This technology report presents the main features of a free digital educational tool which aims to introduce middle school level students to the comprehension of the particulate nature of matter in the three states (i.e., solid, liquid, and gas) and during phase transitions. This digital tool is available and is freely accessible online. Its utility was tested during a pilot study involving several classes of two middle schools with 103 students of sixth and seventh grades. Didactic activities were designed to introduce the particle model of matter and to reach several educational objectives, such as the understanding of the thermal motion of particles and the role of temperature, the changes occurring at phase transitions, and the state diagrams. An activity was also tested with seventh grade students where the solidification and melting of tert-butyl alcohol were first investigated during a cooperative inquiry-based laboratory and then elaborated on and consolidated by using this digital tool. This activity demonstrates how introducing digital tools into the learning process can help students better understand and visualize abstract concepts such as state diagrams. The level of students' engagement and the appreciation of schools' teachers confirmed the usefulness of this digital tool to teach and learn a key concept of chemistry as the particulate nature of matter.