A text-book of inorganic chemistry / by George Senter.

  • Senter, George, 1874-
Date:
1913
Catalogue details

Licence: In copyright

Credit: A text-book of inorganic chemistry / by George Senter. Source: Wellcome Collection.

    22/678 (page 8)
    CHAPTER II CONSERVATION OF MASS AND OF ENERGY- CHEMICAL ATTRACTION /Conservation of Weight in Chemical Changes -So far, we have considered chemical changes from the qualitative point of view only, but it is also necessary to consider them from the point of view of the relative a?noun/s of the reacting substances. As a preliminary to this inquiry, it is first necessary to ascertain whether chemical changes are associated with changes in weight of the reacting substances. The indispensable instrument in such in- vestigations is the balance, by means of which the mass, or quantity of matter, is determined. The balance (Fig. 2) consists essentially of a long beam, supported on a knife-edge at its middle point, and provided with scale-pans supported on knife-edges at either extremity of the beam. The substance of unknown weight is put on one scale-pan and known weights on the other until the beam is exactly horizontal, as indicated by a pointer in front of a scale. The balance does not indicate directly the mass or quantity of matter in a body, but its weight, that is, the force with which it is attracted towards the earth. The weight of a substance is the product of its mass and the force of gravity at the place of observation. Hence, as at any one place the weight and the mass bear a constant ratio to one another,1 two bodies of the same weight have equal masses or contain equal quantities of matter. We are now in a position to investigate the question as to whether there is any alteration in mass (or in weight) when substances enter into chemical combination. Some well-known experiments appear at first sight to show that such changes in weight actually occur. Into an ordinary crucible provided with a lid some pieces of magnesium ribbon are placed, and the crucible and contents weighed. The covered crucible is then strongly heated over a Bunsen flame, 1 The ratio between weight and mass, in other words the force of gravity, is different at different parts of the earth's surface.