If you have 4 moles of Os(NO3)2 and 6 moles of H2O do you have the right stoichiometric ratio, or is there one reactant in excess and other other limiting? How can you tell? The balanced equation:
2Os(NO3)2 + H2O -> Os2O3 + 4NO2 + H202

answer is: c. the experiment shows that the red substance experienced a chemical change.

this is chemical change (chemical reaction), because two new substances are formed (metalig liquid and gas), the atoms are rearranged and the reaction is followed by an energy change.

this is example of chemical decomposition.

chemical decomposition (analysis) is the separation of a single chemical compound into its two or more elemental parts or to simpler compounds.  

chemical decomposition is opposite of chemical synthesis.

c- cold periods in earth's history during which glaciers covered much of the surface.

explanation:

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