Three chemistry doubts. Please help!?

Question

1)Why is Plaster of Paris written as CaSO4•½ H2O? How is it possible to have half a water molecule attached to CaSO4?


2) How does Baking Soda help to make cakes and bread soft and spongy? Also, how is it a good fire extinguisher?


3) When electricity is passed through an aqueous solution of sodium chloride, three products are obtained. Why is the process called chlor-alkali?

Answer 1

1) it's not !!! there is no CaSO4•½ H2O molecule but two molecules attached together sharing a water molecule.
for example, it can be written : H2+½O2->H2O but in fact the reaction is - 2H2+O2->2H2O.

2)baking soda produces CO2 with water, that's how the bread is soft and spongy and that's why it is a good fire extinguisher (more CO2 -> less oxygen -> no fire).

3)sodium = Na is an alkali metal, chloride =Cl

Answer 2

1. For some crystalline solids, water molecules are part of their structure. This includes Plaster of Paris. In this particular substance, the structure is such that there is one molecule to every two sets of ions constituting the calcium sulfate. The water is a part of the substance; then again, it isn't. Because the focus is so strongly on the substance that is being hydrated rather than the hydration, this is one of the few occasions in which a fraction is permissible in a chemical formula. Calcium sulfate is not unique in having a partial water molecule being associated with the primary substance in its hydrated form, but it isn't that common either.

2. Baking soda releases carbon dioxide when in the presence of acid. The bubbles of carbon dioxide permeates the dough before and during the early stages of baking. This causes the dough to rise. By the time the dough toughens up enough to retain its shape, the bubbles have forced the dough into a latticelike network representing the spaces between the bubbles.

When making bread, yeast microbes are deliberately introduced into the dough. The carbon dioxide produced from the yeasts' partial breakdown of some of the sugars and starches serves a similar function as the baking soda added to cake.

Sodium bicarbonate also breaks down and releases carbon dioxide in the presence of intense heat. This carbon dioxide displaces the oxygen a fire needs to burn. If oxygen can't get to the fire, the fire dies. By the way, the other product of sodium bicarbonate breakdown is sodium hydroxide or sodium oxide that becomes sodium hydroxide on prolonged exposure to air. Sodium hydroxide is a corrosive alkali that can attack skin, so wear gloves when cleaning up the mess afterward.

3. In practice, when passing electricity through an aqueous solution of sodium chloride, it is the water that breaks down rather than the sodium chloride. At the negative electrode, hydrogen is produced. In theory, the sodium ions would also be reduced at the negative electrode, but these promptly react with the water to form hydrogen and sodium hydroxide. The hydrogen bubbles off and the sodium hydroxide, a strong alkali, remains in solution.

When electricity is passed through many aqueous salt solutions, the water also breaks down at the positive electrode, releasing oxygen and acidifying the surrounding water. In the presence of the chloride ion, chlorine gas or hydrogen chloride might also be released in small quantities but I expect the hydrogen chloride to remain in solution. The chlorine products at the positive electrode would explain the "chlor" prefix and the involvement of an alkali metal and subsequent production of a strong alkali at the negative electrode provides the rest of the name.