What method of testing substances is the best way to identify a chemical reaction

The chemical reaction comprises chemical changes, that is, changes in the composition of the molecules, the formation of new compounds. As a consequence of the chemical reaction, new substances are produced, which cannot be reversed by any physical means.  

One of the best ways to identify a chemical reaction is observing the change in color, or changes in the color of the substance. This is one of the most general observable chemical reaction known, the basic example is the changing of litmus paper color when it comes in contact with the acid.  

answer : third trial had the contaminated sample and the correct value for dhrxn should be 1000 kj/mol

explanation :

let us write down the given equations

equation 1 :

equation 2 :

equation 3 :

let us use hess's law to find out which 2 equations can be added to get the third equation.

let us reverse equation 2 and multiply it by 2. the new equation that we get is,

equation 4 :

let us add equation 1 & 4

the above equation is similar to equation 3 but has different value for dhrxn.

therefore third trial had the contaminated sample and the correct value for dhrxn should be 1000 kj/mol