|
Alkalis are the opposite of acids. They have very low concentrations of hydrogen ions
when dissolved in water. their pH values are above 7.
pH 7 is neutral. A solution with a pH of 7 is neither acidic nor alkaline.
Alkalis will turn red litmus paper to blue.
When food is squirted from your stomach into your small intestine, an alkaline liquid
called bile is added to it. We say that bile neutralises the stomach acid, but this is not
strictly true. In fact so much bile is added that the mixture of food, bile and enzymes is
turned into an alkaline solution. It is about pH 12. The enzymes in your small intestines
work best at pH 12.
The alkalis which you are most likely to use at school are sodium hydroxide and calcium
hydroxide. Take care when using sodium hydroxide because it is very caustic (it will burn
your skin). Your teacher will probably tell you to wash your hands after using it, or may
even ask you to wear rubber gloves. Sodium hydroxide is a strong alkali, i.e. it has a
high pH. Calcium hydroxide is a much weaker alkali; it is also much safer to use since it is
not caustic. Your biology teacher will probably call it "lime water". You can
use it to test for carbon dioxide, which will turn lime water cloudy. If you breathe out
through a straw bubbling your breath through a test tube of lime water, it will go cloudy:
this proves that we breathe out carbon dioxide.
Lithium, sodium, potassium, rubidium, caesium and francium are all very reactive
metals: they will dissolve in water to make alkaline solutions. These metals are called
alkali metals and belong to group I of the periodic table. When they are dissolved in
water they make hydroxides: lithium hydroxide, sodium hydroxide, potassium hydroxide and
so on. You can find out more about these
elements on my GCSE web site.
Beryllium, magnesium, calcium, strontium, barium, and radium will also make
hydroxides. These metals are called alkaline earth metals. they belong to group II of the
periodic table.
You might find some alkaline chemicals in your kitchen. You can test chemicals to find
out if they are acids or alkalis using indicators: have a look at my page on indicators to find out how to make your own "litmus
paper".
I hope that you will be able to remember most of the facts on this page, but you must
memorise the definition of alkalis. Write it on a small index card. Put the red words on
one side of the card and the blue ones on the other. Add the card to your revision
pack.
Alkalis are the opposite of acids. They have
a pH of more than SEVEN.
|
