Question: Why does the priest break off a particle of the host and drop it in the chalice?
Answer: What you are describing happens during the Communion Rite after the breaking of the bread, and just before the Lamb of God. It is called “commingling” and dates back many centuries. In the early church, there was only one eucharistic celebration in each city on Sunday. The entire Christian community gathered together and the bishop celebrated the Eucharist. As Christianity grew, it became physically impossible to gather for one liturgy and Mass was celebrated throughout the area. The presider or priest was appointed by the bishop. A small portion of the host consecrated by the bishop was taken to each of the eucharistic celebrations over which the priest was presiding. This fraction was mingled into the chalice.
Today the priest breaks off a small fraction of the larger host used at Mass and places it into the chalice with the prayer: “May this mingling of the Body and Blood of our Lord Jesus Christ bring eternal life to us who receive it.” The commingling symbolizes the Lord’s resurrection. In death, body and blood are often separated, but in life they are united. This small symbol serves to remind us that the sacrifice of Jesus is one that gives us all life. In the Mass, we join ourselves to his death, so that we might also rise with him to life.