, a chemical agent which disrupts protein
Common chaotropes which are generally guaranteed to unfold a protein, especially at high concentrations, are urea and guanidine hydrocholride. Sometimes salt or even pH changes can be used to disrupt proteins.
Chaotropes are used when you wish to work with the unfolded protein, either for its own sake, or to acheive some end. If you wish to break up a protein assembly and make it reassemble with new partners, you often need a chaotrope to encourage the process along. Chemical modifications to the inside of the protein often require unfolding prior to chemical treatment. Chaotropes, particularly urea and guanidine hydrochloride are used extensively in the protein folding field to understand how proteins recover their native structure after being denatured chemically.
Chaotropes such as urea and guanidine hydrochloride function by occupying hydrogen bonds which are normally used to stabilize the protein. At high concentrations, these chaotropes outcompete the protein for hydrogen bonding sites, causing it to fall apart. Salt can disrupt electrostatic interactions which are stabilizing the protein. In particular, salt can be used to separate protein multimers without unfolding the individual proteins themselves. pH may change the charge equilibrium on residues that are involved in electrostatic stabilization of the protein. Sometimes pH changes can cause a buried uncharged group to become charged (histidine does this), resulting in an unfavorable buried charge.