ａｃｉｄ　和　ａｋａｌｉｓ

an acid is a molecule or compound which ionizes or dissociates in water to give H+ as the only positive ion.

an alkali is a soluble base. A base in turn means a species which reacts with an acid to form salt and water only.

strong acid or alkali means one which ionizes or dissociates completely in water e.g. HCl, HNO3, NaOH, KOH

a weak or alkali is one which only ionizes or dissociates partially in water e.g. CH3COOH, NH3

Strong acid or strong alkali means the acid or alkali can completely ionize when dissolve in water. example NAOH and HCL/// when NAOH dissolve in water, It will form Na plus and OH minus. HCL will form H plus and Cl minus when dissolve in water.

Weak acid: CH3COOH
It will form CH3COOH and CH3COO minus and H plus
That means some molecules will not form ions.

Acid means the pH value that is smaller than 7
Alkali means the pH value that is greater than 7