Does chocolate harden by itself on pretzels?

Yes, chocolate can harden by itself on pretzels if the conditions are right. Chocolate contains cocoa butter, which is a solid at room temperature. When chocolate is melted, the cocoa butter liquefies and the chocolate becomes a liquid. When the chocolate is cooled, the cocoa butter solidifies again and the chocolate becomes a solid.

If chocolate is melted and poured over pretzels, the chocolate will harden as it cools. This is because the pretzels provide a surface for the chocolate to cool against. The chocolate will also harden more quickly if the pretzels are cold.