The two changes you mentioned (rust forming on a metal gate and snails growing shells) have the following in common:
- Both conserve mass: In both cases, the mass of the material is conserved during the processes. Rusting involves a chemical reaction between iron, oxygen, and moisture, and the mass of the iron and oxygen used to create rust is equal to the mass of the rust produced. Similarly, snails use the calcium from their environment to build their shells, which also conserves mass.
The other options do not apply:
- Both are only physical changes: This is incorrect because rusting is a chemical change, while the growth of a shell is also a biological or physical process but involves the intake of materials and a transformation.
- Both are chemical changes: This is only true for rusting; snails growing shells is primarily a biological process.
- Both are changes in state: This does not apply to either scenario as they are not changes of state (like solid to liquid), but rather transformations involving chemical and biological processes.