Mixing water and acid is very exothermic. Potentially enough heat is generated to cause the water to instantly vaporize, which can result in splattering (and messing up any volume calculations you may have already done). For this reason, ALWAYS add acid to water, NOT the other way around. The water acts as a heat sink and can absorb any heat that is generated. For best results, add the acid slowly while gently swirling the flask/beaker of water. You can also submerge the water flask/beaker into an ice bath.