Acids, Bases, and Salts - Class 7 Notes & Olympiad Questions

Acids

Substances that have a sour taste and can cause damage or corrosion are known as acidic substances. Some common examples of acidic substances include sour fruits like lemons or oranges, vinegar, and certain chemicals like hydrochloric acid.

Properties of Acids

Understanding the properties of acids helps us identify them, predict their reactions, and comprehend their role in various chemical processes.

1. Solubility in water: Acids can dissolve or mix with water, forming a solution.

2. Sour taste: The sour taste is a characteristic property of acids.

3. Corrosive nature: Acids can be corrosive, which means they have the ability to damage or eat away certain materials.

4. Turning blue litmus paper red: Acids have the ability to change the colour of blue litmus paper to red. Litmus paper is a special type of paper that can detect whether a substance is acidic or basic.

5. Presence of hydrogen atoms: All acids contain hydrogen atoms. The hydrogen atoms in acids are responsible for their acidic properties. When acids dissolve in water, they release hydrogen ions (H⁺) into the solution.

6. Formation from non-metallic oxides: Acids can be formed when certain gases called non-metallic oxides react with water. For example, when sulfur dioxide gas (SO₂) dissolves in water, it forms sulphurous acid (H₂SO₃). This shows how acids can be created from other chemical substances.

SO₂ + H₂O → H₂SO₃

7. Reaction with metals: Dilute acids (HCl) can react with certain metals, such as zinc (Zn), to form a salt (ZnCl₂) and release hydrogen gas (H₂). This reaction shows that acids have the ability to break down metals and produce gas.

Zn + 2HCl → ZnCl₂ + H₂

8. Reaction with bases: Acids can react with bases, such as sodium hydroxide (NaOH), to produce salt and water. This process is called neutralization and results in the formation of a new substance (salt) and water.

HCl + NaOH → NaCl + H₂O

Types of Acids

1. Organic acids

a) Organic acids are acids that are naturally present in plants and animals.
b) Examples of organic acids include citric acid (found in citrus fruits like lemons and oranges), tartaric acid (found in grapes and wine), and formic acid (found in ants and some other insects).

2. Mineral or Inorganic acids

a) Mineral acids, also known as inorganic acids, are acids that are not derived from living organisms. They are usually synthesized through chemical processes.
b) Examples of mineral acids include hydrochloric acid (HCl), nitric acid (HNO₃), and sulfuric acid (H₂SO₄). These acids are widely used in various industrial applications.

3. Weak acids

a) Weak acids are acids that do not completely dissociate or break apart in water, resulting in a relatively low concentration of hydrogen ions (H⁺).
b) Examples of weak acids include tartaric acid (found in grapes), acetic acid (found in vinegar), and citric acid (found in citrus fruits).
c) Weak acids have a milder effect compared to strong acids.

4. Strong acids

a) Strong acids are acids that completely dissociate or break apart in water, producing a high concentration of hydrogen ions (H⁺).
b) Examples of strong acids include nitric acid (HNO₃) and hydrochloric acid (HCl).
c) Strong acids have a more pronounced and powerful effect in chemical reactions compared to weak acids.

Acids found in nature

1. Citric acid: It is found in citrus fruits such as oranges and lemons. These fruits have a sour taste due to the presence of citric acid.

2. Tartaric acid: It can be found in various fruits like tamarind, grapes, and unripe mangoes. Tartaric acid gives a tart taste to these fruits.

3. Formic acid: It is present in the sting of ants. When an ant bites or stings, it injects formic acid, which causes a burning sensation.

4. Lactic acid: It is found in milk and dairy products like curd. Lactic acid is responsible for the tangy taste of fermented milk products.

5. Oxalic acid: It occurs naturally in certain plants, with spinach being a notable source. Oxalic acid contributes to the slightly bitter taste of spinach.

6. Amino acid: Amino acids are the building blocks of proteins. They are found in various food sources, particularly in proteins derived from animals and plants.

Uses of Acids

1. Hydrochloric acid: Hydrochloric acid is used to make aqua regia, a mixture of acids used for various purposes including dissolving noble metals like gold and platinum. It is also used to clean the inside of boilers and other industrial equipment.

2. Nitric acid: Nitric acid has several uses. It is used in the manufacturing of explosives, such as dynamite, and fertilizers like ammonium nitrate. Nitric acid is also used to extract metals from their ores through a process called leaching.

3. Sulphuric acid: Sulphuric acid is widely used in various industries. It is used in the manufacturing of fertilizers, as well as in the production of drugs, plastics, dyes, paints, and detergents.

4. Boric acid: Boric acid is commonly used as an eye wash to soothe eye irritations and infections. It has mild antiseptic properties and is used as an eyewash solution.

