Acetone absorbs at lambda max (in nm) 279 in hexane but at lambda max (in nm) 264.5 in water.

This difference in absorption maxima between acetone in hexane and water can be attributed to the different polarity of the solvents. Acetone is a polar molecule, and it interacts differently with polar and nonpolar solvents.

In hexane, which is a nonpolar solvent, the interactions between acetone molecules and the solvent molecules are weaker. This results in a redshift in the absorption spectrum, with a lambda max of 279 nm.

In water, which is a polar solvent, the interactions between acetone molecules and the solvent molecules are stronger. This results in a blueshift in the absorption spectrum, with a lambda max of 264.5 nm.

Overall, the difference in absorption maxima between acetone in hexane and water can be explained by the different solvation effects of the polar and nonpolar solvents on the acetone molecule.

This shift in the absorption maximum is due to the different polarity of the two solvents. Hexane is a nonpolar solvent, while water is a polar solvent. Acetone is a polar molecule, so it interacts more strongly with water than with hexane. This interaction causes the acetone molecule to become more polar in water, which in turn shifts the absorption maximum to a shorter wavelength.