Although the specific impulse has units equivalent to velocity, it almost never corresponds to any physical velocity. In chemical and cold gas rockets, the shape of the nozzle has a high impact on the energy-to-momentum conversion, and is never perfect, and there are other sources of losses and inefficiencies (e.g. the details of the combustion in such engines). As such, the physical exhaust velocity is higher than the "effective exhaust velocity", i.e. that "velocity" suggested by the specific impulse. In any case, the momentum exchanged and the mass used to generate it are physically real measurements. Typically, rocket nozzles work better when the ambient pressure is lower, i.e. better in space than in atmosphere. Ion engines operate without a nozzle, although they have other sources of losses such that the momentum transferred is lower than the physical exhaust velocity.
although the specific impulse has units equivalent to velocity it almost never corresponds to any physical velocity in chemical and cold gas rockets the shape of the nozzle has a high impact on the energy-to-momentum conversion and is never perfect and there are other sources of losses and inefficiencies (eg the details of the combustion in such engines) as such the physical exhaust velocity is higher than the "effective exhaust velocity" ie that "velocity" suggested by the specific impulse in any case the momentum exchanged and the mass used to generate it are physically real measurements typically rocket nozzles work better when the ambient pressure is lower ie better in space than in atmosphere ion engines operate without a nozzle although they have other sources of losses such that the momentum transferred is lower than the physical exhaust velocity