Can I easily compress the water in a jar to reduce its volume?

You can definitely compress the volume of water but it is not true that this can be done easily. The volume of gas can be easily reduced mainly because of the molecules that are far away from each other. Take note that the different molecules of water will still have molecular energies which will hold the water together.

The only way that you can compress water is if you would change the volume of water into smaller quantities. If you really want to compress water, remember that you have to exert a lot of pressure in order to start to feel the compressive strain. There are different things that you can do in order to test this.