You may be familiar with the term "acid" to describe a substance that has a pH lower than
7, turns blue litmus paper red, and turns universal indicator solution or paper red
or orange depending on its strength. An acid is simply a substance that dissociates
to release a hydrogen ion or proton, in water. HA(g/l/s) <->H+(aq) + A-(aq)
In fact, you may even be familiar with the names, properties, and reactions of certain
acids, such as hydrochloric acid and sulphuric acid.
In this lesson, we will learn about other acids � and you may be surprised at the
numerous naturally occurring acids and their uses in everyday life.
Knowledge Core Let�s start this lesson by thinking about
citrus fruits � oranges, lemons, and lime. In particular, let�s think about lemons.
What �taste� is associated with lemons? Pause, think, and continue when ready.
Many citrus fruits have a defining sour taste to them.
This sour taste is due to the presence of a certain weak acid, citric acid.
Now, let�s think about apples. If you slice an apple and leave it out for
a while, what do you think will happen to the apple?
Pause, think, and continue when ready. The answer is that the apple will start to
turn brown. If you slice an apple and leave it out for
a while, it will start to turn brown. Why do you think this is the case?
Pause, think, and continue when ready. The answer is that it because it reacts with
the oxygen in the air. This �browning� that occurs is actually
an oxidation reaction. There are certain enzymes in your apple that
react with the oxygen in air to produce a brown pigment called melanin.
Melanin is the same pigment that makes tanned skin tan, and brown eyes brown!
To slow down this oxidation process, you can add lemon juice to your apple slices.
As citric acid is soluble in water, this can be washed away when ready to serve, if you
don�t want your apple slices to have a �citrus� taste to them.
It is a common misconception that citric acid is �vitamin C�.
In fact, vitamin C is ascorbic acid, and although it is found in citrus fruits, kiwifruit and
red peppers have a much higher ascorbic acid concentration.
Another food item that you may be familiar with is vinegar.
Vinegar is a dilute solution of ethanoic acid, also known by its common name, acetic acid.
Like citrus fruits, vinegar has a distinctive sour taste.
Although it is not a good idea to taste a substance to determine its properties, most
acidic food items have a sour taste to them. Rhubarb leaves are toxic owing to a high concentration
of oxalic acid. Oxalic acid, however, is still found in low
concentrations in spinach, parsley, and chives. The concentration is low enough so it does
not present any immediate dangers when consumed. In our minds, let�s open a cold soft drink
can or bottle. What image comes to mind?
Pause, think, and continue when ready. Most likely, you imagined bubbles rising from
the drink, and maybe even a �fizz� sound. This happens because carbonic acid is formed
when carbon dioxide is dissolved in soft drinks during the manufacturing process.
The reason why it is done is to give it its characteristic taste and �fizz�.
Challenge Here is a challenge for you.
You know that carbon dioxide dissolves in water to give carbonic acid.
Can you write down the equation for this reaction? Let�s try this � grab a piece of paper
and a pencil, pause the lesson, and continue when ready.
The reaction equation is: CO2(g) + H2O(l) <->H2CO3(aq)
Carbon dioxide + Water <-> Carbonic acid Did you get this right? Did you remember to
include the state symbols? Notice the reaction arrow that denotes that
this is a reversible reaction. Carbonic acid can dissociate to give hydrogen
carbonate and a hydrogen ion, also known as a proton.
In fact, an acidic solution is made because of the presence of these protons.
Soft drink cans and bottles are pressurized so that equilibrium favours the side of the
reaction with fewer moles, in this case, the right hand side.
Opening the tab or bottle cap decreases the pressure, favouring the side with more moles,
thereby releasing carbon dioxide. When you see bubbles being released and hear
the �fizz� sound, it is actually carbon dioxide being released.
Phosphoric acid is also sometimes added to soft drinks to give additional sour taste.
Carbonic acid is one of the components of acid rain.
Carbon dioxide in the atmosphere dissolves in rainwater to create carbonic acid, and
hence, acidic rain. Acidic rain can also decrease the pH of soils,
and of large bodies of water. This is a serious environmental issue because
it affects living organisms that can only function and survive at a specific pH or pH
range. A similar reaction occurs when carbon dioxide
dissolves in rainwater to form carbonic acid. Carbon dioxide in the atmosphere can dissolve
in rainwater to form carbonic acid. This makes rain slightly acidic with a pH
around 6. This is natural but unfortunately, increased
SO2 and NOx emissions from the combustion of fossil fuels have made rain a lot more
acidic when dissolved in rainwater to form H2SO4 and HNO3.
Acidic rain can weather limestone structures, which are then eroded by water and carried
into the rivers and seas. Acidic rain can also decrease the pH of soils,
and large bodies of water. This is a serious environmental issue because
it affects living organisms that can only function and survive at a specific pH or pH
range. In fact, we are one of those living organisms
whose existence depends on maintaining a certain pH, though our health is not directly affected
by acid rain. Our blood has a very specific pH � 7.41
If it is lower than this pH, even at 7.35, the medical condition is known as acidosis.
If it is higher than this pH, even at 7.45, then the condition is known as alkalosis.
However, for most healthy individuals, our blood pH is maintained by a very precise and
accurate buffer system, catalyzed by an enzyme called carbonic anhydrase.
CO2(g) + H2O(l) <-> H2CO3(aq) Carbon dioxide + Water <-> Carbonic acid
A key component of this buffer system, as you can see, is carbonic acid.
The extent of the dissociation of carbonic acid is critical in maintaining the optimal
pH. If the concentration of protons is too high,
it will increase the acidity of blood. If the concentration of protons is too low,
it will decrease the acidity of blood. If you exercised for an extended period of
time, your muscles will begin to cramp. If you continued to exercise, it would be
too painful to continue. This cramping feeling is due to the presence
of lactic acid, produced by your muscle cells when there is insufficient oxygen.
Remember how we learned that many acidic foods have a sour taste to them?
Yogurt and sour cream have a sour taste as well � due to the presence of lactic acid
and curdled casein. Milk contains lactose, and certain types of
bacteria can ferment lactose into lactic acid. The lactic acid then reacts with a certain
milk protein called casein. The combination of lactic acid and curdled
casein gives yogurt and sour cream their texture and taste.
The acidity acts as a natural preservative - certain bacteria and molds cannot grow in
solutions with a low pH, so the milk is preserved. When we pickle foods in vinegar, this too,
is a preservation method. The last acid that we will learn about is
nitric acid. It is a strong acid and is highly toxic at
high concentrations as it can decompose to nitrogen dioxide.
of explosives, a common one being TNT, or trinitrotoluene.
not just hydrochloric and sulfuric acid. A lot of these acids can be found in everyday
food items, in our bodily processes, and in many other real-life applications.