Juice Cans and Boxes

Think back to pre-school or Kindergarten and you will probably remember large cans of apple juice (or some other juice) that was poured out for snack time.

Inside that can is a lot of juice and a little bit of air. Because this is juice (not soda) the air is not at high pressure. It is just whatever the pressure of the factory was when it was sealed. If we put a hole in the top of the can, the pressure of the air in the can will be exactly the same as it is in the room. (If it was higher in the can, some air would be pushed out and vice-versa.

If we could turn the can over instantaneously, the air in the can would push down on the juice with exactly the same force as the air outside the can is pushing in. (Remember that air pushed in all directions.)

Normally when forces are equal, we expect that things won't move, but in this case there is another force to be dealt with – gravity. Gravity provides an extra force down. Thus, the force down is greater than the force up and the juice will begin to come out of the can.

However, all of the juice will not come out. That is because as the juice leaves the can, the volume of space at the top of the can gets larger. Since no additional air can get in, the pressure will go down (Boyle's Law). As a result, the force down gets weaker and weaker until the total force down (gravity and pressure of air inside can) is equal to the force up, at which point the juice will stop coming out.

If an additional hole is placed in the can at the bottom (now the top since the can is upside down) air will be pushed in by the atmosphere until the pressure inside the can and outside are equal. As a result, the total force down (air in can plus gravity) is again more than the force up and the juice will pour out.

This is why, when people pour juice out of cans, they generally put two holes in the top, so that when the pour the juice, one hole is at the top and one at the bottom. The hole at the top allows air in, so that the pressure of the air in the can and outside the can are always equal. That way gravity wins.

The same thing can be seen if you buy a coffee or other hot drink and put a plastic top on it. These tops have one opening for you to drink through, but they have another (very small) hole on the opposite side of the lid that allows air in, so that your coffee pours smoothly.

Without that second opening for air to enter the cup, the coffee may still come out (especially if the opening is large), but it will do so in a very uneven way.

Some will pour out, perhaps so quickly that the pressure inside drops enough so that the force up is greater than the force down. As a result, air will rush in. This causes the pressure inside to increase, pushing some liquid out, etc. This is what is occurring when a drink goes “glug, glug, glug.”

Juice Boxes
Juice Boxes have their own issues. If you squeeze a juice box, the juice will come squirting out of the straw. This is a simple application of Boyle's Law. When you squeeze the box, the volume of the air inside goes down. When the volume goes down, the pressure of the air in the box increases. Since the air in the box is the air that is around the straw, if the pressure around the straw is greater than the pressure in the straw, the liquid will be pushed up the straw.

You can also see gas laws working if you blow into the straw of a juice box. If the seal around the straw is good, the box will puff out (increased amount leads to increased pressure – Un-named law). Taking it a step further, if you blow as much air as possible into the juice box and then suddenly let go (of the straw with your mouth) juice will squirt out of the straw. This is because the extra air you added to the box causes an increase in pressure (Un-named Law). Thus, the pressure around the straw will be greater than the pressure in the straw and the liquid will be forced up the straw.