.90 dL is 90 mL because 1 dL is 100 mL
The way I would explain it is quite difficult to understand, so this is what Google says. "The wavelength (or equivalently, frequency) of the photon is determined by the difference in energy between the two states. These emitted photons form the element's spectrum. The fact that only certain colors appear in an element's atomic emission spectrum means that only certain frequencies of light are emitted." I hope this helped.
The density of ice is less than the density of water (liquid). We generally observe that the density of a solid substance is more than its liquid form as volume of a solid is generally less than the liquid, However in case of water this is not true.
The volume of ice is less than that of liquid water due to an open cage like structure in ice which gives its a wide structure. This cage like structure is due to presence of hydrogen bond (more extensive) in ice.
The maximum density of water is observed at 4 degree celsius
1 mole is 6.02x 10^23 molecules
3 * 6.02*10^23 =1.806*10^24 molecules
