The above picture shows a type of igneous rock called obsidian. why is the top rock so shiny and smoother than the others?

The rock is shiny and smoother due to how it was formed and the composition.

Explanation:

Obsidian is an example of an extrusive igneous rock. They are categories as a volcanic glass family. The contents of obsidian is mostly silica(SiO2).

Like most extrusive igneous rocks , obsidian is formed when magma migrate to the earth surface as lava. This lava, due to the temperature on the earth surface it will solidify rapidly denying the rocks adequate time to form crystals.

Obsidian are majorly found in the area where volcanic activities is prevalent . The volcanic eruption releases the lava required for it formation to the earth surface . The reason for the top rock so shinny , glassy and smoother is the rapid cooling of the lava which denies the rock adequate time to form crystals. The inability of this crystal to grow gives the rock the shinny, glassy , smoother, and conchoidal fracture it possess.

Obsidian's are majorly black . Although they might possess other color like blue but it is rare . The picture above is an obsidian with a very smooth texture and glassy nature.  

the law of conservation of mass states that in a chemical reaction, the total mass of reactants is equal to the total mass of products. because atoms are only rearranged in a chemical reaction, there must be the same number of sodium atoms and chlorine atoms in both the reactants and products

explanation: no but i do not usually comment