This is a very simple online application which can be built using basic HTML and JavaScript/JQuery. The application will be like this. 

At the top of the page images of all the P-Block chemicals will be displayed (You can use images of the chemical/ particular colors to indicate the chemical). Then there will be two/ three drop down lists to select the chemicals. Once the chemicals are selected, the result will be displayed in box at the bottom (Color of the result / any gas emitted or any other result).

This is just a suggestion how you can accomplish it. It is all up to you to put your creativity and come up with a nice and useful application. 

Useful links :

http://www.w3schools.com/html/

http://www.w3schools.com/js/

http://jquery.com/

Please feel free to ask any questions.