Once our travelers cross the Red Sea and find themselves on the Arabian Peninsula, they watch the Egyptian soldiers drown, sing a song of praise to God, and get the heck out of there.
Moses led Israel away from the Red Sea, and they went out into the desert of Shur; they walked for three days in the desert but did not find water.Exodus 15: 22-23
They came to Marah, but they could not drink water from Marah because it was bitter; therefore, it was named Marah.
Here we have the first instance of the Hebrews having trouble finding fresh water. They have (barely) enough food still, but have run out of water for themselves and their flocks.
The people complained against Moses, saying, What shall we drink?Exodus 15: 24
We might not be surprised that the people are, once again, complaining. But in this case not having water is pretty serious. Now they’ve finally found water, but it is bitter.
What is bitter water? It’s not a term that comes up much in modern conversation. But it generally means the water is very hard (usually with a high pH aka alkaline) which means it has more minerals than usual, and not the nice tasting ones we might seek out. Dissolved copper is a common cause (and copper is abundant in that region, hence the multiple copper mines).
So he [Moses] cried out to the Lord, and the Lord instructed him concerning a piece of wood, which he cast into the water, and the water became sweet. There He gave them a statute and an ordinance, and there He tested them.Exodus 15: 25
Is there wood that will remove bitterness from water? Not really. Some commentators speculate that the wood itself was bitter, to cure like with like.
Another interpretation (of Exod. 15:25): AND THE LORD SHOWED HIM A TREE. What was it? R. Joshua says: It was an olive tree; R. Nehemiah says: A willow tree. Some say: The roots of a fig tree; and others say: The roots of a pomegranate, since there is nothing as bitter as those. But the sages say: It was ivy wood, and there is nothing as bitter as that. R. Ishmael the son of R. Johanan ben Baroqah said: See how great are the miracles of the Holy One! [Those of] flesh and blood cure the bitter with the sweet, but the Holy One cures the bitter with the bitter.Midrash Tanchuma Buber, Beshalach 18:2
Other texts (various commentaries on Sefaria) posit that the wood was acacia wood. I wondered if the species was as important as the state of the wood. If the wood was charred enough to become charcoal, would that not purify the water? Turns out I’m not the only one to have thought of that.
As for the purification of the water, Dr. Humphreys asserts that the piece of wood that Moses threw into the spring may have been charred Acacia seyel wood. Charcoal has been used across many civilizations to purify water, and even simply by putting some charcoal into the water, and allowing the coal’s porous surface to absorb the minerals and other contaminants in Marah’s bitter spring.The Bitter Spring of Marah