5. Carbonic acid: Carbonic acid is not commonly used directly, but it is involved in the process of carbonation, which is used in the beverage industry to add bubbles and provide a fizzy taste to drinks like soda or sparkling water. The carbonic acid formed from dissolved carbon dioxide gives these drinks their characteristic flavour.

6. Acetic acid: It is used in household cleaning products.

Bases

Substances that have a bitter taste and feel soapy when touched contain bases. We refer to these substances as basic substances.

Properties of Bases

1. Solubility in water: Not all bases are soluble in water, but the bases that are soluble in water are called alkalis.

2. Bitter taste: Bases have a bitter taste. For example, if you ever accidentally taste baking soda, which is a base, it will have a bitter taste.

3. Soapy texture: Bases feel slippery or soapy to the touch.

4. Turns red litmus paper blue: Bases turn red litmus paper blue. Bases make the paper turn blue.

5. Contains hydrogen and oxygen: Bases typically contain the hydroxide ion (OH^-), which consists of one oxygen atom and one hydrogen atom. Bases dissociate in water and release hydroxide ions.

6. Metallic oxides: Bases are often metallic oxides. When certain metals burn in the presence of oxygen, they form metallic oxides that have basic properties. For example, when magnesium (Mg) burns in oxygen, it forms magnesium oxide (MgO), which is a base.

2Mg + O₂ → 2MgO

7. Reaction with acids: When a base and an acid react, they form salt and water.

HCl + NaOH → NaCl + H₂O

Types of Bases

1. Weak Base

a) Weak bases are substances that do not completely dissociate or break apart in water. They release only a small number of hydroxide ions (OH^-) when dissolved in water. These bases have a relatively weaker alkaline effect.
b) Examples of weak bases include zinc hydroxide (Zn(OH)₂) and ammonium hydroxide (NH₄OH).

2. Strong Base

a) Strong bases are substances that completely dissociate or break apart in water, releasing a large number of hydroxide ions (OH^-). These bases have a strong alkaline effect.
b) Examples of strong bases include sodium hydroxide (NaOH) and potassium hydroxide (KOH).

Uses of Bases

Calcium hydroxide

a) Calcium hydroxide, also known as slaked lime, is used for various purposes. It is commonly used for whitewashing or painting buildings, as it provides a protective and decorative coating.
b) It is also used in agriculture to neutralize acidic soils and adjust the pH levels.
c) Additionally, calcium hydroxide is used in the treatment of wastewater to remove impurities.

Sodium hydroxide

a) Sodium hydroxide, also known as caustic soda, has numerous industrial applications. It is a key ingredient in the manufacturing of soap, paper, rayon, textiles, and medicines.
b) It is also used in the production of various chemicals, including detergents, dyes, and synthetic fibres.

Ammonium hydroxide

a) Ammonium hydroxide, commonly referred to as ammonia solution, has several uses. It is effective in removing grease stains from clothes, making it a common ingredient in laundry detergents.
b) Ammonium hydroxide is also used in the manufacturing of fertilizers, plastics, dyes, and other chemical products.

Magnesium hydroxide

a) Magnesium hydroxide is widely used as an antacid to relieve symptoms of indigestion and heartburn. It works by neutralising excess stomach acid.

Salts

a) Salts are compounds formed by the reaction between an acid and a base.
b) When an acid reacts with a base, they undergo a neutralisation reaction. During this reaction, the acid and base cancel out each other's properties, resulting in the formation of salt and water.

Properties of Salts

1. Solid Crystals: Salts are solid substances that often have a crystal-like structure. They can form beautiful patterns and shapes when they solidify.

2. Dissolving in Water: Many salts dissolve in water, which means they can mix with water to create a solution. This property allows us to make salty water or solutions of different concentrations.

3. Melting and Boiling Points: Salts have high melting and boiling points, which means they require high temperatures to melt or boil. This property is why we use salt to melt ice on roads during winter.

4. Stability: Salts are usually stable and do not easily break down or change in normal conditions. This stability allows salts to be stored for a long time without losing their properties.

Types of Salts

Salt can be acidic, basic or neutral.

1. Neutral salts

a) Salts formed from the reaction between a strong acid and a strong base are called neutral salts.
b) These salts do not exhibit either acidic or basic properties.
c) An example is sodium chloride (NaCl), which is formed from the neutralization of hydrochloric acid (HCl) and sodium hydroxide (NaOH).

2. Acidic salts

a) Salts formed from the neutralization of a strong acid and a weak base are called acidic salts.
b) These salts have a slightly acidic nature.
c) An example is ammonium chloride (NH₄Cl), which is formed from the neutralization of hydrochloric acid (HCl) and ammonium hydroxide (NH₄OH).

3. Basic salts

a) Salts formed from the neutralisation of a strong base and a weak acid are called basic salts.
b) These salts have a slightly basic nature.
c) An example is sodium acetate (NaCH₃COO), which is formed from the neutralisation of sodium hydroxide (NaOH) and acetic acid (CH₃COOH).

