IS A difficult question help!!

acid是酸(具體定義是一種hydrogen ion's concentration高過hydroxide ion's concentration既溶液)
alkali是鹼(具體定義是一種hydrogen ion's concentration低過hydroxide ion's concentration既溶液)

indicator是指示劑，是一些當放到大部份溶液中能顯示該溶液的酸鹼度(pH)的物品

two indicator:methyl orange and phenolphthalein
color in acid for methyl orange :red
color in acid for phenolphthalein :colorless
color in alkali for methyl orange :yellow
color in alkali for phenolphthalein :pink

2013-07-24 15:38:15 補充：
但除了pH meter，pH paper同universal indicator外，大部份其他indicators(包括上述的2種)，也只能顯示大概的pH值(例如由pH 0-7左右 phenolphthalein也是colorless)

2013-07-24 19:42:32 補充：
但是除了pH meter,pH sensor,pH paper同universal indicator外，大部份indicator也是不能反映出實際的pH值的，因為它們在一個範圍中的pH的顏色是一樣的(例如phenolphthalein在pH0-8左右也是colorless)

If you want to determine whether the solution is acid and alkali, I do NOT consent
to the answer given by "Yc".

Methyl orange and phenolphthalein are normally used to detect the end point
during titration. It is NOT used for determinating whether the solution is acid or
alkali.

In weak acid (pH4.4-6.9) and alkali (pH7.1-14), if methyl orange is used, it shows
the same colour (yellow). You can only determine whether the solution is strong
acid or not.

In acid (pH0-6.9), weak alkali (pH7.1-8.2) and very strong alkali (pH>12), if
phenolphthalein is used, it shows the same colour (colourless). You can only
determine whether the solution is super strong acid (red in colour when pH<0) or
strong alkali (pH8.2-12) or not.

Instead, it is advisable to use universal indicator and litmus solution. It can
indicate whether the solution is acid or alkali.

Of course, using unicersal indicator will be better as it shows a wide range of
colour to detect the pH or the solution. Red, orange and yellow show that it is an acid. Green shows that it is neutral and blue and purple show that it is an alkali.

Or you can simply use litmus solution which turns red in acid and blue in alkali.
In neutral solution, it is purple.

In theory, the solution which has more H+ than OH- is called acid. Oppositely, it
is an alkali if it contains more OH- than H+.

If you don't understand, you can think in the following way. The compound which
has H+ is an acid (e.g. *HCl, *HNO3, *H2SO4, CH3COO*H, the H with * will form
H+ in aqueous solution.) The compound which has OH- is called a alkali
(e.g. Na*OH, K*OH, #NH3, the OH with * will form OH- in aqueous solution. # is a
special one because it forms NH4+ and OH- in aqueous solution,)

Or, you can think in this way too. The solution which has pH lower than 7 is an
acid. If its pH is higher than 7, it is an alkali.

Indicator is the chemcal that helps you to determine whether the substance
tested contains a kind of ingredients or not.