Powdered sugar, also called confectioners' sugar, icing sugar, and icing cake, is a finely ground sugar produced by milling granulated sugar into a powdered state. It usually contains a small amount of anti-caking agent to prevent clumping and improve flow. Although most often produced in a factory, powdered sugar can also be made by processing ordinary granulated sugar in a coffee grinder, or by crushing it by hand in a mortar and pestle.
Powdered sugar is utilized in industrial food production when a quick-dissolving sugar is required. Home cooks use it principally to make icing or frosting and other cake decorations. It is often dusted onto baked goods to add a subtle sweetness and delicate decoration.
Powdered sugar is available in varying degrees of fineness, most commonly XXX, XXXX, and 10X: the greater the number of Xs, the finer the particles. Finer particles absorb more moisture, which results in caking. Corn starch or tricalcium phosphate is added at 3 to 5% concentration to absorb moisture and to improve flow by reducing friction between sugar crystals.Because of these anticaking agents, it cannot always be used as a substitute for granulated sugar, such as in coffee or tea.
Caster sugar (also referred to as superfine or baker's sugar) has a larger particle size than powdered sugar, approximately half that of granulated sugar. It is commonly used in baking and cold mixed drinks because it dissolves faster than granulated white sugar. Caster sugar can be easily prepared at home by grinding white sugar in a food processor to make it finer. The most common food caster sugar is used in is meringue.
Snow powder (or snow sugar) is a non-melting form of icing sugar usually consisting of dextrose, starch and anti-binding agents, useful for retaining its structure when dusted onto cakes or pastries that require refrigeration. It is mostly used for decorative purposes. There is also titanium dioxide in it, which gives it a vibrant white colour. Use of this sugar is for visual appeal without the sugar melting into the pastry from moisture. It is mostly used on baked goods that are slightly wet like fruit bars and tarts. It will not melt even if it is sprinkled on whipped cream or ice cream. Snow sugar is less sweet than regular powdered sugar because dextrose (a type of sugar made when starchy plants are broken down into monosaccharides using enzymes) around 205% less sweet than regular sugar. 
- Media related to powdered sugar at Wikimedia Commons
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