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- Describe neutrality and relative acidity and alkalinity in terms of pH (whole numbers only) measured using full-range indicator and litmus.
Acids are sour in taste and are corrosive nature. Examples: hydrochloric acid (found in our stomachs), citric acid (found in ‘citrus’ fruits such as oranges and lemon) and ethanoic acid (found in vinegar)
A base is a substance that dissolves in water to produce hydroxide.
Bases are bitter in taste and have a soapy touch. Bases that are dissolved in water are called alkalis. Examples: calcium carbonate (limestone), sodium chloride (salt), calcium hydroxide (limewater) etc.
The real distinction between acids and alkalis is shown on the pH scale (pictured here) acids have pH values from 0 to 6, 0 being the strongest acids and 6 being the weakest acids. Bases have pH values from 8 to 14, 8 being the weakest bases and 14 being the strongest bases. So, as you go from left to right, it goes from strongly acidic to strongly basic. A pH value of 7 is neutral. Water has a pH value of 7- it is neither acidic nor basic.
To know whether a substance is acidic or basic, we can use the litmus paper test.
Litmus paper is a thin strip of paper which comes in colours of blue and red. You can dip it in the solution you want to test. If the blue litmus paper turns red or if red litmus paper stays red the solution must be acidic . If the red litmus paper turns blue or the blue litmus paper stays blue, the solution must be basic.
Basically, red means acidic and blue means basic. Simple!
Another acid-alkali indicator is the universal indicator. The universal indicator turns different colours based on substances’ pH values. It will turn red-yellow shades when tested with acids, and turn purple-blue shades when tested with alkalis. Shown on the left is the universal indicator’s colours based on pH (from acidic to basic) and examples of substances that have those pH values.
- Describe the characteristic reactions of acids with metals, bases (including alkalis) and carbonates.
Here are the products that are produced when acids react with different substances. Learn the general formula.
- acid + metal —> salt + hydrogen
hydrochloric acid (acid) + magnesium (metal) —–> magnesium chloride (salt) + hydrogen
- acid + alkali —> salt + water – this is the neutralization reaction formula. the acid and alkali cancel each other to form salt and water.
nitrate acid (acid) + sodium hydroxide(alkali) —-> sodium nitrate (salt) + water
- acid + metal oxides (bases) —> salt + water
sulfuric acid (acid) + copper(II)oxide (metal oxide) ——> copper(II)sulfate + water
- acid + carbonate —> salt + water + carbon dioxide
sulfuric acid (acid) + copper(II)carbonate (carbonate) —–> copper(II)sulfate + water + carbon dioxide
(How to memorise: Acids react with alkalis and metal oxides the same way- salt and water is produced. When acids react with carbonates carbon-dioxide is produced along with the salt and water).
- Describe and explain the importance of controlling acidity in the environment (air, water and soil).
- Air: Burning fossil fuels releases gases into the air, such as nitrogen oxides and sulfur dioxide. They react with water and air, leading to acid rain. Acid rain causes buildings to erode and will be harmful for soil, plants and water. This is why burning of fossil fuels need to be controlled. It can damage the environment.
- Water: Factory waste is often acidic, and it can leak into water. This waste is treated with slaked lime to neutralise it so that when it leaks into water, it doesn’t pollute it.
- Soil: Soil is used to grow crops, so it is important for it to be neutral. If it is too alkaline or acidic, the crops won’t grow properly. Acidity is usually the problem for soil, so a base can help neutralise it. Bases like limestone, slaked lime, or quick lime are usually sprayed in the soil to neutralise it before planting crops.
Notes submitted by Lintha
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