acids, bases & salts Flashcards
indicator
An indicator is a substance that is one colour in acids and a different colour in alkalis. they can be used either in solution or the solution can be dried on paper to make test papers.
universal indicator
universal indicator is a mixture of several different indicators. Universal indicator can indicate the strength of the acid or alkali
Red litmus paper
Red litmus paper turns blue when alkalise are added. it’s used to test for alkalis.
blue litmus paper
blue litmus paper turns red when acids are added. it is used to test for acids
The ph scale
understanding the PH scale
It is the hydrogen iron H that causes acidity. the pH scale is a measure of the concentration of hydrogen ions in solution and hence its acidity. So therefore, a solution of PH 1 contains 10x more hydrogen ions than a solution that has a pH 2. it is 10x as acidic. PH 1 is 100x more acidic than PH 3.
How to determine the ph of a solution precisely
by using a digital pH metre which can determine the PH to 1 or 2 d.p.
Acids
Acids can be regarded as a substance containing hydrogen and giving a pH of less than seven when dissolved in water.
Salts
Salts are a ionic substances where hydrogen is replaced by other positive ions (normally metals)
e.g. Mg + 2HCl → MgCl₂ + H₂
concentrated and dilute acids
refers to the amount of acid molecules dissolved in water a concentrated acid will have a high amount of acid molecules dissolved in a certain volume of water. a delete acid will have a relatively low number of acid molecules dissolved in the water.
strong and weak acids
refers to how many of the acid molecules have been split apart into ions this is called dissociation or ionisation
Strong acids (more detail)
strong acid will completely dissociate into its ions - all of the acid molecules have broken apart and formed into hydrogen ions. acid such as HCL, HNO₃ and H₂SO₄ are all strong acids. we can show that acid is fully dissociates by writing an equation – HCl ⇌ H+ + Cl-
weak acids (more detail)
In a weak acid only a small proportion of the molecules dissociate into ions this means the solution contains mainly undissociated compounds and just a low number of ions acids such as ethanoic acid, CH₃COOH all weak acids we can show that they only partially dissociate by writing an equation - CH₃COOH ⇌ CH₃COO- + H+
Bases
- all substances whether soluble or not that neutralise acids.
- normally metal oxides or metal hydroxides
alkali’s
- subgroup of bases - also neutralise acids but must be soluble in water
- generally soluble hydroxides
- all alkalise will either dissolve in water or react with water to produce hydroxide ions (OH-) in solution. these ions cause alkalinity.
- Like acids alkalise can also be concentrated or dilute and weak or strong
