Acids and Bases (4) Indicators and pH

Most solutions of acids and alkalis are colourless. How to tell if a solution is acidic or alkaline?

An indicator is a substance that has different colours in acidic and alkaline solutions. Common examples:  Litmus  Phenolphthalein  Methyl Orange

 Grown in acidic soil – they are blue.  Grown in alkaline soil – they are pink/red

 A set of numbers used to indicate whether a solution is acidic, neutral or alkaline. A measure of the acidity or alkalinity of a solution is known as pH.

The pH of a solution is calculated based on the concentration of H + or OH - present in a solution. More acidicMore alkaline Neutral

 pH < 7: Acidic  Contains H +  The smaller the pH, the more acidic the solution, the more H + it contains  pH = 7: Neutral. This is the pH of pure water.  pH > 7: Alkaline  Contains OH -  The greater the pH, the more alkaline the solution, the more OH - it contains

 Manufacturers have come up with lipsticks that change colour when you put it on. Many believe that the colour of the lipstick depends on the wearer’s mood.  The colour of the lipstick depends on the pH of our skin (which is affected by diet, stress and physical activity level)

1. Universal Indicator  A mixture of indicators  Gives different colours at different pH values  Approximate pH values

2. pH meter  Accurate pH values  pH probe is dipped into solution and pH is shown on the meter either on a scale or digitally

3. pH sensor and computer  A pH sensor connected to a computer through an interface can be used to measure the pH of a solution

 pH and the body  Substances in your body have different pH values. ▪ Stomach (pH ~1.5) ▪ Small intestine (pH ~8.4) ▪ Blood (pH ~7.4)  E.g. If the pH of the blood is changed by one unit during an injection, the person will die!

 pH and food preservation  Fresh food go bad due to microorganisms (bacteria) present.  Microorganisms do not grow well in solutions of low pH.  E.g. Acids can be used to preserve food. ▪ Ethanoic acid (vinegar) ▪ Benzoic acid ▪ Citric acid

 pH and hair  Shampoos are alkaline – to dissolve grease.  Alkalis damage hair – we must always rinse away shampoo after use.  Hair conditioners – contain weak acids, to restore pH of hair to its normal value (weakly acidic: pH ~5)

 pH and agriculture*  The pH of the soil is important for good plant growth.  Most plants grow best when the pH of the soil ~5.6 (weakly acidic).  Most plants will not grow well in soils that are too acidic/alkaline.

 pH and agriculture*  Soils become acidic from: ▪ Extensive use of chemical fertilisers ▪ Acid rain  Excess acidity can be neutralised by adding calcium hydroxide (‘slaked lime’) – ‘liming the soil’.

 Bases are the oxides or hydroxides of metals.  Contains either oxide ions (O 2- ) or hydroxide ions (OH - ) BaseFormulaIons present Sodium oxideNa 2 O Zinc oxideZnO Copper(II) oxideCuO Magnesium hydroxideMg(OH) 2 Aluminium hydroxideAl(OH) 3 Na + ; O 2- Zn 2+ ; O 2- Cu 2+ ; O 2- Mg 2+ ; OH - Al 3+ ; OH -

Oxides AcidicBasicAmphotericNeutral

 Oxides of non-metals (non-metal + oxygen)  React with water to form acids  React with alkalis to form salt and water Acidic Oxide Formula Acid Produced with Water sulphur trioxideSO 3 sulphuric acid, H 2 SO 4 sulphur dioxideSO 2 sulphurous acid, H 2 SO 3 carbon dioxideCO 2 carbonic acid, H 2 CO 3 phosphorous(V) oxide P 4 O 10 phosphoric acid, H 3 PO 4

 Oxides of metals (metal + oxygen)  React with acids to form salt and water Basic Oxide Formula magnesium oxideMgO sodium oxideNa 2 O calcium oxideCaO copper(II) oxideCuO

 Oxides of metals  Behave as acidic oxide or basic oxide  React with acids and alkalis to form salts Amphoteric Oxide Formula aluminium oxideAl2O3Al2O3 lead(II) oxidePbO zinc oxideZnO

 Do not react with acids or bases  Do not form salts  Examples:  Water, H 2 O  Carbon monoxide, CO  Nitrogen monoxide, NO

So, what have you learnt?