CHEMISTRY SEMINAR

INTRODUCTION • Acids : is a chemical substance whose aqueous solutions

are characterized by a sour taste, the ability to turn blue litmus red, and the ability to react with bases and certain metals (like calcium) to form salts. An aqueous solution of an acid has a pH of 7 or less . An acid usually contains a hydrogen. A lower pH means a higher acidity, and thus a higher concentration of positive hydrogen ions in the solution. Chemicals or substances having the property of an acid are said to be acidic.

EXAMPLE:-• Zinc reacting with hydrochloric acid.

• Bases :- In chemistry, bases are substances that, in aqueous solution, are slippery to the touch, taste astringent, change the colour of indicators (e.g., turn red litmus paper blue), react with acids to form salts, promote certain chemical reactions (base catalysis), accept protons from any proton donor, and/or contain completely or partially displaceable OH− ions. Examples of bases are the hydroxides of the alkali metals and alkaline earth metals (NaOH, Ca(OH)2, etc.).

EXAMPLE

• Soaps are weak bases formed by the reaction of fatty acids with sodium hydroxide.

• Salt :In chemistry, a salt is an ionic compound that

results from the neutralization reaction of an acid and a base. Salts are composed of related numbers of cations (positively charged ions) and anions (negative ions) so that the product is electrically neutral (without a net charge). These component ions can be inorganic, such as chloride or organic, such as acetate and can be monatomic, such as fluoride or polyatomic such as sulphate.

EXAMPLE:-• Copper sulphate

ARE THE CRYSTALS OF SALT REALLY DRY ?

• Copper sulphate crystals which seem to be dry contain water of crystallisation. When we heat the crystals, this water is removed and the salt turns white. If you moisten the crystals again with water, you will find that blue colour of the crystals reappears. Water of crystallisation is the fixed number of water molecules presenting one formula unit of a salt. Five water molecules are present in one formula unit of copper sulphate. Chemical formula for hydrated copper sulphate is Cu SO4. 5H2O. One other salt, which possesses water of crystallisation is gypsum. It has two water molecules as water of crystallisation. It has the formula CaSO4.2H2O. Gypsum is widely mined and is used as a fertilizer, and as the main constituent in many forms of plaster, blackboard chalk and wallboard.

• On heating gypsum at 373 K, it loses water molecules and becomes calcium sulphate hemihydrate. This is called Plaster of Paris, the substance which doctors use as plaster for supporting fractured bones in the right position. Plaster of Paris is a white powder and on mixing with water, it changes to gypsum once again giving a hard solid mass. Plaster of Paris is used for making toys, materials for decoration and for making surfaces smooth.

QUESTIONS

1. On heating what temperature does gypsum lose its water molecules and becomes calcium sulphate hemihydrate ?Ans. 373 k 2. What are the uses of Plaster of Paris ?Ans. Doctors use as plaster for supporting fractured bones in the right position, Plaster of Paris is used for making toys, materials for decoration and for making surfaces smooth. 3. What are the uses of gypsum?Ans. Gypsum is widely mined and is used as a fertilizer, and as the main constituent in many forms of plaster, blackboard chalk and wallboard.

THE END