Lesmahagow High School Acids and Bases
Lesmahagow High School
S3 Chemistry
Acids and Bases
Properties of acids and bases. pH adjusting and significance of pH in everyday life.
SCN 3-18a
The effect of soluble oxides on the pH of water - Acid Rain.
National 4
Learning Outcomes – Acids and Bases
Circle a face to show how much understanding you have of each statement: J if you fully understand enough to do what the outcome says, K if you have some understanding of the statement, and L if you do not yet understand enough to do what the statement says. Once you have completed this, you will be able to tell which parts of the topic that you need to revise, by either looking at your notes again or by asking for an explanation from your teacher or classmates.
Level 3 Learning Outcomes
By the end of this topic I will be able to:
1. Describe practical activities to compare the
properties of acids and bases.
2. Demonstrate ways of measuring and adjusting pH.
3. Describe the significance of pH in everyday life
e.g in soils, insect stings, toothpaste, indigestion
remedies etc.
4. Participate in practical activities to extract
useful substances (pH indicators) from natural
resources.
5. Contribute, through research and discussion, to
evaluations of media items about pollution and
acid rain, with regard to scientific content and
ethical implications.
Acids and Bases Notes
Level 3 Notes
pH is a measure of how many H+ ions there are in a solution. The more H+ ions there are, the lower the pH value will be. Hence, acids will have low pH and alkalis will have high pH, because acids have many more H+ ions in solution.
pH below 7 ® Solution Acidic
pH equals 7 ® Solution Neutral.
pH above 7 ® Solution Alkaline.
If you are using pH paper or universal indicator to test a solution, acids will usually be red to yellow and alkalis will be dark green/blue to purple. Neutral solutions like water will give green. Not all acids will give the same colour because some acids are stronger (more acidic) than others, and the same applies for alkalis.
Definitions
Acids are substances which have a pH less than 7 when dissolved in water.
Alkalis are substances which have a pH greater than 7 when dissolved in water. A base can neutralise an acid, but may not necessarily be soluble.
Indicators of all types and colours are available, and some can be made from natural sources like berries, flowers or vegetables like red onion or red cabbage. Different indicators work best over different pH ranges.
Level 4 Notes
Uses of Acids and Bases
Laboratory Acids
Sulphuric Acid (H2SO4): Paints, dyes, fertilisers, soaps, car batteries.
Nitric Acid (HNO3): Nitrate fertilisers, explosives (TNT).
Hydrochloric Acid (HCl): Metal cleaning, plastics.
Ethanoic Acid (CH3COOH): Vinegar, pickling food.
Common household acids include vinegar, lemon juice, window cleaner, tomato sauce, hydrogen peroxide (used in hair dye).
Laboratory Bases
Magnesium Hydroxide, Mg(OH)2: milk of magnesia used for indigestion relief.
Calcium Hydroxide, Ca(OH)2: Lime used in cement and plant feed to reduce soil acidity.
Sodium Hydroxide, NaOH: Caustic Soda used in cleaning products and soaps.
Common household bases include indigestion tablets, bleach, soap, oven cleaner, toothpaste, dishwasher tablets.
National 4 Notes
Acid Rain
Gases like sulphur dioxide, nitrogen dioxide and carbon dioxide dissolve in water to make acids. When they are released into the atmosphere they dissolve in water droplets to form clouds of acid rain. Rain containing these acids erodes buildings and damages freshwater fish and trees. The main source of sulphur dioxide is the burning of fossil fuels.
Neutralisation
A base is a substance that reacts with an acid, decreasing the acidity. This kind of reaction is known as neutralisation (since the pH of the acid moves towards 7, the pH of a neutral solution). Water is one of the products of a neutralisation reaction and the other is a salt.
ACID + BASE SALT + WATER
Neutralisation of an acid involves the reaction of H+ (aq) ions with the base.
H+ (aq) + base salt + H2O
An alkali can also be neutralised by an acid.
OH- (aq) + acid salt + H2O
Many neutralisation reactions occur in everyday life. For example, bicarbonate of soda is a base used to neutralise acidic bee stings. Wasp stings, however, are basic and so acidic vinegar is used to neutralise a wasp sting. Toothpaste neutralises mouth acid to reduce tooth decay, and indigestion tablets neutralise excess stomach acid. Gardeners and farmers in Scotland can add basic 'gardener's lime' to the soil to reduce its acidity and make it suitable for growing different plants and crops.
Acids and Bases - Glossary
Word / MeaningAcid / A substance with pH less than 7, because it contains more H+ than pure water.
Alkali / A substance with pH greater than 7, because it contains less H+ than pure water.
Household Acid / Substance used in the home with a pH less than 7. e.g. lemon juice.
Laboratory Acid / Substance used in the lab with a pH less than 7. e.g. sulphuric acid.
Household Alkali / Substance used in the home with a pH greater than 7. e.g. Bleach
Laboratory Alkali / Substance used in the lab with a pH greater than 7. e.g. sodium hydroxide.
pH scale / A measure of the acidity of a substance, ranging from 0 to 14.
pH chart / A colour chart showing the colour produced by an indicator at different pH values.
Acidic gases / Oxides of non metals which produce acid solution when they dissolve in water in the atmosphere.
Acid Rain / Rain, fog, snow or hail which has a pH less than 7.
Neutralisation / The reaction of an acid to produce a salt and water.
Base / Any substance which neutralises an acid.
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