解：$(1)Q_{放}=mq=0.5\ \text{kg}×4.6×10^7\ \text{J/kg}=2.3×10^7\ \text{J}$
$ (2)Q_{吸}=50\%Q_{放}=1.15×10^7\ \text {J}$
$ m_{水}=\frac {Q_{吸}}{c_{水}∆t}=\frac {{ 1.15×10^7 }\ \text {J}}{4.2×10^3\ \text {J/}(\ \text {kg}·℃)×40 ℃}={ 68.45 }\ \text {kg}$
