Pure substances can be converted from elements to compounds ONLY in a chemical reaction. Likewise, compounds can be broken down to the elements ONLY if a chemical reaction occurs.

chemical change

elements

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compounds

Hydrogen can be reacted with the oxygen to produce water. The particle combinations are different in the two substances. The element hydrogen contains only hydrogen particles or molecules, H₂. The element oxygen contains only oxygen particles or molecules, O₂. The pure water contains only H₂ O particles.

Elements(reactants)

Compound(products)

The combination of hydrogen and oxygen in water is always H₂O. There are two hydrogen atoms for every one oxygen atom. Water always has this definite composition. All compounds have a specific combination of elements. The combination determines the identity of the substance. Table salt always has the combination NaCl. 

Exercise: Which of the following is a pure substance salt water or 24 carat gold ?

Answer: Salt water is a mixture of water and salt. The mixture can be separated by evaporating the water away from the salt. Gold is usually alloyed or mixed with other metals to improve durability and wear resistance. A piece of 24 carat gold is pure gold with no other metals.

Water is H₂O and under different conditions you can cause hydrogen and oxygen to form hydrogen peroxide which is H₂O₂. These are wildly different substances. This is an important characteristic of compounds. The space filling models show of a water, H₂O, and a hydrogen peroxide, H₂O₂, molecule.

H₂O

H₂O₂

Diamonds are an example of a pure substance. Diamonds are relatively pure, the color in diamonds results from trace impurities. Graphite is another form of carbon. Which form of carbon is more stable diamonds or graphite? The lead block is the same form of matter in fish line sinkers.

Diamond, carbon

Graphite, carbon

Lead