Uses of Salts

1. Sodium Chloride/ NaCl (Table Salt)

a) Sodium chloride is used to season food and enhance its flavour.
b) It is also used in food preservation methods like pickling and curing.
c) Sodium chloride is used for melting ice on roads and sidewalks during winter.
d) In sports drinks, it helps replenish electrolytes lost during physical activity.

2. Calcium Carbonate/ CaCO₃ (Chalk, Limestone)

a) Chalk, made of calcium carbonate, is used for writing on blackboards or sidewalks.
b) Calcium carbonate is an ingredient in toothpaste for cleaning teeth.
c) It is used in gardening to neutralize soil acidity and improve plant growth.

3. Sodium Bicarbonate/ NaHCO₃ (Baking Soda)

a) Sodium bicarbonate is a baking ingredient that helps dough rise when making bread and cakes.
b) It is a household cleaning agent used to remove stains and odours.
c) Sodium bicarbonate can be used as an antacid to relieve indigestion or heartburn.

4. Sodium Hydroxide/ NaOH (Caustic Soda)

a) Sodium hydroxide is used in the manufacturing of soap, detergents, and cleaning products.
b) It is a common ingredient in drain cleaners used for unclogging pipes.
c) Sodium hydroxide is used for pH adjustment in various industrial processes.

5. Calcium Hydroxide/ CaOH

a) CaOH is used to whitewash buildings.
b) It can be added to soil to make it less acidic and better for plants.
c) It's used to treat wastewater and remove harmful substances.

6. Magnesium Hydroxide/ MgOH

a) Magnesium hydroxide is used as an antacid to help with troubles like heartburn and indigestion.

Neutralisation Reaction and Its Uses in Daily Life

Neutralization is a special type of chemical reaction that occurs when an acid and a base react with each other.
This neutralization reaction is essential in various situations.

1. Treatment of acidity and indigestion: When we experience acidity or indigestion, taking antacids like milk of magnesia helps neutralize the excess stomach acid (hydrochloric acid) and provides relief.

2. Treatment of ant sting: Ants inject formic acid into the skin when they sting. Rubbing moist baking soda or calamine solution on the affected area helps neutralize the acidic effect of the sting and provides relief.

3. Treatment of soil acidity: Excessive use of fertilizers can make the soil acidic. To neutralize the acidity, bases like quicklime (calcium oxide) or slaked lime (calcium hydroxide) are added to the soil. If the soil is already basic, organic matter can be added to balance its pH.

4. Treatment of waste: Factory waste often contains acidic substances that can harm the environment. Before being discharged, the waste is treated with basic substances to neutralize its acidity and minimize environmental damage.

Indicators

An indicator is a substance that can be used to determine whether a substance is an acid, a base, or neutral. It changes colour when it comes into contact with an acid or a base, indicating its presence.

There are different types of indicators, including natural indicators that are found in plants or substances, and synthetic (chemical) indicators that are created in laboratories.

Natural Indicators

a) Natural indicators are dyes or substances derived from natural sources, like plants.
b) Some examples of natural indicators are:

1. Litmus Paper

a) Litmus paper is a natural indicator extracted from lichens. It comes in blue and red forms.
b) Blue litmus paper turns red in the presence of an acid and remains blue in the presence of a base.
c) Red litmus paper, on the other hand, turns blue in the presence of a base and remains red in the presence of an acid.

2. Red Cabbage Indicator

a) Red cabbage contains a pigment called anthocyanin that changes colour depending on whether the substance is acidic or basic.
b) When boiled in water, it creates a purple solution that turns red in the presence of acids and green or yellow in the presence of bases.

3. Turmeric

a) Turmeric is a yellow spice commonly used in cooking.
b) It can act as an indicator, turning red in the presence of bases and remaining yellow in the presence of acids.

4. China Rose (Hibiscus) petals

a) China rose petals can be used as a natural indicator.
b) They produce a dark pink or red colour in the presence of acids and turn green or blue in the presence of bases.

Chemical Indicators

Chemical indicators are synthetic compounds or substances created in a laboratory. They can also help determine the acidity or basicity of a solution.

1. Phenolphthalein: Phenolphthalein is a commonly used chemical indicator. It is colourless in acidic solutions but turns pink or magenta in basic solutions.

2. Methyl Orange: Methyl orange is another chemical indicator. It is red in acidic solutions and turns yellow in basic solutions.

pH Scale

pH is a scale that measures the acidity or basicity of a solution. It indicates the concentration of hydrogen ions (H⁺) present in the solution. The pH scale ranges from 0 to 14, with 7 being considered neutral. A pH value below 7 indicates an acidic solution, while a pH value above 7 indicates a basic solution.

The pH of a solution can be measured using pH indicators, pH paper, or electronic pH meters.