Is wood soluble in water? How do you know?
No, it's not soluble in water. If it was, for one thing trees (which depend on water to stay alive) would dissolve! Furthermore, so would wooden boats!
Wood is a mixture of cellulose and hemicellulose (as is found in plant cells in general) impregnated with lignin which is a naturally occurring cross linked phenolic polymer.
Neither cellulose nor hemicellulose are water soluble - if they were, they would not be able to form cell walls because they would dissolve in the water that makes up plant cells. Lignin is cross linked and therefore does not dissolve in water.
The reason that we know wood is not soluble in water is by simple observation. Trees take in water through their roots and up into the trunk. If wood was water soluble trees would simply dissolve when water was drawn up by roots (or in the rain!). Also, wood has been used for thousands of years to build boats. If it was soluble in water this would not have been possible.