When you pluck a guitar string, you are causing the string to vibrate. This vibration causes the air around the string to vibrate, which creates sound waves. The pitch of the sound depends on the frequency of the vibration, which is determined by the tension of the string and its length.

The physical change that occurs when you pluck a guitar string is that the string is stretched and then released. This causes the string to vibrate back and forth, which creates the sound waves. The sound waves travel through the air and reach your ears, where they are converted into electrical signals that are interpreted by your brain as sound.

In addition to the physical change of the string vibrating, there is also a chemical change that occurs. When the string is plucked, it causes the molecules of the string to collide with each other, which creates heat. This heat causes the molecules of the string to move faster, which increases the frequency of the vibration and the pitch of the sound.