Carbon and hydrogen are examples of pure substances or

Elements

Explanation:

Carbon and Hydrogen are pure substances or elements. All the atoms of carbon have same number of protons. It cannot be broken into new elements.

Contrary to elements, there are compounds which are formed by the chemical bonds between two or more elements like in water. In water, Hydrogen and oxygen elements combine. All the atoms of water do not have the same number of protons. Then, there are mixtures where elements are not chemically bound. For example, air.

chemical energy --> gravitational potential energy --> kinetic energy --> elastic potential energy --> gravitational potential energy

explanation:

when emma starts the hike, the chemical energy stored in her body is used to climb up the mountain, and so she gains gravitational potential energy (because at the end, she will be at a higher height h above the ground). then she goes bungee jumping: as she falls, this gravitational potential energy is converted into kinetic energy (because her speed increases as she falls down), until the point when the rope stretches, and she stops moving. at that moment, all the kinetic energy is converted into elastic potential energy due to the stretching of the rope. then, this energy is released because the rope returns to its natural length, and emma gains a small increase in altitude , so she gains some gravitational potential energy.