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Factors Affecting Acid Strength
Two key factors contribute to overall strength of an acid:

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  polarity of the molecule
  
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  strength of the H-A bond
  

These two factors are actually related. The more polar the molecule, the more the electron density within the molecule will be drawn away from the proton. The greater the partial positive charge on the proton, the weaker the H-A bond will be, and the more readily the proton will dissociate in solution.
Acid strengths are also often discussed in terms of the stability of the conjugate base. Stronger acids have a larger K_a and a more negative pK_a than weaker acids.

Metal and acid reaction: Zinc reacting with hydrochloric acid to form hydrogen gas.
Bases
There are three common definitions of bases:

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  Arrhenius base: any compound that donates an hydroxide ion (OH^–) in solution.
  
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  Brønsted-Lowry base: any compound capable of accepting a proton.
  
• 
  Lewis base: any compound capable of donating an electron pair.
  

In water, basic solutions will have a pH between 7-14.
Base Strength and Strong Bases
A strong base is the converse of a strong acid; whereas an acid is considered strong if it can readily donate protons, a base is considered strong if it can readily deprotonate (i.e, remove an H⁺ ion) from other compounds. As with acids, we often talk of basic aqueous solutions in water, and the species being deprotonated is often water itself. The general reaction looks like:
A−(aq)+H2O(aq)→AH(aq)+OH−(aq)A−(aq)+H2O(aq)→AH(aq)+OH−(aq)
Thus, deprotonated water yields hydroxide ions, which is no surprise. The concentration of hydroxide ions increases as pH increases.
Most alkali metal and some alkaline earth metal hydroxides are strong bases in solution. These include:

• 
  sodium hydroxide (NaOH)
  
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  potassium hydroxide (KOH)
  
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  lithium hydroxide (LiOH)
  
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  rubidium hydroxide (RbOH)
  
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  cesium hydroxide (CsOH)
  
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  calcium hydroxide (Ca(OH)₂)
  
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  barium hydroxide (Ba(OH)₂)
  
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  strontium hydroxide (Sr(OH)₂)
  

The alkali metal hydroxides dissociate completely in solution. The alkaline earth metal hydroxides are less soluble but are still considered to be strong bases.