Why ice cans are always tapered?

This is something related to casting.

concept you must understand is about "Draft Angle".

Any part with significant depth of draw should use a cavity draft angle.In traditional molding where the core is the moving half of the mold,it is desirable for the part to ‘stick’ the core side for ejection.Material shrinkage helps with this in two ways.Shrinkage causes pressure and friction between the part and the core, and the part shrinks away from the cavity of the tool.Draft is typically added to the cavity side of the tool with ½ degree minimum.
Draft also reduces abrasion and wear between the part walls and the cavity during the mold opening.The picture below helps to explain draft angles.

you will understand it in better way from animation of pattern removing from mould click the Link provided below for same
http://www.nptel.iitm.ac.in/courses/Webcourse-contents/IIT-ROORKEE/MANUFACTURING-PROCESSES/metalcasting/lecture4.htm#

why ice is sometimes white & some times transparent

There is a Henry's law in chemistry.
"At a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid."
in simple way
"Gas solubility in water increase as the temperature decrease."
whiteness of ice is mainly dependant on two things 1) Method of ice making 2) what kind of water you are using for ice making i.e.Properties of water
when we put water in ice can it might have dissoved some gases from air or air itself get dissolved in water.
when you start ice making process, temperature of water drops and as its temperature decreases it dissoves more gases/air in it.
these bubbles of gases get enclosed inside ice help to give ice white appearance.
when actual ice formation starts ,first ice formed near to walls of ice can,at the same time these gas bubbles moves to centre part where water is presents,that is why you find ice is more white at centre and more transparent or clear at surfaces.
you can make clear ice by using boiled (deaerated) water.

Why Ice cubes float over water?

question : Why Ice cubes float over water?
Answer : usualy solids have more weight than liquids.
If we take 2 things seperately, like take 1 kg of water and 1 kg of ice, you will find that evevthough mass is same ice occupies larger volume.
water is made up of hydrogen & oxygen, molecules of hydrogen and oxygen are bound together by valancy bounding.
hydrogen and oxygen are having different properties like
oxygen atoms have more affinity towards electrons than hydrogen atoms.
the oxygen atom in a water molecule has a slightly negative charge
and the hydrogen atoms have a slightly positive charge.
So water molecules which are closer to each other are through the slightly negatively charged oxygen atoms and the slightly positively charged hydrogen atoms.
this is nothing but hydrogen bonding.
Ice has a diamond structure due to the hydrogen bonding.
Water does not have such an orderly structure, but water molecules are squeezed close to one another because of the hydrogen bonding.
There is more space in ice than in water!
This is the reason why ice is lighter than water. Therefore, ice can float on water.
Ice floats because it is about 9% less dense than liquid water. In other words, ice takes up about 9% more space than water.

When water is in its solid state (ice), the water molecules are packed close together preventing it from changing shape. Ice has a very regular pattern with the molecules rigidly apart from one another connected by the hydrogen bonds that form a crystalline lattice. These crystals have a number of open regions and pockets making ice less dense than liquid water. This is why ice floats on water. Ice forms when the temperature is below freezing (0°Celsius or 32°Fahrenheit).