishing

The chemical reaction between silver and hydrogen sulfide can be represented by the following equation:

$\text{2Ag (s)} + \text{H}_2\text{S (g)} \rightarrow \text{Ag}_2\text{S (s)} + \text{H}_2\text{ (g)}$

In this equation:

• $\text{Ag}$ represents Silver.
• $\text{H}_2\text{S}$ represents Hydrogen Sulfide.
• $\text{Ag}_2\text{S}$ represents Silver Sulfide.
• $\text{H}_2\text{}$ represents Hydrogen gas.
• (s) indicates the substance is a solid.
• (g) indicates the substance is a gas.

The silver sulfide ($\text{Ag}_2\text{S}$) formed on the surface of the silver article is a black substance. This black layer is the tarnish you see.

Why Other Substances Are Not the Primary Cause of Blackening

Let's consider the other options provided and why they don't cause the typical blackening (tarnishing) of silver:

• Oxygen: Silver does react with oxygen, especially at high temperatures, but this reaction is generally slow and forms silver oxide, which isn't the primary cause of the widespread black tarnishing seen at room temperature. Tarnishing is mainly due to sulfur compounds.
• Carbon dioxide: Carbon dioxide is present in the air, but silver does not readily react with it to form a black product under normal atmospheric conditions.
• Phosphorus: Phosphorus is a reactive element, but it is not commonly found in the air in a form that would react with silver articles under normal conditions to cause blackening.

Therefore, sulphur compounds, particularly hydrogen sulfide, are the main culprits behind silver tarnishing and its black appearance.

Revision Table: Key Concepts

Additional Information: Preventing and Cleaning Silver Tarnishing

Preventing silver tarnishing completely in a sulphur-containing environment is difficult, but you can slow it down:

• Store silver in airtight bags or containers, preferably with anti-tarnish strips.
• Avoid exposing silver to things that contain sulphur, like rubber, wool, and certain foods (eggs, onions).
• Handle silver with cotton gloves to avoid transferring oils and residues.

To clean tarnished silver, you can use commercial silver cleaners or home remedies like baking soda paste or a method involving aluminum foil, baking soda, and hot water, which reverses the tarnishing reaction.