Jupiter and Saturn are gas giants, which basically means that their atmospheres are mainly composed of Hydrogen and Helium, the former being in a greater proportion. Jupiter has no defined solid surface. It is just layers of atmosphere, increasing in density as we go towards the center of the planet. So, if you were to grab a parachute and dive into Jupiter’s atmosphere from space, you would die before you reach the planet’s core due to increasing pressure or temperature or a variety of other factors.
The core is theorized to be composed of either hot molten liquid or a solid rock. Theorized being the key word because we can’t really tell for sure as of now. This core is also thought to be surrounded by a layer of metallic hydrogen which extends upward till up to 78% of the planet’s radius. Above this layer is an atmosphere composed mainly of hydrogen. But this layer is neither gas nor liquid. Due to the high temperature and pressure on the planet, hydrogen is said to be in a supercritical fluid state. It can be considered to be gaseous in the upper layers and then moving towards the core, it starts to appear in liquid form. Basically, there is no boundary, you just traverse through layers of gas of varying densities.
We don’t know a lot about the planet’s structure. The Juno spacecraft, launched in 2011, reached Jupiter’s orbit in July 2016 and one of it’s mission is to study the composition of the planet.