Design activities are gaining interest as rich contexts for learning science, technology, engineering, and mathematics (STEM) subjects. STEM teachers may find this challenging however, as designing requires support that they are not used to providing. In a subject like chemistry, teachers would have to balance creativity and responsibility for the students with concept learning and safety in the classroom. In a case study, we analyzed the verbal interactions of three teachers with their students during design activities in the classroom, with the aim of understanding what teachers and students talk about and how chemistry teachers support the students with their designs. During the lesson, students worked on the design of a self-heating or self-cooling cup, while also performing chemistry experiments to learn about the energy effects of reactions. Such a lesson reflects what design activities in the chemistry classroom could look like. We described the topics that teachers and students talk about, revealing that teachers support students through several types of feedback and questions. We also found that teachers support design tasks in a more open, constructive, and encouraging way than is used for experiments and chemistry concepts, which are supported in a closed, clarifying, and steering manner.