Q. Can you substitute milk chocolate for semisweet?
Chocolate is chocolate a mixture of cocoa butter and cocoa solids that are ground, beaten, flamed, bullied, and coerced from the cocoa bean. Unsweetened chocolate is a pure or nearly pure combination of these two ingredients, called chocolate liquor, although the ratio of cocoa butter to the solids may vary. There may be 1% or less of vanilla and lecithin (for added smoothness), but unsweetened chocolate is essentially 99% chocolate liquor.
Other types of chocolate are less pure. In extra bittersweet chocolate, about 30% of the total weight is sugar. In bittersweet, semisweet, and sweet chocolate, the amount of sugar ranges from 35% to 55%. Milk chocolate starts out with a high percentage of sugar, and is further diluted with the addition of milk solids, generally accounting for 12% or more of the total weight. Milk chocolate may have as little as 10% chocolate liquor, which means it is substantially different from unsweetened chocolate, and even quite a bit different from semisweet chocolate.
What are you making that you want to substitute milk chocolate for semisweet? Because of its relatively high concentration of sugar and because the milk solids are easy to burn, milk chocolate doesn't work very well in baking projects. If you're making candy, however, milk chocolate works great. But because it has a lower melting point than darker chocolates, it may be harder to work with.
At the end of this long-winded sermon, you will have to decide whether adding quite a bit of extra sugar, the milk solids, possible problems with melting and viscosity and substantially less chocolate flavor will be satisfactory in your chocolate project. Most cooks would probably not make the substitution.