After studying about conservation of mass, you can use this activity as practice for your students. Students could complete their work in their science journal, either independently or with a partner. When they are done, they then check their work by scanning the QR code on the card. I suggest that students are not given access to their method of scanning until after all work is completed. The type on the task cards is in the Open Dyslexic font in order to make the cards easier to read for all students. This activity typically takes 10-15 minutes.