The origin is what makes it change
        
             
        
        
        
Explanation:
A physical change is a change that alters the physical properties of matter most especially the state and the form. This change has the following attributes. 
- it is easily reversible
 - leads to the production of no new kinds of matter
 - involves no change in mass
 - requires little energy
 
Therefore, some of the examples of physical changes are: 
- boiling 
 - melting 
 - freezing 
 - condensation
 - sublimation
 - magnetization of metals 
 - breaking of glass
 - cutting of wood
 
 
        
             
        
        
        
Answer is: Prepare to measure pH change.
For example for strong acid-base titration, sodium hydoxide and hydrochloric can be used.
Balanced chemical reaction: HCl + NaOH → NaCl + H₂O.
In this reaction pH of equivalence point will be always 7.
Equivalence point is the point which there is stoichiometrically equivalent amounts of acid and base.  
Chemist can draw pH curve (graph showing the change in pH of a solution, which is being titrated) for titration and determine equivalence point.  
Near equivalence point indicator should change color, so we must pick indicator who change color near pH of equivalence point.
 
        
                    
             
        
        
        
Answer:
Conservation of Charge is the principle that the total electric charge in an isolated system never changes. The net quantity of electric charge, the amount of positive charge minus the amount of negative charge in the universe, is always conserved.