Science problem

Fluoride is added to tap water to prevent tooth decay occurs in our mouth cavity.
First,let me introduce the causes of tooth decay.
Bacteria grows in our mouth.After having a meal,some food debris may trapped between teeth/adhered on the teeth surface.
During digestion of carbohydrates in our mouth,bacteria acts on the sugar and starch in food,this reaction is sped up by saliva due to the presence of enzyme,to produce an acid.
Enamel is a non-living layer which mainly contains calcium salt.
As acid can dissolve calcium,the enamel is then dissolved by the acid."Holes" appears on the surface of teeth.
Dentine is a calcified tissue,thus acid will dissolve it after dissolving the enamel.
Hence,if no further treatment has been done on the small "hole",
the "hole" will become larger and deeper.
Finally,the acid will reach the pulp cavity.Pulp cavity contains blood vessels and nerves.Acid irritates the nerves and causing pain.

Fluoride is alkaline.It can neutalise the acid in our mouth.This can prevent tooth decay.

2012-03-01 19:43:50 補充：
SORRY-.-我睇唔到要用中文答,我用中文簡單咁打多次
蛀牙係因為口腔入面有acid(酸).琺瑯質(enamel) contain calcium(鈣),所以acid 會 dissolves 琺瑯質,之後再dissolve埋牙本質(dentine).
咁樣d acid就會reach 到髓腔(pulp cavity),髓腔入面有神經線,acid會刺激到d神經,造成痛楚.......
Fluoride係alkaline(鹼性),佢可以中和口入面既acid,無acid咁enamel,dentine就唔會被dissolve,咁就可以prevent到tooth decay.

2012-03-02 18:00:17 補充：
我記得F2教acid and alkali個陣老師講過toothpaste(contain fluoride)係
alkaline,所以我以為水都係=.=....
唔好意思,我上網再幫你check一check

2012-03-02 19:31:02 補充：
reason of adding fluoride into water.
How does the addition of fluoride prevent tooth decay?
http://www.animated-teeth.com/tooth_decay/t4_tooth_decay_fluoride.htm

2012-03-02 19:34:50 補充：
thx Billyionaire for pointing out my mistake

2012-03-02 19:43:13 補充：
http://210.60.226.25/science/content/1981/00070139/0006.htm
(provided by billyionaire)

氟具有許多特性，因此能防治蛀牙，主要特性如下：

一、氟離子與羥基磷灰石反應形成氟磷灰石：

Ca10(PO4)6(OH)2+2F-→Ca10(PO4)6F2+2OH-

氟離子比羥基離子更適合填入磷灰石的結晶格子中，因此形成比較穩定而且對酸的溶解度更小的晶體。氟磷灰石在乳酸中的溶解度約為羥基磷灰石的百分之一。

二、在琺瑯質中，本來就含有少部分雜質，其中以碳酸鹽較易溶於酸中，氟磷灰石生成後也相對地減少碳酸鹽的含量。

2012-03-02 19:43:21 補充：
三、氟也可以殺死齒斑中的細菌或干擾其代謝與生長，因而減低糖轉變為有機酸的量。

四、氟可能會抑制多醣類的形成，這些多醣類會幫助細菌附著在琺瑯質表面。

+fluorine除左令琺瑯質堅固之外都會幫手殺菌