Which of the following is a true statement about acids and bases?
an acid has a ph of 7.1.
an acid accepts hydrogen ions.
a base has a ph of 7.
a base gives off hydroxide ions.

A base gives off hydroxide ions.

Explanation: According to Arrhenius concept, a base is defined as a substance which donates hydroxide ions when dissolved in water and an acid is defined as a substance which donates hydronium ions in water.

According to the Bronsted Lowry conjugate acid-base theory, an acid is defined as a substance which donates protons and a base is defined as a substance which accepts protons.

Acids have pH values ranging from 1 to 6.9, neutral solutions have pH of 7 and bases have pH ranging from 7.1 to 14.

A substance that increases the hydroxide ion concentration of a solution is an Arrhenius base.

a:

by the combination of an unlimited number of elements

  the   balanced   equation   for   reaction of   ca metal   with   o2 to   produce   cao   is as below

2 ca+ o2→ 2 cao2

  explanation

the reaction is   balanced     since the   number of atoms   in reactant side is   equal to   number of atoms   in   product side.   for   example   they   are 2   atom of   ca in   reactant   side   and   2   atoms of ca   in   the product side.

  the   process   is a   meet the   requirement   of   redox   reaction because :     calcium   is   oxidized   from   oxidation state 0   to 2+   while   oxygen   is reduced from   oxidation   state   0   to 2-