2.28: Describe the Use of Litmus, Phenolphthalein and Methyl Orange to Distinguish Between Acidic and Alkaline Solutions

LITMUS:

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• Litmus indicator solution turns Red in Acidic solutions, Blue in Alkaline solutions, and Purple in Neutral solutions
• Litmus paper is usually more reliable - comes as Red litmus paper or Blue litmus paper

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	RED LITMUS	BLUE LITMUS
ACIDIC SOLUTION	Stays Red	Turns Red
NEUTRAL SOLUTION	Stays Red	Stays Blue
ALKALINE SOLUTION	Turns Blue	Stays Blue

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PHENOLPHTHALEIN & METHYL ORANGE:

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• Phenolphthalein changes colour sharply at approximately pH 8, and is commonly used in titrations
• Methyl Orange is a solution and changes into a range of warm colours

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INDICATOR	ACID	NEUTRAL	ALKALI
PHENOLPHTHALEIN	Colourless	Colourless	Pink
METHYL ORANGE	Red	Yellow	Yellow

2.29: Understand How to Use the pH Scale, From 0-14, can be Used to Classify Solutions as Strongly Acidic (0-3), Weakly Acidic (4-6), Neutral (7), Weakly Alkaline (8-10) and Strongly Alkaline (11-14)

UNIVERSAL INDICATOR: Solution or paper indicator that changes into a range of colours to specifically identify pH of solution

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• In Solution form, drops of Indicator is added and colour change is observed
• In Paper form, paper is dipped into solution and colour change is observed

pH SCALE:

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2.30: Describe the Use of Universal Indicator to Measure the Approximate pH Value of an Aqueous Solution

UNIVERSAL INDICATOR: Solution or paper indicator that changes into a range of colours to specifically identify pH of solution

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USE OF UNIVERSAL INDICATOR TO MEASURE pH OF AQUEOUS SOLUTION:

• If using universal indicator solution, a few drops of this indicator solution is added to the Aqueous Solution and colour change is observed and matched to specific colour on pH scale (to identify pH)
• If using universal indicator paper, this paper is dipped into Aqueous Solution and colour change is observed and matched to specific colour on pH Scale (to identify pH)

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UNIVERSAL INDICATOR AND pH SCALE:

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Colour change of universal indicator is matched to a specific colour on pH Scale, identifying approximate pH

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2.31: Know that Acids in Aqueous Solution are a Source of Hydrogen Ions and Alkalis in a Aqueous Solution are a Source of Hydroxide Ions

ACIDS AND ALKALIS

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• When atoms or groups of atoms lose or gains Electrons, they form charged particles called Ions - either positively or negatively charged

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*ACIDS:

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• When Acids dissolve in Water to form an Aqueous Solution, they produce Hydrogen ions (H+) which makes the Aqueous Solution Acidic

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    Example: Hydrochloric Acid

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HCl (aq)   →    H+ (aq)    +    Cl- (aq)

    

    *Acids are often produced from Non-Metal Oxides, E.g Sulfur Oxides produce    

     Sulfuric Acid

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*ALKALIS:

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• When Alkalis dissolve in Water to form an Aqueous Solution, they produce Hydroxide ions ( OH- ) which makes the Aqueous Solution an Alkali

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    Example: Sodium Hydroxide

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NaOH (aq)   →    Na+ (aq)    +    OH- (aq)

2.32: Know that Alkalis can Neutralise Acids

NEUTRALISATION: Reaction which removes Acidity or Alkalinity of solution

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• When Alkali is added to an Acid, the pH of mixture rises as the Alkali reacts with Acid to form a neutral solution
• When an Acid is added to an Alkali, the pH of the mixture falls as the Acid reacts with Alkali to form a Neutral Solution

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NEUTRALISATION REACTION:

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Neutralisation reaction involving an Acid and base (Alkali) will always produce a Salt and Water:

Acid    +    Base (Alkali)   →    Salt    +    Water

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When H+ ions from an Acid reacts with the OH- ions from an Alkali, this forms Water, causing the neutral solution to be formed:

H+ (aq)    +    OH- (aq)   →    H2O (l)

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The Name of the Salt Produced can be Predicted:

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ACID REACTED	SALT
HYDROCHLORIC ACID	Salt will be a Chloride
NITRIC ACID	Salt will be a Nitrate
SULFURIC ACID	Salt will be a Sulfate

2.33C: Describe How to Carry Out an Acid - Alkali Titration

TITRATION: Method to determine the volume of one solution that is required to react with a given volume of another solution - used to determine concentration of Acid or Alkali

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TITRATION

Diagram showing the Setup of a Titration

METHOD: Use pipette to measure Alkali solution into conical flask Add drops of indicator (Phenolphthalein or Methyl Orange) Add Acid into burette and note the starting volume Add Acid in burette into Alkali in flask until indicator changes to appropriate colour Note the final volume of Acid in burette

INDICATORS As universal indicator has no end-point (colour will constantly change depending on pH), universal indicator is not used for titration Instead, Phenolphthalein or Methyl Orange is used as there is a end-point, allowing colour change to be observed more easily

CALCULATION Volume of Acid Added   =   Starting Volume of Acid   -   Final Volume of Acid