Chemistry Revision

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CHEMISTRY REVISION

How to safely handle acids and bases:
-Acids are corrosive, and therefore potentially dangerous
-Strong bases can also be harmful as they are caustic
-Do not combine unknown acids or bases
-Only work under supervision
-Immediately report any spills
-Do not consume any unknown substances
-Always wear safety glasses coats
-Dispose of substances as directed, not directly down the sink
-Keep away from flames unless directed

Chemical and Physical Changes:
Physical Chemical
Change of state Chemical reaction occurs
Reversible Irreversible (or difficult to do so)
Substances stay the same Form a new substance
1. Water freezing to ice 1. Some metal rusts or tarnishes
2. Dissolving sugar in water 2. Cooking
3. Melting gold 3. Vinegar and sodium bicarbonate bubbles
4. A piece of chalk is ground to dust 4. Sugar in grapes ferments
5. Glass breaks 5. Wood burns to form ash

Properties of Acids and Bases:
Acids Bases
Taste sour Taste bitter
Are corrosive Are caustic
Reacts with some metals = hydrogen React with oil so feel slippery to touch
Turn litmus red Turn litmus blue
pH is less than 7 pH is more than 7

Examples of Acids and Bases:
-Hydrochloric acid: HCl
-Sulphuric Acid: H2SO4
-Nitric Acid: HNO3
-Acetic Acid: HC2H3O2
-Sodium Hydroxide: NaOH
-Potassium Hydroxide: KOH
-Calcium Hydroxide: Ca(OH)2
-Magnesium Hydroxide: Mg(OH)2

The Use and Example of Indicators:
Indicators are used to determine whether a substance is acidic, basic (alkaline), neutral. However, the following indicators do not indicate the strength of the acid or base.

Indicator Colour in Acid Colour in Neutral Colour in Base/Alkali
Methyl Orange Reddish Orange Yellowish Yellow
Litmus (lichens) Red Green Blue
Bromothymol Blue Yellow Yellow Bluish purple
Phenolphthalein Colourless Colourless Pink
Red Wine Red Red Green
Red Cabbage Juice Red Blue Yellow
*Alkalis are bases that are soluble in water

The pH Scale and What the Numbers Indicate in Terms of Acid or Base:
-The pH scale is used the measure the strength of an acid or base
-A pH of less than 7 indicates that the substance is an acid
-A pH of more than 7 indicates that the substance is a base
-If the pH is 7, the substance is considered to be neutral – neither acid nor base
-Strong Acid: 0-3, HCl, H2SO4, HNO3
-Weak Acid: 3-7, vinegar, citric acid
-Weak Base: 7-11, Baking Soda, bicarbonate of soda
-Strong Base: 11-14, KOH, NaOH
-Universal indicator is added to substance, the colour degrees can be matched and interpreted.

Reactions of Acids and Bases:
(Reactants → Products)
1. Neutralisation: Acid + Base → Salt + Water
e.g HCl + NaOH → NaCl + H2O
2. Acid + Metal carbonate → Salt + Water + Carbon Dioxide
e.g 2HCl + CaCO3 → CaCl2 + H2O + CO2
3. Acid + Reactive metal → Salt + Hydrogen gas
e.g 2HCl + Mg → MgCl2 + H2
*Salt is a solution, so the state symbol is (aq) for aqueous. Others are (s), (l), (g)
**Test for CO2 is bubbled through limewater, hydrogen is ‘pop’ test

Interpret and Analyse Data Related to pH and acid-base:
-Remember not to put too much universal indicator
-Cloudy result means solid is made
-Bubbles or fizzing means gas is made
-In the chemical symbols, H at the beginning usually indicates an acid, and an OH at the end usually indicates a base

Acid Rain – What it is, Causes, Problems, Solutions:
- Definition:
Acid rain is any form of precipitation, such as rain or snow, which has been polluted by acids.
- Causes:
Acid rain is caused when water vapour in the air reacts with chemical compounds given off by cars, factories, and power plants (burn fossil fuels). These chemicals include Sulphur and nitrogen oxides which form into sulphuric and nitric acids.
*Carbon dioxide, which is naturally present in air, dissolves in rainwater to form a light acid
- Problems:
Acid rain directly attacks leaves and needles of plants which prevent photosynthesis. It also soaks into soil which affects chemical balance, damaging plants and crops. It can run off into rivers and lakes polluting drinking water and killing aquatic life. Acid rain also attacks stonework including buildings and statues.
- Solutions:
1. Decrease fossil fuel usage so as to limit sulphur and nitrogen compounds in the air. Find other, renewable energy sources.
2. Lime can be added to lakes in order to neutralise the acid rain.
3. Smokestacks, high tower-like chimneys, reduce local air pollution, but transfer the issue elsewhere.

Antacids in the Treatment of Indigestion:
-Your stomach contains hydrochloric acid which assists in food digestion
-When it becomes too acidic, it causes a burning feeling called indigestion
-Treatment is to take an antacid powder or tablet
-The antacid contains a base which neutralises excess acid in the stomach and relieves the pain
-Salt and water is produced
-Common antacid is milk of magnesia
-Consists of solid base, magnesium oxide suspended in water
-The base neutralises the hydrochloric acid in your stomach
magnesium oxide base + hydrochloric acid → magnesium chloride salt + water

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