Rapid Revision – Class 7 Science – Chapter 5-Acids, Bases and Salts

Revision Notes – Chapter 5-Acids, Bases and Salts

• Acids derive from the Latin “acere” meaning sour, and bases feel soapy and taste bitter.

Natural Acids And Bases Examples

• Acids- Citric acid in citrus fruits, acetic acid in vinegar, formic acid in ant stings, lactic acid in curd, oxalic acid in spinach, ascorbic acid in fruits like amla, and tartaric acid in grapes.
• Bases- Calcium hydroxide from heated limestone, sodium bicarbonate in mineral springs, potassium hydroxide in wood ash, ammonium from protein decomposition, magnesium hydroxide in antacids.

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Indicators

• Indicators like turmeric, litmus, and china rose petals are used to identify acids and bases by their colour change.

Practical Application and Safety

• Acids are often identified by taste, but using indicators is safer and more reliable.

Acid Important Properties

• Acids have a sour taste, while bases are bitter and soapy to the touch.
• Common acids and their sources include acetic acid in vinegar, formic acid in ant stings, citric acid in citrus fruits, lactic acid in curd, oxalic acid in spinach, ascorbic acid in fruits, and tartaric acid in tamarind.
• The role of these acids ranges from flavouring and preservation to essential nutrients and pain infliction in stings.

Bases and Their Common Examples

• Calcium Hydroxide- Found in lime water, used for water treatment and food preparation.
• Ammonium Hydroxide- Present in window cleaner, it’s used for its cleaning properties.
• Sodium and Potassium Hydroxide- Strong bases found in soap, helping to remove dirt and oils.
• Magnesium Hydroxide- Found in milk of magnesia, used as an antacid for stomach relief.

Natural Indicators

Litmus as an Indicator

• Sourced from lichens, litmus displays a purple colour in distilled water, turns red in acidic solutions, and blue in basic solutions.
• Available in solution form or as paper strips.
• Activity involves testing substances with litmus paper to observe colour changes indicating the substance’s nature as acidic, basic, or neutral.

Class 7 – Chapter 4- Acids, Bases, and Salts- An Overview of Activities

Turmeric as an Indicator

• Preparation involves making a paste from turmeric powder and water, applying it to paper, and drying to create indicator strips.
• Testing with turmeric involves observing colour changes when exposed to different solutions. Acidic substances (like lemon juice, orange juice, and vinegar) show no change, while basic substances (like milk of magnesia, baking soda, and lime water) turn turmeric paper red or brownish-red. Neutral substances (like sugar and salt) show no change.

China Rose as an Indicator

• Preparation involves infusing warm water with China rose petals until it is coloured.
• Testing includes adding the indicator to solutions and observing the colour change- acidic solutions turn magenta, basic turn green, and neutral may show no significant change.

Effects of Indicators on Acids and Bases

• Understand the response of litmus, turmeric, and China rose solutions to acids (turns red for litmus, no change for turmeric, magenta for China rose) and bases (blue for litmus, red for turmeric, green for China rose).
• Neutralisation reactions can be observed using indicators such as phenolphthalein, which turns colourless in acid and pink in a basic solution.

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Neutralization Reaction with Phenolphthalein Indicator

• The concept of neutralisation where an acid reacts with a base to form water and salt.
• Phenolphthalein changes from colourless in acidic solutions to pink in basic solutions, and the reaction is observed by the change in colour when acid or base is added.

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Neutralisation Explained

• Neutralisation is a reaction where an acid and a base mix, resulting in the formation of salt and water, and the release of heat.
• The reaction neutralises the properties of both the acid and the base, resulting in a mixture that is neither acidic nor basic.

Observations During Neutralization

• The reaction is exothermic, as touching the container will reveal warmth.

End Products of Neutralization

• The reaction always yields salt and water, where the nature of the salt can be acidic, basic, or neutral.

General Neutralization Equation-

• Acid + Base → Salt + Water (+ Heat)

Examples of Neutralization

• Hydrochloric acid reacts with sodium hydroxide to form sodium chloride and water.
• Sulphuric acid reacting with lime water also illustrates a neutralisation reaction.

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Neutralisation in Everyday Life

• Indigestion- Overproduction of stomach acid can be neutralised by antacids like milk of magnesia, forming water and a salt and relieving discomfort.
• Ant Bites– Formic acid injected by ant bites can be neutralised by applying bases like baking soda or calamine, alleviating irritation.
• Soil Treatment- To counteract the effects of acidic or basic soil, substances like quicklime (for acidity) or organic matter (for basicity) are used to adjust the soil pH for optimal plant growth.
• Factory Wastes- Acidic wastes from factories are neutralised with basic substances before being released to protect aquatic life.

Acid Rain

• Acid rain is formed when pollutants like CO2, SO2, and NO2 dissolve in rain, forming carbonic, sulfuric, and nitric acids, resulting in more acidic rainwater.

Effects of Acid Rain

• Buildings and Monuments- Corrosion and damage to materials and loss of structural integrity and aesthetic value.
• Plants and Animals- Damage to vegetation and disruption to the survival of aquatic life.
• Soil- Lowered pH affecting nutrient availability and plant growth.

Components of Acid Rain

• Carbonic Acid from CO2, Sulfuric Acid from SO2, and Nitric Acid from NO2.

Remember that the examples and products of neutralisation may vary depending on the reactants’ strength and concentration, and the effects of acid rain can have broad environmental impacts. Always handle acids and bases with care, and consider the broader implications of chemical reactions in environmental contexts.

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