1: What are Acids and Bases?
General Properties of Acids and Bases
Acids and bases were first recognized by simple properties, such as taste. Vinegar and lemon juice taste sour because they contain acid. A basic solution has a bitter taste and feels slippery; an example of a basic solution is soapy water. An interesting and important property of acidic and basic solutions is their effect on the color of certain dyes and vegetable matter, which can be used as indicators. Litmus, a vegetable substance obtained from lichens, turns red in acidic solutions and blue in basic solutions (fig. 1).
Figure 1: Litmus paper is an indicator for acids and bases. When litmus paper is placed in acid, it turns red (right). When litmus paper is placed in base, it turns blue (left). [This picture is public domain; modified.]
Exercise: Black Tea Indicator Experiment
If you feel like performing an acid/base indicator test yourself, add lemon juice to black tea and observe the color change (fig. 2). Afterwards you may want to enjoy some pepped-up tea.
If you are feeling adventurous, try some common bases such as baking soda or bleach on the black tea, but definitely do NOT drink this mixture afterwards!
Figure 2: Black tea can be used as an indicator for acids and bases. [CC0 Public Domain]
Definition of Acids and Bases for Environmental Scientists
An acid is a substance that produces or donates protons (H+). For example, hypochloric acid (HCl) dissociates to form H+ and Cl-:
HCl(aq) ↔ H+(aq) + Cl-(aq)
(aq stands for aqueous. It means the ions are dissolved in water.)
H+ is the hydrogen ion, which is often referred to by the term “proton” because a neutral hydrogen atom contains one proton and one electron, so that a H+ ion is simply a proton. Naked protons H+ do not exist in aqueous solution because they are strongly attracted to water molecules. Instead, the H+ is hydrated, which means it is chemically bonded to a water molecule. This is represented by the formula H3O+ and called a hydronium ion.
Consequently, the previous equation could also be written as
HCl + H2O ↔ H3O+ + Cl-
As there is no uniform convention which notation (H+ or H3O+) should be used and you will probably stumble across both notations in your further studies, we will also use both notations in this course.
A base is a substance that produces hydroxide ion (OH-) and/or accepts H+. Many bases consist of a metal cations and OH-, for example sodium hydroxide (NaOH) dissociates in water to release OH-:
NaOH(aq) ↔ Na+(aq) + OH-(aq)
Another example of a base is ammonia (NH3). When NH3 is dissolved in water it accepts a proton to form NH4+. Additionally, OH- is produced in this reaction.
NH3(aq) + H2O ↔ NH4+(aq) + OH-(aq)
Let’s look at the water molecule in this reaction. The water molecule donates a proton to form OH-. Donating a proton makes H2O an acid in this reaction.
As acid-base reactions are equilibrium reactions (link to chemical equilibrium), it is interesting to look at the reverse reaction. In the reverse reaction NH4+ loses a proton, and thus acts as an acid. It is called a conjugate acid. OH- acts as a base since it accepts H+ to form H2O. It is called a conjugate base.
In the following equation, identify each reactant as an acid or as a base. Don'r forget to consider the reverse reaction.
HNO3 + H2O ↔ H3O+ + NO3-
Answer: see end of page
Have you noticed that in the previous examples water acted as an acid as well as a base? (If water reacts with NH3 it acts as an acid. When reacting with HCl it is a base.) This property is called amphoterism. An amphoteric substance can act as both a proton donor and a proton acceptor, that is, as an acid and as a base.
Another example of amphoteric substance is the hydrogen carbonate ion HCO3-. It can act as a base
HCO3- + H3O+ ↔ H2CO3 + H2O
or as an acid
HCO3- + OH- ↔ CO32- + H2O
Acids and bases can neutralize each other.
An amazing example of a neutralization reaction is the reaction between HCl and NaOH. Both substances are very corrosive and they can cause dangerous damage to human tissue. However, if combined the two reactive and hazardous substances react to form sodium chloride (NaCl) and water, which is harmless table salt dissolved in water.
HCl + NaOH ↔ NaCl + H2O
- I. Williams, 2001. Environmental Chemistry, pp. 319, John Wiley & Sons Ltd.
- S.E. Manaham, 2008. Fundamentals of Environmental Chemistry, 3rd edition, pp. 210. CRC Press, Boca Raton (FL).
- D.A. McQuarrie, 2011. P.A. Rock, E.B. Gallogly, General Chemistry, 4th edition, pp. 729. University Science Books.
- D.D. Ebbing & S.D. Gammon, 2012. General Chemistry, Cengage Learning, 10th edition, pp. 521. Cengage Learning.
- M.S. Cracolice, E. Peters, 2015. Introductory Chemistry: An Active Learning Approach, 6th edition, pp. 487. Cengage Learning.
Nitric acid (HNO3) is an acid because it donates a proton to form NO3-,its conjugate base. H2O acts as a base because it accepts a proton to form H3O+, its conjugate acid.
Info: Nitric acid is an acid contained in acid rain. You will learn more about acid rain later on this unit.