Elements, compounds, acids, and bases
Classify materials by composition and use the pH scale as a measure of acidity in aqueous systems.
Pure substances and mixtures#
An element cannot be broken into simpler substances by ordinary chemical means. A compound has different elements chemically bonded in a fixed proportion. A mixture combines substances without a fixed chemical bond, so physical methods may separate them.
Acids and bases#
In an introductory aqueous model, acids increase hydrogen-ion concentration and bases increase hydroxide-ion concentration or accept hydrogen ions. Indicators change color over ranges of acidity.
The pH scale commonly runs from 0 to 14 for dilute aqueous solutions, with 7 neutral under standard conditions. Lower values are more acidic and higher values more basic.
Logarithmic scale#
A change of one pH unit represents a tenfold change in hydrogen-ion concentration. A solution at pH 3 is not merely “one unit” more acidic than pH 4 in a linear sense.
Neutralization#
An acid and base can react to produce salt and water, though real reactions and hazards depend on substances and concentrations.
Safety and mistakes#
Never identify chemicals by taste or touch. “Natural” does not mean harmless, and strong or concentrated acids and bases can cause severe injury. Classroom work requires appropriate supervision and procedures.
Key points
- Elements contain one type of atom; compounds contain chemically bonded elements.
- Mixtures retain components that can be separated physically.
- Acids and bases show characteristic behavior in water.
- The pH scale is logarithmic, so each whole step represents a tenfold change.