what are the two main methods of combination of elements? what is the force that results because of each kind of combination called?

This chapter discusses the combination of elements. Chemistry involves putting elements together to make new substances. Metals combine with non-metals to form compounds. An exothermic reaction is a chemical change in which heat is given out. Combustion is an exothermic reaction. An explosion that occurs during rapid combustion is caused by a sudden release of heat energy that produces rapid expansion of large volumes of gaseous products. Spontaneous combustion occurs when a material appears to ignite of its own accord. A fuel is a material that is burned to supply heat energy. Most of the common fuels are mixtures of compounds, which were derived from living plants and animals. Common methods of rust prevention depend upon protection of the metal surface from contact with air and water. An endothermic change is one in which heat is absorbed. The metals that combine easily with other elements are difficult to extract from their compounds. Such compounds are said to be the most stable.

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