Suppose you are recording a neuron’s resting membrane potential. If you added more KCl to the external medium, what would happen to the normal resting membrane potential? What if you decreased KCl in the medium?

addition to the external medium.

Genearlly, potassium is an intracellualr ion, therefore there is higher concentration of potasium ions with its positive charges inside than outside.In addition potassium ion has many leaking potassium  channels in the cell membrane,which permit K+ to escape down the gradient to the extracellular medium.

Therefore when  KCl is added to the external medium of a resting neuron, the concentration of external K+ increase than intracellular medium, preventing K+ from diffusing out  to the external medium, retaining their positive charges, therefore favouring depolarization of the neuron.This  decreases the RMP, leading  to depolarization.

If the KCl is reduced, less k+ are available outside  compare to the inner medium.Therefore  electrochemical gradients  is sets up for leaking of K+out of the intracellular layers, caring  the positive charges along, leaving the inside more negative (together with the negative charges of the  anions of the intracellular layer). This is reversal of charges.And it  increases the RMP.

Explanation: