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Acids and Bases

The word "acid" comes from the Latin acidus meaning "sour" because acids have a sour taste. Lemon juice is sour because it is an acid.

Bases generally have a bitter taste and have a soapy or slimy feel. The word alkaline is almost a synonym for basic, although in chemistry the word alkali refers to just the soluble salts of alkali metals or alkali-earth metals. At secondary school level we normally use alkaline to mean a solution made from a base.

Acidity is measured with pH (note capitalisation), which normally ranges from 0 to 14, or -5 to 15 for particularly concentrated solutions. Strong acids have a low pH, while strong bases have a high pH. Neutral solutions such as distilled water (at 25 °C) is 7.

The pH scale is logarithmic, which means that each step along the scale is an increase or decrease of ten times. For example, going from pH 3 to pH 1 increases the acidity by 100 times.

If you measure the pH of tap water with a pH meter, you may be surprised at how far from a pH of 7 it is because of dissolved substances in the water. Distilled water is necessary to get a pH very close to 7. Water companies sometimes deliberately make their water a bit alkaline (pH 8.0 - 8.2) so plumbing is not eroded by acidic water.

If water is left to stand exposed to the air it will absorb CO2 from the air, which will form carbonic acid, lowering the pH of the water to about pH 5.7 over time. This is why water can taste a little sour when it has been left out. Water which has had CO2 forced into it (like with a Soda Stream machine) has an even lower pH.

 
Representative pH Values
Substance pH
Hydrochloric acid, 10M
-1.0
Hydrochloric acid, 1M
0.0
Car battery acid
0.5
Gastric acid
1.5 – 2.0
Lemon juice
2.4
Cola
2.5
White vinegar
2.6
Orange or apple juice
3.5
Beer
4.5
Acid Rain
<5.0
Coffee
5.0
Tea or healthy skin
5.5
Milk
6.5
Pure water
7.0
Healthy human saliva
6.5 – 7.4
Blood
7.34 – 7.45
Sea water
7.7 – 8.3
Hand soap
Baking soda
9.5
Household ammonia
11.5
Bleach
12.5
Household lye
13.5
Caustic soda 1M
14.0
Caustic soda 10M
15.0

Mostly from Wikipedia: pH.

Advanced – the pH scale

To get technical, pH is a measure of the activity of H+ ions, or more correctly, hydronium, H3O+ ions. Activity is not the same as concentration, but for strong acids and bases they can be taken to be about the same, because a strong acid almost completely dissociates (splits apart into ions). Weak acids and bases only partially dissociate.

For example, with the strong acid hydrochloric acid, all the HCl molecules will dissociate into H+ ions and Cl - ions. However, with the weak acid carbonic acid, only some of the H2CO3 molecules dissociate into the ions H+ and HCO3-.

If the H+ ions in a solution outnumber hydroxide (OH-) ions the solution will be acidic and have a low pH. If the OH- ions outnumber the H+ ions the solution will be basic and have a high pH.

Pure water has equal numbers of these ions (from its molecule, H2O) and so is neutral, with a pH of 7 at room temperature. However, at 50°C pure water has a pH of 6.55.

2H2O ⇌ H3O+ + OH-

The pH values of some common acids and bases at different concentrations can be found here.

Some of the stronger acids

Hydrochloric acid: Also known as muriatic acid, this is a highly corrosive acid and is often used (quite diluted) to clean calcium carbonate buildup from the inside of kettles or from around water faucets and from shower heads, and also to lower the pH of swimming pools.

Sulphuric acid: This is a common acid in both the laboratory and industry. It is both highly corrosive and economical to manufacture, which makes it the reagent of choice for many applications. It is the acid in "lead acid" batteries, like those used in cars. It used to be called oil of vitriol.

Phosphoric acid: This acid is used to remove rust and rust stains from metal tools and from car bodies undergoing repairs. Also present in soft drinks like Coca Cola (along with carbonic acid).

Nitric acid: This is another common laboratory acid used as a reagent in many chemical tests and experiments due to the fact that almost all of its products (salts) are soluble in water. Nitric acid was first made about 1200 years ago.

Hydrofluoric acid: While technically classed as a "weak" acid, this acid is extremely corrosive and has the unique property of being able to etch (eat away) glass. Consequently it is used in industry to write signs on glass windows in stores and office buidings or on glass products. Particularly nasty, it is a contact poison and will eat into skin and keep eating. Pain from serious burns may not be immediately evident.

Some of the stronger bases

Sodium hydroxide: Also known as caustic soda or lye. Used (quite diluted) to unblock drains. Used to make soap.

Potassium hydroxide: Used to make flowing soap.