Properties of Acids
What is an Acid -
A chemist named Svante Arrhenius defined acids and bases in 1887. He saw that when you put molecules into water, sometimes they break down and release an H+ (hydrogen) ion. At other times, you find the release of an OH- (hydroxide) ion. When a hydrogen ion is released, the solution becomes acidic. When a hydroxide ion is released, the solution becomes basic. Those two different ions determine whether you are looking at an acid or a base.
H+ concentration shifts away from neutral when an acid or base is added to an aqueous (water-based) solution. An acid is a substance that increases the concentration of hydrogen ions H+ in a solution, usually by donating one of its hydrogen atoms through dissociation. A base, in contrast, reduces the number of free H+ ions by providing hydroxide OH- or another ion or molecule that scoops up hydrogen ions and removes them from solution.
Dissociation of Water
Water is a neutral solution as the number of H+ ions is equal to the number of OH- ions
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The pH Scale
The pH scale is used to measure the concentration of free H+ and OH- ions. At all pH values there are both hydrogen ions H+ and hydroxide ions OH-. It is the relative concentrations of each ion that determines the pH for a solution.
An acid is a substance that releases hydrogen ions(H+) when it is dissolved in water.
In a strong acid such as hydrochloric acid(HCl) all the acid molecules releases their hydrogen ions(H+) when in water - they completely dissociate
In a weak acid such as ethanoic acid (CH3COOH) only some of the acid molecules release their hydrogen ions(H+) when in water
Acids have a pH range of 0 -3 (strong acid - Red colour Universal Indicator) - pH range 4 - 6 (weak acid Orange/Yellow colour Universal Indicator)
Acids have a sour taste
Neutral solutions have equal numbers of hydrogen ions(H+) and hydroxide ions(OH-).
Neutral substances have a pH of 7 and are Green colour with Universal Indicator.
A base is a substance that releases hydroxide ions(OH-) when it is dissolved in water.
In a strong base such as sodium hydroxide(NaOH) all the base molecules release their hydroxide ions(OH-) when in water- they completely dissociate
In a weak base such as sodium hydrogen carbonate(NaHCO3) only some of the base molecules release their hydroxide ions(OH-) when in water
Bases have a pH range of 13-14 ( Strong base - Purple colour with Universal Indicator) - pH range 8 - 11 ( weak base - Blue with Universal Indicator)
Bases have a bitter taste
What is an Acid?
Acids dissociate releasing H+ ions. The greater the number( concentration) of H+ ions the more acidic a solution is. Strong acids have a pH of 1 and will turn Universal Indicator dark Red..
What is a Base?
Bases dissociate and release OH- ions. The greater the number of OH- ions the more basic a substance is. Strong bases have a pH of 14 and will turn Universal Indicator dark Purple(Violet)
Quizlet
Use this Quizlet to help learn key terms for this topic
Acid - Base Indicators
Indicators are used to show visually if a substance is an acid a base or neutral.
The main indicators used are universal indicator or litmus.
Universal indicator paper(or liquid) can be used to tell the pH of a solution
Litmus can only be used to tell if a solution is acidic, basic or neutral. It does not establish the pH of the solution
Litmus Paper
Red Litmus Paper
- stays red(pink) in an acid
- turns blue in a base
Blue Litmus Paper
- stays blue in a base
- turns red in an acid
Rainbow fizz Experiment
The range of colours as acid is added to a basic solution. ROYGBIV colours shown by the universal Indicator - Red, Orange, Yellow, Green, Blue, Indigo(light purple) and Violet. You need to be able to identify these colours and the pH value they represent in the exam.
