Effect of Air and Water on Metals
You would often have noticed that iron objects develop brown deposits when left out in open for a few days. This brown deposit is called rust.
Rusting:
The process of formation of rust on objects made of iron is called rusting.
This can be understood through an activity.
Activity: Testing conditions in which iron objects develop rust
Take a few shining iron nails and place them in three different clean test tubes. Label them as test tubes 'A', 'B', and 'C'.
- In test tube 'A', pour enough water to partially immerse the iron nail, leaving some air space at the top.
- In test tube 'B', pour enough clean water (without dissolved air) to completely immerse the iron nail. Then, add a layer of oil on top of the water to cover it.
- In test tube 'C', add some calcium chloride (a drying agent) and seal it tightly with a lid.
Testing of iron nails in different conditions
Observation:
Iron nails rust in test tube 'A', but not in test tubes 'B' or 'C', as you will observe. The nails in test tube 'A' are exposed to both air and water. The nails in test tube 'B' are exposed to water only, whereas those in test tube 'C' are only exposed to dry air. Hence, rusting of iron requires both air and moisture.
Result:
The chemical process of rusting is as follows,
\(Iron\) \(+\) \(Oxygen\) \(+\) \(Water\) \(\to\) \(Iron\) \(oxide(rust)\)
\(Fe + O_2 + xH_2O \to Fe_2O_3.xH_2O\)
\(Iron\) \(+\) \(Oxygen\) \(+\) \(Water\) \(\to\) \(Iron\) \(oxide(rust)\)
\(Fe + O_2 + xH_2O \to Fe_2O_3.xH_2O\)
Many other metals also exhibit discolouration when kept open in the atmosphere.
i. Copper, when exposed to moist air, undergoes corrosion and forms a greenish layer called patina (copper carbonate). The green coating is primarily due to copper reacting with carbon dioxide (\(CO_2\)) and moisture (\(H_2O\)) in the air to form basic copper carbonate \(CuCO_{3}\).
ii. Silver metal reacts with hydrogen sulfide gas present in the air to form silver sulfide. As a result, silver objects appear dull and black due to the layer of silver sulfide \(Ag_{2}S\) that is created on their surface.
Corrosion:
Gradual deterioration of metal surfaces caused by air, water, or other substances is known as corrosion.
Prevention of corrosion:
- Painting, oiling, greasing, chrome plating, and anodising are all effective methods for preventing iron from rusting.
- Galvanisation is a process that coats steel and iron with a thin layer of zinc to prevent rusting. Even if the zinc coating on the galvanised item is damaged, it remains protected against rust. Because zinc is more reactive than iron, so zinc corrodes in preference to iron and protects it.
Galvanised iron sheets
Let us learn the reactions of metals with air. Observing these reactions helps us understand the unique properties of different materials.
Activity:
Step 1: Take a magnesium ribbon (2 to 3 cm long) and clean it with sandpaper. This cleaning will remove the oxide layer on the magnesium ribbon, making it inactive.
Step 2: Hold the magnesium ribbon over a watch glass with tongs and burn it in the air using a burner. Using a pair of dark-coloured goggles, watch the burning magnesium ribbon.
Step 3: Take a watch glass and collect the the ashes.
Burning of magnesium ribbon
Observation:
- We observe that the magnesium ribbon burns with a dazzling white flame.
- The magnesium ribbon is converted into a white powder (collected in the watch glass).
Result:
The white powder collected after burning the magnesium ribbon is magnesium oxide (MgO).
\(2Mg + O_{2} → 2MgO\)
\(Magnesium + Oxygen → Magnesium\ oxide\)
Litmus test:
- Transfer the white powder to a beaker with a small amount of distilled water, then mix it thoroughly.
- Pour a few drops of the mixture into the watch glass, and place the blue and red litmus paper on it.
Litmus test of magnesium oxide
Among the red and blue litmus papers, the colour of red litmus paper changes to blue. The colour of blue litmus paper remains the same (no change).
The difference in the colour of red litmus paper to blue suggests that the aqueous solution of magnesium oxide is basic in nature. Most metals react with oxygen to form metal oxides. These metal oxides are basic in nature.
Sodium is a metal which is stored in kerosene because it reacts vigorously with oxygen and water. A lot of heat is generated in the reaction. Storing sodium in kerosene prevents its exposure to moisture and air.