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Unit 10: Acids, Bases, and the pH system
People have known that substances like vinegar and juice from citrus fruits like lemons and limes possess a sour taste for thousands of years. But we have only known why they taste sour for the last couple of hundred years and that’s because these substances are acids. The term acid itself comes from the Latin word “acere” which means “sour”. Throughout the ages, people have loved the sour flavor of acids – we certainly see that today as well: take a look at a candy counter in a convenience store; how many varieties of sour candy do you see? A bunch! And some of that stuff is so sour it makes your whole face pucker up! I’ll bet some youngster has probably offered you a piece of this super sour stuff and waited and watched in the anticipation of your gnarly reaction.
For a long time, the substances, which would later be called bases, were only considered in conjunction with acids. Acids were the stars of the show and bases were the supporting actors. Because of that, the study of bases was overshadowed by the study of acids. Nevertheless, bases have also been known for a long time. An early word for “base” was “alkaline” and it has Arab roots. The term originally referred to ashes, because the first alkaline substances studied were obtained from the ashes of organic material. The origin of the word “base” was introduced a lot later, maybe because bases were the basic or fundamental compounds used to form salts with acids.
Let’s consider acids first
An acid is a substance that yields hydrogen ions (H +1) to water
Properties of acids:
- Acids have a sour taste (as we have discussed above – but that doesn’t mean you should check that out with just any old acid you come across! Many acids would do irreparable damage to your mouth if you did!)
- Acids affect indicators (for example: litmus a blue vegetable dye turns from blue to red in the presence of an acid)
- Aqueous solutions of acids conduct electric current (they are electrolytes)
- Acids neutralize bases to form water and a salt (salt is not just NaCl, a salt is defined as the other product of a neutralization reaction between an acid and a base besides water)
- Hydrogen gas is often produced when an metal is placed in an acid
Examples, formulas, and uses of some common acids:
Strong acids (release a lot of hydrogen ions in water)
|Sulfuric||H2SO4||vehicle batteries, pickling metals, petrochemical products|
|Hydrochloric||HCl||major component of gastric juice|
|Nitric||HNO3||explosive manufacture, indicator for proteins|
Weak acids (release fewer hydrogen ions in water)
|Carbonic||H2CO3||soda pops, buffer in the blood stream|
|Boric||H3BO3||eye drops, roach-killer(!), wood preservative|
|Acetic||HC2H3O2||vinegar is a 5% solution of acidic acid|
|Citric||H3C6H5O7||citrus fruits, preservative|
A base is a substance which yields hydroxide (OH-) ions in water
Properties of bases:
- Bases have a bitter taste – but please take my word for it, don’t eat any to see for yourself! (Ever have soap in your mouth?)
- Bases give your skin a slippery feel because they are liberating your skin’s natural oils
- Bases affect indicators (litmus stays blue or turns from red to blue, phenolphthalein turns pink-red)
- Bases neutralize acids
- The aqueous solutions of bases conduct electricity (are electrolytes)<
- Bases have a caustic action on hair
Examples, formulas, and uses of common bases:
|Sodium hydroxide (lye)||NaOH||drain cleaner, soap production|
|Calcium hydroxide (lime)||Ca(OH)2||mortar, depilatory, tanning hides|
|Magnesium hydroxide||Mg(OH)2||milk of magnesia = antacid|
|Ammonium hydroxide||NH4OH||ammonia water = grease cleaner|
|Potassium hydroxide||KOH||liquid soap production, wart solvent|
Concentrations of acids and bases
When we’re dealing with acids and bases, we’re largely dealing with solutions, so let’s take a look at some important terms to understand:
Solution – a mixture consisting of a solute and a solvent
Solute – the part of a solution that’s present in the lesser amount
Solvent – the part of a solution that’s present in the greater amount
Concentration – the amount of a solute present in a solution per standard amount of solvent
Turns out that there are a lot of ways of expressing concentrations. We’re going to stick to molarity.
Molarity (M): The moles of solute per liter of solution. (remember Moles from stoichiometry?)
Molarity = Moles of solute/ liter of solution
Ex: A one molar solution (1M) is one that contains one mole of solute per liter of solution.
Ex: A six molar solution of hydrochloric acid (written: 6M HCl) means there are 6 moles of HCl per liter of solution. If the a solvent is not specifically listed, it’s most likely water.
Ex: A four molar solution of sodium hydroxide (4M NaOH<) would have 4 moles of NaOH per liter of solution.
The pH system: a way to measure the concentrations of acids and bases
In 1909, a Danish chemist named Sorensen developed a method of measuring the relative strengths and weaknesses of acids and bases. In his language it was the potenz of H+ or the power of the hydrogen ion that he was measuring. That’s how he came up with the symbol we still use today:
pH (the p is lower case and the H is upper case)
pH measures the concentration of hydrogen ions in a solution. The pH scale goes from 0 – 14. Because Sorensen used inverse logarithms to come up with his scale, low pH values (from 0 – just below 7) indicate solutions with high concentrations of hydrogen ions and low concentrations of hydroxide ions (acids), while high pH values (from just above 7 – 14) indicate solutions with low concentrations of hydrogen ions and high concentrations of hydroxide ions (bases). Pure water has a pH of 7.0, since pure water has a balance between H+ and OH-.
So, acids are found from 0 – just before 7 and the lower the number the stronger the acid.
Bases are found from just above 7 – 14 and the higher the number the stronger the base.