Final answer:
During a dehydration synthesis or condensation reaction, a molecule of water is removed as a byproduct, consisting of an atom of hydrogen from one reactant and a hydroxyl group from another.
Step-by-step explanation:
In a dehydration synthesis or a condensation reaction, the substance removed is a molecule of water (H2O). During this chemical process, one reactant relinquishes an atom of hydrogen (H) while another reactant gives away a hydroxyl group (OH). When these reactants combine to form a new bond in the product, the H and OH combine to form water, which is released as a byproduct. Such reactions are instrumental in forming polymers from monomers, for example, in the synthesis of proteins where amino acids are joined together by peptide bonds, each formation releasing a molecule of water.