Benchmark 4D
The Physical Setting: Structure of Matter
Grades 3-5, page 77

No matter how parts of an object are assembled, the weight of the whole object made is always the same as the sum of the parts; and when a thing is broken into parts, the parts have the same total weight as the original thing.

The most closely related standard is at a higher level of sophistication.

NSES Content Standard B 
Physical Science: Properties and changes of properties in matter 
Grades 5-8, page 154 
Substances react chemically in characteristic ways with other substances to form new substances (compounds) with different characteristic properties. In chemical reactions, the total mass is conserved. Substances often are placed in categories or groups if they react in similar ways; metals is an example of such a group.