In The Old Testament, what is the reason for the particular Ten Plagues on Egypt?
The Ten Plagues in Egypt served two purposes: first, they punished the Egyptians for their brutal slavery of the Hebrew people; second, they demonstrated the power of the true God over idols. Each plague was meant to punish a specific sin or act, and each was unique in its power over even the court magicians, who could replicate only a few small parts of the plagues.
For example, the plague of Blood punished the worship of the Nile River, which was an important part of Egyptian culture. To show that the Nile was simply a river and nothing more, its waters turned to blood, removing the benefit of yearly flooding. It also represented the use of sacrificial blood in religious ritual, and the unnecessary spilling of slave blood by the Egyptians.
Another example is the plague of locusts, which destroyed crops; this represented the abundance of crops produced and stored by Joseph, who served as king of Egypt for a time. However, the blessings brought onto Egypt by the Hebrew people were rewarded with slavery and hatred by the new Pharaoh, so God destroyed his crops in symbolic punishment.
Most commentaries include specific reason for each plague. The links below will help with identifying and understanding their purpose, both in Scripture and in history.