sentences: lichens can produce acid to break down rocks & acid can cause rocks to weather by chemical reaction & chemical weathering means rocks weathering because of chemical reactions ->  lichens can cause rocks to break down by chemical weathering  chemical weathering is a kind of weathering & soil is formed by weathering ->  soil can be formed by chemical weathering  lichens can cause rocks to break down by chemical weathering & soil can be formed by chemical weathering -> lichens breaking down rocks can form soil