Several definitions of a base are used in the contemporary chemistry:
- Arrhenius base – a substance that increases the concentration of hydroxide anions when dissolved in water;
- Brønsted-Lowry’s base – a substance that takes up proton when reacting with acid;
- Lewis base – a substance that yields an electron pair of another substance, when reacting with acid.
The most widely used is the Brøndsted- Lowry’s definition.
Bases in the broad sense include three groups of substances:
- Water-soluble metal hydroxides: NaOH, Ca(OH)2, etc.;
- Water-insoluble oxides or hydroxides that can react with acid: FeO, Al(OH)3, etc.;
- Other compounds which, when dissolved in water, interact with it and release hydroxide ions: NH3, CH3NH2, etc.
Some of the general properties of the bases are:
- Soapy or slimy touch;
- Bitter taste;
- Electric conductivity;
- Violent reaction with reducible or acidic substances; caustic on organic matter;
- Turn red litmus paper blue.