Download NaCl (aq) + AgNO3 (aq) → NaNO3 (aq) + AgCl (s)

by CHUNG, Yik-Sham
INTENDED READERS:
FORM 4 OR ABOVE STUDENTS
STUDENTS
Nowadays, more and more people are aware of the
importance of their health. They would try to have a healthy
diet, by eating more vegetables and less meat. However,
eating too much vegetable would inhibit the absorption of
calcium ions, as well as increasing the chance of formation
of kidney stones. These surprising facts can be explained by
a simple chemical reaction: PRECIPITATION.
Precipitation Reaction
Precipitation
is the formation of insoluble solids in a
solution1. An example is the reaction, which should have
been taught in F.3 chemistry, between silver nitrate solution
(AgNO3 (aq)) and halide solution, such as sodium chloride
solution (NaCl (aq)). White precipitates are formed because
the product, silver chloride (AgCl), has very low solubility in
water.
White
Precipitates
NaCl (aq) + AgNO3 (aq) NaNO3 (aq) + AgCl (s)
Precipitation reactions do not occur only in test tubes. They
can be carried out in our bodies, and the one between
calcium and oxalate ions, in fact, plays an important role in
the formation of kidney stones and the inhibition of the
absorption of calcium ions2.
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Mismatches between Food
Oxalate and Calcium
Many vegetables (Fig 1) are rich in oxalic acid. Examples include spinach,
parsley and carambola1. Oxalic acid in these vegetables can be absorbed into
our bodies in the form of oxalate, with two protons being dissociated out1, 2.
Fig 1: Some common vegetables with
high oxalic acid content. spinach (top
left), parsley (top right), and carambola
(bottom)@ 2003 green recipe.com and
@ 2001-2007 PCC Natural Markets
Calcium ions in our bodies come from a variety of food, including cheese,
milk and butter1, 3, 4. They can be absorbed simply by passing through the
intestinal wall into the bloodstream. Some of the absorbed ions would then
deposit in bones with the help of magnesium ions5. Therefore, in addition to
the dietary source, calcium ions in blood can be from dissolved bone calcium
ions1. The following table shows the calcium content of some common food:
Food
Cow’s milk
Swiss cheese
Yogurt
Ice-cream/frozen yogurt
Canned salmon
Egg
Soybean curd (tofu)
Almonds
Cooked beans
Serving Size
1 cup (250ml)
1 oz. (slice)
1 cup (175ml)
1/2 cup (125ml)
3 oz.
1 medium
4 oz.
1/2 cup (125ml)
1 cup (250ml)
Calcium Content (mg.)
300
250-270
240-300
90-100
170-210
55
145-155
200
90
Table 1: Calcium content in some common food3, 4
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Mismatches between Food
Formation of Kidney Stone
One of the possible causes of the
formation of kidney stone is the
intake of vegetable with high oxalic
acid content. The acid in vegetable
can be absorbed in form of oxalate
and can react with calcium ions in
blood to form calcium oxalate, which
is insoluble and would precipitate
out1, 2, 6.
Fig 2: Structure of human urinary system.
Note that ureter is the narrow tubule connecting
bladder and kidney © 2001- 2007 wikipedia
Ca2+(aq) + C2O42-(aq) CaC2O4(s)
precipitates
Most of the small calcium oxalate
solids can pass through kidney and
bladder in urine, thus can be
excreted out of the bodies. However,
if a large quantity of vegetables with
high oxalic acid content is taken,
calcium oxalate will be formed in a
significant amount and the solids will
grow into a large size before
excretion and block a narrow tubule
called ureter (see Fig 2), which
connects the outlet of kidney with
the bladder1,6.
The solids obstructed ureter are
called kidney stones1,2,6 (Fig 3).
Patients suffering from kidney stones
will result in acute flank pain,
vomiting, restlessness and even fever
if an infection happens1,6.
Fig 3: Photo of kidney stone.
Note that the kidney stone is large in size, with
8mm diameter. © 2001 – 2007 wikipedia
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Mismatches between Food
As a result, over-intake of vegetables
which are rich in oxalic acid can lead
to the formation of kidney stones
and bring a severe painful sensation
to the patients.
Wrong Matching in Diet
Oxalic acid in vegetables can inhibit
absorption of calcium ions, too1,2.
Some Chinese dishes, such as ‘bean
curd and spinach soup’ (Fig 4) and
‘bean curd with chopped green
onion’, are rich in both calcium and
oxalic acid. Taking these dishes will
not lead to formation of kidney
stones, but our bodies are impossible
to absorb the nutritive calcium ions.
Fig. 4: Bean curd and spinach soap
© 1999 – 2000 recipe. goolu.com
When both calcium and oxalic acid
reach our stomach, they will combine
to form calcium oxalate, which
precipitate out as solids and cannot
pass through intestinal wall. These
solids would pass through the gut
and then be excreted out of the
bodies in stool. Therefore, the
the bodies in stool. Therefore, the
calcium ions cannot be absorbed into
the bodies and wasted1,2.
Simple Science Principles:
Simple yet IMPORTANT!
In conclusion, taking too much
vegetable with high oxalic acid
content alone can result in the
formation of kidney stones by the
reaction of absorbed dietary oxalate
ions
with
calcium
ions
in
bloodstream, either from previously
absorbed calcium ions or dissolved
bone calcium ions. On the other
hand, having dishes with both
calcium and oxalic acid would result
in inhibition of absorption of calcium
ions. This is common to Chinese
because a number of Chinese dishes
contain food with high content of the
above two. Both of them are
governed by a simple chemical
reaction
called
precipitation.
Therefore, if we look into science,
even with simple principles, it is
possible to have some important
discoveries!
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Mismatches between Food
References:
1. Wikipedia. Home page. [online]. Available:
http://en.wikipedia.org/wiki/Main_Page 2007. Accessed: 2007 March 6
2. Oxalic acid and Foods. Home page. [online]. 2007. Available:
http://growingtaste.com/oxalicacid.shtml 2007. Accessed: 2007 March 2
3. iVillage. Calcium content of various calcium-rich foods [online]. Available:
http://health.ivillage.com/eating/ewdairy/0,,bh5x,00.html 2007. Accessed:
2007 March 2
4. Manbir online. Calcium content of some common food items [online].
Available: http://manbir-online.com/htm2/osteo.4.htm Accessed: 2007
March 2
5. Magnesium and Calcium absorption. Home page [online]. Available:
http://www.mgwater.com/calmagab.shtml 2002. Accessed: 2007 March 2
6. MedlinePlus. MedlinePlus Medical Encyclopedia: Kidney stones [online].
Available: http://www.nlm.nih.gov/medlineplus/ency/article/000458.htm
2007. Accessed: 2007 March 6
In-Text Image Sources:
Green Recipe.com. Home page [online].
Available: http://www.greek-recipe.com/static/ingredients/spinach.html
2003. Accessed: 2007 April 12
PCC Natural Markets. Carambola [online]. Available:
http://www.pccnaturalmarkets.com/ 2007.Accessed: 2007 April 12
Recipe.goolu.com. Home page [online]. Available:
http://recipe.goolu.com/01_catalog/index.php?php=prototype/h020_070_a.
htm 2000. Accessed: 2007 April 12
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