Final answer:
The acid etchant is placed on dentin for a shorter period of time to prevent over-etching and excessive demineralization.
Step-by-step explanation:
When preparing a cavity preparation for a composite resin restoration, the acid etchant is placed on the dentin for a shorter period than on the enamel because dentin is less mineralized and more sensitive than enamel. Dentin also contains a higher proportion of organic material and water, making it more susceptible to the acidic effects of the etching process. Acid etchant removes some of the mineral content to create a rough surface to which the resin can bond. If the etchant is left on the dentin for too long, it can cause over-etching, leading to the potential for post-operative sensitivity and weaker bond strengths. For these reasons, dentists need to carefully control the etching time on dentin.