Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne NaCO3 as Acid and Base
Name:  Lindsey
Status: other
Grade: other
Location:CO
Country: N/A
Date: 4/26/2005


Question:
Why is NaCO3 both an acid and a base?


Replies:
First, the formula for the species above (NaCO3) is incorrect. It should be (Na2CO3), (NaHCO3), and (H2CO3). That is, the carbonate anion is di-valent (CO3)(-2). In water these equilibria occur simultaneously. So if acid (H+) is added to the solution, some Na2CO3 is converted to NaHCO3 and H2CO3. The relative amounts being determined by how much acid (H+) is added. On the other hand if base (OH)(-1) is added to the solution, some H2CO3 is converted to HCO3(-1) + H2O, and (OH)(-1) + HCO3(-1) ---> H2O + CO3(-2), again, the relative amounts depend upon how much (H+) or (OH-) is added. So a solution of the sodium salts of carbonate can react with either an acidic (H+) [meaning it is a base] or with (OH-) [meaning it is an acid.

Vince Calder


Hi Lindsey-

A detail: NaCO3 is an unbalanced formula. CO3(2-) needs two (+)'s, and Na(+) only has one. So you probably meant Na2CO3 or NaHCO3. (adding an Na(+) or an H(+)). This is important because the H does a lot of the work.
Na2CO3 is only a base:
    CO3(2-) + H2O  <-->  HCO3(-)  + OH(-)
    CO3(2-) + H(+)  -->  HCO3(-)
    HCO3(-) + H(+)   -->  H2CO3  <-->  H2O + CO2(gas)

NaHCO3 is both acid and base like you mentioned. (Do not forget the H.) In neutral solutions it ionizes to Na(+) + HCO3(-). The HCO3(-) can either donate its proton (acid) or accept another (base). in strong basic solution:

HCO3(-) + OH(-) --> CO3(2-) + H2O

in strong acidic solution:

HCO3(-) + H(+) --> H2CO3(temporary) <--> H2O + CO2(gas).

When a substance adds acid to basic solutions and base to acid solutions, such as NaHCO3 does, it is called a buffer. It tends to damp out either extreme, tries drag them to the pH it considers "the middle". Buffers are good to have around in case of spills of strong acids or bases.

Jim Swenson



Click here to return to the Chemistry Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory