Is the combination of all forces acting on an object
        
             
        
        
        
Alkenes must undergo addition because they have easily broken tt bonds. 
Markonikov's rule states in the addition of HX to an unsymmetrical alkene, the H atom bonds to the less substituted carbon atom.
 alkenes are unsaturated hydrocarbons because they have fewer than the maximum number of hydrogen atoms per carbon.
Alkyl halides have good leaving groups and therefore readily undergo substitution and elimination reactions. 
In hydroboration, the boron atom bonds to the substituted carbon.
Hydroxides, amines and alcoxides undergo substitution and elimination, but can do so only when the heteroatom is made into a good leaving group.
 
        
             
        
        
        
The answer to that is mass and chemical
        
             
        
        
        
The law of definite proportions agrees with Dalton atomic theory.
What is Dalton atomic theory?
It state that all matters is made of very tiny particles called atom. atoms are individual particles which can not be created or be destroyed in a chemical reactions. Atoms of given elements are identical in mass and chemical properties. Atoms of
different elements have different masses and chemical properties.
The law of definite proportions also known as proust's law ,state that a chemical compound contain the same proportion of elements by mass.this law is one of the stoichiometry .
Thus ,
This is the reason why it is agrees with dalton atomic theory.
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The methane molecule undergoes oxidation and the carbon is oxidized to carbon dioxide and hydrogen to water. 
In this reaction there is cleavage of four C-H bonds and two O=O bonds
there will be formation of two C=O bonds and four O-H bonds.
Overall due to more bond cleavage energy there will be evolution of energy. thus combustion is an exothermic reaction.