A physicist will tell you that a true vacuum is a space that contains nothing. No one can create a true vacuum. For most applications the term vacuum means the absence of air or any other gas. Create a vacuum at home and observe the effects of air pressure with an inexpensive vacuum pump and a science fair bell jar.
Place the bell jar vacuum plate on a sturdy, flat surface. The vacuum plate is what the vacuum pump will connect to in order to create your vacuum.
Take the small balloon and blow a little air into it. Now tie off the end of the balloon and place it in the centre of the vacuum plate. The balloon will help to demonstrate the effects of reduced air pressure.
Rub a small amount of petroleum jelly around the bottom edge of the bell jar and place the jar carefully down onto the vacuum plate. The petroleum jelly will make the seal between the bell jar and the vacuum plate stronger.
Attach one end of the 1/4 inch tubing to the nipple on the vacuum base. Slide on firmly to make an airtight connection.
Place the pressure vacuum hand pump on the table near the bell jar and attach the other end of the tubing to the nipple under the vacuum gauge.
Hold the hand pump with one hand and raise and lower the handle several times to remove the air inside the bell jar. After several pumps the needle on the pressure gauge will move to indicate the negative pressure inside the bell jar. Stop pumping when it nears 25.
Observe the difference in the balloon and you will see that the air inside the balloon has expanded to make the balloon grow larger. The vacuum inside the bell jar created a space of much lower atmospheric pressure than the air inside the balloon.
For another demonstration of vacuums and atmospheric pressure place a marshmallow inside the bell jar. Under a vacuum the marshmallow will expand quickly and then shrink back under normal air pressure.
Do not use high pressure vacuum pumps for bell jar experiments. Too much negative pressure can cause the glass to break or shatter.