"If the double bond is stronger than the single bond, why alkenes are more reactive than alkanes?
When reagents add across the double bond in ethene, the pi bond is broken, this require energy. The energy used in breaking the one pi bond, however, is more than repaid by the energy released when two new bond are made.
Hence, alkenes are energetically unstable relative to their products in an addition reaction. They are also kinetically unstable, because the high electron density in the double bond tends to attract electrophiles, thus initiating addition reactions. "
The statement above is what my lecture tell me. But I can't understand it.
What is energically unstable and kinetically unstable? 
When we do experiments , such as the experiment to investigate the effect of temperature on the rate of reaction of an enzyme, gas will be produced and, through a delivery tube, transfer to another test tube that is filled with distilled water. In the distilled water, the gas forms bubbles.
Why does the gas, once it meets water, get wrapped into bubbles? How does the water form a membrane to wrap the gas?