Matter can be classified as an element, a compound, or a mixture. The elements sodium (Na)
and chlorine (CI) react with each other to form the compound sodium chloride (NaCl), also
known as table salt. The reaction between sodium and chlorine releases energy. This energy
causes the temperature of the substances in the reaction to increase. When table salt
dissolves in water, a solution is formed, which is a type of mixture.
Which of these changes must happen for elements to become a compound?
elements dissolve in water
a gas becomes a liquid
elements are heated
chemical bonds form between atoms