How can you tell if a chemical reaction has taken place between two substances? After having been mixed, new substances are formed, which have different properties than the original substances. After having been mixed, the size of one substance changes while the properties of both substances remain the same. After having been mixed, there are no visual changes in either substance and both substances maintain their identities. After having been mixed, both substances change phase and both keep their original identities.

The allotropes of carbon include a variety of structures that include three-dimensional tetrahedral lattices, planes of hexagonal rings, cylindrical tubes of hexagonal rings, and spheres of five- and six-membered rings. similar shapes of network covalent atomic solids are possible with carbon nitride, boron, and pure silicon (e.g., silicene is a graphene-like allotrope of pure silicon). in contrast, silicates exist as either highly ordered or amorphous (more random) three-dimensional lattices. what could explain why there are there no naturally occurring sheets, stacked sheets, cylindrical tubes, or spheres of network covalent atomic solids composed of silicon and oxygen (sio2)? would pure silicate structures make good lubricants or good electrical conductors?