Why does sound travel faster in warm air

Warm or hot air particles have more energy than that of cool air. As a result they move faster than cool air particles. Since sound propogation involves movement or vibration of particles, sound travels faster in warm air

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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.