A law is statement about an observed concept. A theory involves the explanation of scientific concepts or principles. A hypothesis is the predicted explanation about some concepts that has to be tested in order to prove it to be right. An observation is the observing the results of a scientific experiment carried out to test an hypothesis.
Here the given statement 'A chemist mixes sodium with water and witnesses a violent reaction between the metal and water,' can be classified as an observation as it explains what the chemists observes as a result of his chemical experiment or test..
 
        
             
        
        
        
Answer:
crust, mantle, inner core, outer core
 
        
             
        
        
        
Answer:
No
Explanation:
given that, enthalpy is a state function, that means it depends only on the initial and final states,  there is no difference between the enthalpy of a phase transition versus the enthalpy of a heating or cooling process, when the cooling or heating process finish in a change of phase.
It does not  matter which way we take to cool or heat the substances the Enthalpy of this process will be the same. 
 
        
             
        
        
        
5) state
when the state of the chemical changes that is the most important