# A Virtual Experiment for Learning the Principle of Daniell Cell Based on Augmented Reality
## 2 System Development
### Operating Principle  
The design of the virtual experiment is implemented by AR and VR technologies based on the operating principle of a redox reaction, which is defined as: \"If a substance loses electrons in a chemical reaction, it is called oxidation; if a substance receives electrons, it is called reduction.\" Since the activity of zinc is higher than that of copper, zinc atoms lose electrons easily. Therefore, the zinc strip is the negative electrode and the copper strip is the positive electrode. The zinc ion Zn\({}^{2+}\) (colorless) formed by the loss of electrons is dissolved in the zinc sulfate solution, so the mass of the zinc strip decreases gradually during the oxidation reaction.  
The user can see the symbolic representation as well as the submicroscopic view of the reaction in the virtual experiment, where the electrons emitted by the zinc atoms flow from the zinc strip through the galvanometer to the copper strip. The electrons received by the copper strip combine with the copper ion Cu\({}^{2+}\) (blue) in the copper sulfate solution and the copper ion Cu\({}^{2+}\) is reduced to the copper atom Cu (red), so the copper strip gradually increases its mass during the reduction reaction. Because the concentration of copper ion Cu\({}^{2+}\) is decreasing, the user can see the color of the copper sulfate solution becomes lighter as time goes on (Figure 3).  
Figure 3: Operating principle of the Daniell cell’s redox reaction.  
When conducting the virtual Daniell cell experiment, the user can determine if there is current and its direction by observing the pointer on the galvanometer when the zinc strip and copper strip are connected to it. The salt bridge is mounted on the beakers to maintain the neutrality of the electrolytic solutions. The salt solution (KNO\({}_{3}\)) in the salt bridge must be easily dissociated, and it does not react with the electrodes or the electrolytic solutions. The user can see the nitrate ions NO\({}_{3}^{-}\) (anions) move to the zinc sulfate solution (negative electrode), and potassium ions K\({}^{+}\) (cations) move to the copper sulfate solution (positive electrode). The user can also observe if the pointer on the galvanometer moves to determine if there is current flowing through and its direction. If the number of salt bridges is increased, the channels for ion communication can also be increased, and thus the current generated by electrochemical cell is increased.  
In the design of the virtual experiment, the submicroscopic explanation can display the reaction of molecules and ions to meet students' expectation by the application of AR and VR technologies in a chemistry education context. It was implemented by integrating the three levels of chemical concepts in [PERSON]'s triangle to interrelate each new concept in all three domains such that different levels of knowledge can be linked together to provide more complete concepts for better cognition. The macroscopic, submicroscopic, and symbolic chemistry knowledge in the Daniell cell's redox reaction are listed in Table 1, which can be used to design the required AR cards, molecular and ion models, and the script of chemical reactions in the virtual experiment.