Given a scientific passage and an answer, generate a question for the given answer.

[EX Q]: Passage: A metallic bond is the force of attraction between a positive metal ion and the valence electrons that surround itboth its own valence electrons and those of other ions of the same metal. The ions and electrons form a lattice-like structure. Only metals, such as the copper pictured in the Figure below , form metallic bonds. Answer: metallic bond
[EX A]: What is the force of attraction between a positive metal ion and valence electrons?

[EX Q]: Passage: Wavelength is one way of measuring the size of waves. It is the distance between two corresponding points on adjacent waves, and it is usually measured in meters. How it is measured is a little different for transverse and longitudinal waves. Answer: waves
[EX A]: What do wavelengths measure the size of?

[EX Q]: Passage: The fluoride ion is acting as a weak Brnsted-Lowry base. The hydroxide ion that is produced as a result of the above reaction makes the solution slightly basic. Salt hydrolysis is a reaction in which one of the ions from a salt reacts with water, forming either an acidic or basic solution. Answer: water
[EX A]:
Salt hydrolysis is a reaction in which one of the ions from a salt reacts with what?