A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
exo 7:1SECOND INTERVIEW WITH PHARAOH. (Exo. 7:1-25)
the Lord said unto Moses--He is here encouraged to wait again on the king--not, however, as formerly, in the attitude of a humble suppliant, but now armed with credentials as God's ambassador, and to make his demand in a tone and manner which no earthly monarch or court ever witnessed.
I have made thee a god--"made," that is, set, appointed; "a god"; that is, he was to act in this business as God's representative, to act and speak in His name and to perform things beyond the ordinary course of nature. The Orientals familiarly say of a man who is eminently great or wise, "he is a god" among men.
Aaron thy brother shall be thy prophet--that is, "interpreter" or "spokesman." The one was to be the vicegerent of God, and the other must be considered the speaker throughout all the ensuing scenes, even though his name is not expressly mentioned.
exo 7:3I will harden Pharaoh's heart--This would be the result. But the divine message would be the occasion, not the cause of the king's impenitent obduracy.
exo 7:4I may lay mine hand upon Egypt, &c.--The succession of terrible judgments with which the country was about to be scourged would fully demonstrate the supremacy of Israel's God.
exo 7:7Moses was fourscore years old--This advanced age was a pledge that they had not been readily betrayed into a rash or hazardous enterprise, and that under its attendant infirmities they could not have carried through the work on which they were entering had they not been supported by a divine hand.
exo 7:9When Pharaoh shall speak unto you, &c.--The king would naturally demand some evidence of their having been sent from God; and as he would expect the ministers of his own gods to do the same works, the contest, in the nature of the case, would be one of miracles. Notice has already been taken of the rod of Moses (Exo 4:2), but rods were carried also by all nobles and official persons in the court of Pharaoh. It was an Egyptian custom, and the rods were symbols of authority or rank. Hence God commanded His servants to use a rod.
exo 7:10Aaron cast down his rod before Pharaoh, &c.--It is to be presumed that Pharaoh had demanded a proof of their divine mission.
exo 7:11Then Pharaoh also called the wise men and the sorcerers, &c.--His object in calling them was to ascertain whether this doing of Aaron's was really a work of divine power or merely a feat of magical art. The magicians of Egypt in modern times have been long celebrated adepts in charming serpents, and particularly by pressing the nape of the neck, they throw them into a kind of catalepsy, which renders them stiff and immovable--thus seeming to change them into a rod. They conceal the serpent about their persons, and by acts of legerdemain produce it from their dress, stiff and straight as a rod. Just the same trick was played off by their ancient predecessors, the most renowned of whom, Jannes and Jambres (Ti2 3:8), were called in on this occasion. They had time after the summons to make suitable preparations--and so it appears they succeeded by their "enchantments" in practising an illusion on the senses.
exo 7:12but Aaron's rod swallowed up their rods--This was what they could not be prepared for, and the discomfiture appeared in the loss of their rods, which were probably real serpents.
exo 7:14Pharaoh's heart is hardened--Whatever might have been his first impressions, they were soon dispelled; and when he found his magicians making similar attempts, he concluded that Aaron's affair was a magical deception, the secret of which was not known to his wise men.
exo 7:15Get thee unto Pharaoh--Now began those appalling miracles of judgment by which the God of Israel, through His ambassadors, proved His sole and unchallengeable supremacy over all the gods of Egypt, and which were the natural phenomena of Egypt, at an unusual season, and in a miraculous degree of intensity. The court of Egypt, whether held at Rameses, or Memphis, or Tanis in the field of Zoan (Psa 78:12), was the scene of those extraordinary transactions, and Moses must have resided during that terrible period in the immediate neighborhood.
in the morning; lo, he goeth out unto the water--for the purpose of ablutions or devotions perhaps; for the Nile was an object of superstitious reverence, the patron deity of the country. It might be that Moses had been denied admission into the palace; but be that as it may, the river was to be the subject of the first plague, and therefore, he was ordered to repair to its banks with the miracle-working rod, now to be raised, not in demonstration, but in judgment, if the refractory spirit of the king should still refuse consent to Israel's departure for their sacred rites.
exo 7:17Aaron lifted up the rod and smote the waters, &c.--Whether the water was changed into real blood, or only the appearance of it (and Omnipotence could effect the one as easily as the other), this was a severe calamity. How great must have been the disappointment and disgust throughout the land when the river became of a blood red color, of which they had a national abhorrence; their favorite beverage became a nauseous draught, and the fish, which formed so large an article of food, were destroyed. [See on Num 11:5.] The immense scale on which the plague was inflicted is seen by its extending to "the streams," or branches of the Nile--to the "rivers," the canals, the "ponds" and "pools," that which is left after an overflow, the reservoirs, and the many domestic vessels in which the Nile water was kept to filter. And accordingly the sufferings of the people from thirst must have been severe. Nothing could more humble the pride of Egypt than this dishonor brought on their national god.
exo 7:22And the magicians . . . did so with their enchantments, &c.--Little or no pure water could be procured, and therefore their imitation must have been on a small scale --the only drinkable water available being dug among the sands. It must have been on a sample or specimen of water dyed red with some coloring matter. But it was sufficient to serve as a pretext or command for the king to turn unmoved and go to his house.