Take a can of beer, you can shake it and feel it gets noticeably firmer indicating it will spray if you open it, but if left for long enough it returns back to its normal state. What happens inside to make that pressure increase dissipate?
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The carbon dioxide gas dissolves back into the liquid.
When you shake the can you momentarily increase the surface area considerably and a lot of the gas comes out of solution to pressurize the can.
That same pressure then slowly forces the gas back into solution once the agitation stops.
There is a chemical reaction between compressed carbon dioxide gas and water. They react and form carbonic acid. This is why fizzy drinks taste sour, they contain acid. As the carbon dioxide reacts with the water there is less carbon dioxide so the pressure goes down until finally the chemical reaction stops due to low pressure/high carbonic acid concentration.
When you shake the container you are creating areas of low pressure in the liquid. This reverses the chemical reaction and the carbonic acid turns back into carbon dioxide and water. The increased amount of carbon dioxide is increasing the pressure. Even worse the gas is still dissolved into the liquid in tiny bubbles so when you open the container all the tiny bubbles of compressed gas in the liquid expands and pushes the liquid out in a huge spray. But if you leave it unopened over time the carbon dioxide will react with the water and turn into carbonic acid again.
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