The lid on a vacuum storage container gets hard to open because most of the air has been removed from inside, creating lower pressure than the air outside; the normal atmospheric pressure outside keeps pushing the lid inward, and that pressure difference makes it hard to lift by hand.
This doesn't mean the container is faulty; if anything, it's a sign the vacuum seal is doing exactly what it's supposed to and the contents have been cut off from air.
Why does the vacuum create such a strong grip?
In vacuum containers, once the lid is closed, a pump or a hand-operated valve removes most of the air from inside. As the internal pressure drops, the normal atmospheric pressure outside keeps pushing the lid inward, and that force makes the lid almost impossible to budge.
The bigger the pressure difference, the more force it takes to open the lid; that's why a freshly vacuum-sealed container is harder to open than one sealed a few hours earlier.
The wider the lid's surface, the greater the total force that same pressure difference exerts on it; that's why large, wide-mouthed containers tend to be harder to open than small, narrow-necked ones.
Why does hot food make the vacuum even stronger?
When hot food goes into a container and the lid closes, the air inside still takes up more space while it's hot. As the food cools, that air contracts and shrinks in volume; this creates an extra low-pressure zone inside the container, and the lid ends up sealed almost like a real vacuum on its own.
That's why containers sealed while still hot and put straight in the fridge tend to be among the hardest to open once they've cooled.
How do you safely open a vacuum-sealed lid?
Most vacuum containers have a small valve or button to equalize the pressure; pressing it lets air back in, and the lid opens easily within seconds. Without a valve like that, holding the container under warm water for a few minutes helps the trapped air expand and balance out the pressure.
Pressing gently on the edge of the lid and trying to lift one corner can also loosen it; just be careful not to force the container hard enough to crack it.
Sliding the handle of a metal spoon under the lid's edge and gently prying can also work on some containers; but since it can scratch the plastic rim, the valve or warm water methods are worth trying first.
Is every tight lid actually a vacuum seal?
A lid that just fits tightly, without any air actually removed, isn't the same as a real vacuum seal; that kind of lid usually pops open easily with a bit of force. A real vacuum-sealed lid makes a clear "pop" sound as air rushes back in when it opens.
That sound is a good sign the container was genuinely airtight and had kept its contents properly sealed off from air.
The small button in the center of a metal jar lid sinking inward after sealing is a similar vacuum sign; if that button doesn't move when pressed, the jar is still sealed airtight.
In short
A vacuum container's lid is hard to open because the low pressure inside is out of balance with the atmospheric pressure outside. That effect gets stronger as hot food cools down.
Pressing a valve, running the container under warm water, or pressing gently on the edge of the lid are the safest ways to open it without forcing it.