WHAT HAPPENED?
When the balloon is rubbed against the wool, it becomes negatively charged. The salt and pepper are both positively charged, which means they will form a natural attraction to the static from the balloon. When the balloon is slowly placed above the mixture, the pepper particles fly up and attract. Why do the pepper particles attract while the salt doesn't? Pepper particles are much lighter than the salt, so they're quicker to attract to the balloon. If you were to bring the balloon closer to the mixture, the heavier salt would eventually cling to it as well.