6. A solution of hydrochloric acid in benzene does not conduct the electric current. When this solution is treated with zinc, will hydrogen be evolved? Explain. 7. (a) Write equation for preparation of hydrogen from zinc and dilute sulphuric acid. (b) Rewrite the same equation from the standpoint of the theory of electrolytic dissociation, (c) Subtract the common SO_{4} ion from both members of the equation, (d) From the resulting equation, explain in what the preparation of hydrogen consists when examined from the standpoint of this theory. 8. In the same manner as in the preceding exercise, explain in what the action of sodium on water to give hydrogen consists. CHAPTER XI VALENCE ~Definition of valence.~ A study of the formulas of various binary compounds shows that the elements differ between themselves in the number of atoms of other elements which they are able to hold in combination. This is illustrated in the formulas HCl, H_{2}O, H_{3}N, H_{4}C. (hydrochloric acid) (water) (ammonia) (marsh gas) It will be noticed that while one atom of chlorine combines with one atom of hydrogen, an atom of oxygen combines with two, an atom of nitrogen with three, one of carbon with four. The number which expresses this combining ratio between atoms is a definite property of each element and is called its _valence_. DEFINITION: _The valence of an element is that property which determines the number of the atoms of another element which its atom can hold in combination._ ~Valence a numerical property.~ Valence is therefore merely a numerical relation and does not convey any information in regard to the intensity of the affinity between atoms. Judging by the heat liberated in their union, oxygen has a far stronger affinity for hydrogen than does nitrogen, but an atom of oxygen can combine with two atoms only of hydrogen, while an atom of nitrogen can combine with three. ~Measure of valence.~ In expressing the valence of an element we must select some standard for comparison, just as in the measurement of any other numerical quantity. It has been found that an atom of hydrogen is never able to hold in combination more than one atom of any other element. Hydrogen is therefore taken as the standard, and other elements are compared with it in determining their valence. A number of other elements are like hydrogen in being able to combine with at most one atom of other elements, and such elements are called _univalent_. Among these are chlorine, iodine, and sodium. Elements such as oxygen, calcium, and zinc, which can combine with two atoms of hydrogen or other univalent elements, are said to be _divalent_. Similarly, we have _trivalent, tetravalent, pentavalent_ elements. None have a valence of more than 8.