All of the following are examples of chemical reactions EXCEPT * Water changing from liquid to steam
Apple turning brown from oxygen hitting it
baking soda and vinegar creating bubbles
cellular respiration creating energy

Water changing from a liquid to a gas (steam)

Explanation:

When water changes states of matter it is considered a physical change. Not a chemical one.

the answer that best describes the relative refractory period in the neuron is the resting membrane potential is restored, but concentrations of sodium and potassium are not restored to their original state.

the refractory period is the time during which another stimulus given to the neuron will not lead to a second potential. thus, because na+ channels are inactivated during this time, additional depolarizing stimuli do not lead to new action potentials.  

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trachea, alveoli, lung capillaries, arteries