If hard water is alkaline, and washing soda is also alkaline, how does adding it to my laundry help soften the water and protect my machine?
The answer to this has to do with a chemistry concept called equilibrium. Hard water is water that has two main compounds present in it: calcium carbonate and magnesium carbonate. It’s the metal portion of this that causes the hard water (calcium and magnesium), not the carbonate portion. For simplicity I’ll discuss just one of them, but this will hold true both both compounds.
Calcium carbonate is weakly soluble in water, which means only a small amount of what is present will actually dissolve. The rest remains undissolved in solid form, even though you can’t actually see it. The thing is though, individual molecules are converting back and forth between dissolved and nondissolved, but the total ratio of dissolved:nondissolved remains the same. We call this state equilibrium, where the rate of the two reactions is equal (rate of formation equals rate of dissociation/dissolving).
A really interesting thing happens when you’re at equilibrium, where you can “push” the reaction in one direction or another by adding extra amounts of the compounds on one side of the reaction. In this case, by adding baking/washing sods to the water, we are adding extra carbonates to the reaction. This means all the sudden the ratio of solid:dissolved carbonates is out of balence, so to reach equilibrium again a bunch of the dissolved carbonates reform solid calcium cacarbonate. This reduces the total amount of dissolved metals, thus softening the water.
It isn’t the alkalinity that you’re trying to remove with water softening, but rather dissolved minerals (magnesium and calcium usually). Washing powder is sodium carbonate. Dissolved magnesium and calcium ions easily replace sodium, forming insoluble magnesium/calcium carbonate which drops out of the solution.