Why is the outer core made of liquid?
Extreme temperature and pressure, The structure of matter is transitional, from solid at crust (beneath land and sea) to low-viscous liquid in the outer core. .
Towards center of the Earth, temperature, pressure and density gradients are positive, as we move from surface to the center. Despite that there are discontinuities, on the average, all these increase with depth.
The core temperature
As of now, the outer core is bearing low-viscous liquid and is visualized. from the depth of about 2800 km to 5100 km. Beyond nearly this depth, It is a spherical inner core..
It is surmised that matter reverts to very high density ( 13.1 g.cc) solid
form, near the center, under very extreme conditions of temperature and pressure.