Introduction:
Aluminium and silver are both metallic elements that are commonly used in various industries. However, their natural occurrence differs, with aluminium being found in a combined state and silver being found in a free state. This can be attributed to several factors related to their reactivity, abundance, and geological processes.

Aluminium in a Combined State:
- Aluminium is one of the most abundant elements in the Earth's crust, comprising approximately 8% of its composition. However, it is primarily found in combination with other elements rather than in its pure form.
- This is because aluminium is highly reactive and readily combines with other elements to form compounds, such as oxides, sulfates, and silicates.
- The most common compound of aluminium is bauxite, which is a mixture of aluminium oxides, hydroxides, and impurities. Bauxite is the primary source of aluminium used in various industrial applications.

Reasons for Aluminium's Combination:
1. Reactivity: Aluminium is a highly reactive metal, particularly in the presence of oxygen. When exposed to air, aluminium quickly forms a thin layer of oxide on its surface, protecting it from further corrosion. This oxide layer prevents the metal from existing in a pure state in nature.
2. Stability: Aluminium compounds are more stable and less reactive than the pure metal. This stability contributes to their prevalence in nature, as they are less prone to oxidation and degradation.
3. Geological Processes: The formation of aluminium compounds involves various geological processes, including weathering, deposition, and sedimentation. These processes contribute to the accumulation of aluminium in the Earth's crust over millions of years.

Silver in a Free State:
- Unlike aluminium, silver is found in its pure form in nature, known as native silver. It occurs as nuggets, grains, or veins within rocks and minerals.
- Silver is a relatively unreactive metal and does not readily combine with other elements to form compounds. This characteristic allows it to exist in a free state.

Reasons for Silver's Free State:
1. Low Reactivity: Silver has a low reactivity, making it less prone to oxidation and reaction with other elements or compounds. This stability allows silver to exist in its elemental form.
2. Geological Processes: The formation of native silver is associated with specific geological processes, such as hydrothermal activity and volcanic activity. These processes concentrate and deposit silver in specific geological formations.
3. Rarity: Although silver is not as abundant as aluminium, it is still considered a relatively rare metal. The low abundance and favorable geological conditions contribute to the formation of native silver.

Conclusion:
In summary, aluminium is predominantly found in a combined state due to its high reactivity and the stability of its compounds. On the other hand, silver is found in a free state because of its low reactivity, specific geological processes, and relative rarity. Understanding the natural occurrence of these metals is essential for their extraction and utilization in various industries.