pH counts how many hydrogen ions a solution holds, as a power of ten. Pure water has 10⁻⁷ moles of them per litre, so its pH is 7. Each step down is ten times more acidic: lemon juice at pH 2 has 100,000 times the hydrogen ions of pure water. Above 7, a solution has fewer hydrogen ions than water and more hydroxide ions (OH⁻), and is basic.