The Nature of Science 18.5 Conservation of mass Conservation of
Mass In a closed system the total mass will remain the same even if there are physical or chemical changes.
Scientists have studied matter under a wide range of different conditions. From these studies scientists developed the idea known as the principle of conservation of mass. A closed system prevents anything already in the system
from leaving and does not allow anything else to get into the system. For example, in Fig. 15 the Alka Seltzer tablets in the glass of water in A form an open system because the bubbles of gas can leave the system by escaping into the air. The balloon and conical flask in B form a closed system because gas produced in the flask is trapped by the balloon and cannot leave the system. When it appears that the total mass of a system has either
increased or decreased, it must be because the system is not closed and that matter has either entered or exited the system. The atomic nature of matter accounts for the conservation of mass because everything is made of atoms. In a closed system the total number of atoms is fixed. Even if there are chemical or physical changes the total number of atoms does not change. The mass stays the same.
Activity
Does the mass of an object change when the object is physically changed in some other way?
1. Get some lego bricks and stick them all together to make a single object.
2. Place the object on an electronic balance and record the mass.
3. Remove the object and break it up into single bricks.
4. Place all of the bricks on the top pan balance and record the total mass.
Questions
Has taking the bricks apart affected the total mass?
Fig. 16 Does the mass of an object change when it is broken up?
A
B
Fig. 15 The glass in A shows an open system in which mass can escape. The flask in B is part of a closed system in which mass cannot escape.
