Question

what do you mean by solid ice ? why co2 is kept under low pressure

Answer 1

Because solid form of CO2 is dry ice and when dry is come in contact with high pressure it start subliming so to protect it from sublimation,CO2 is kept under low pressure 

Answer 2

Solid ice is solid carbon dioxide.

EXPLANATION:

Solid ice is also known as dry ice in which carbon dioxide solidifies into ice cubes.

\mathrm{co}_{2} is kept under low pressure so that the solid state of dry ice can be maintained. If the pressure is increased, the solid carbon dioxide again starts vaporizing into its gaseous state.

So, to prevent the solid state from converting back to gaseous carbon dioxide, \mathrm{co}_{2} is kept under low pressure.