Introduction to Acid and Base Class 8 Science Notes Maharashtra Board

Introduction to Acid and Base Class 8 Science Notes Maharashtra State Board

Acid

You will notice that some substances have a sweet taste, some are bitter, and some are sour or astringent. Lemon, tamarind, vinegar, or amla-like substances acquire sour taste due to the presence of a typical compound in them. Such compounds imparting a sour taste are called acids. Acids are soluble in water and they are corrosive. Animals and plants also possess acids in them.

Acids present in foodstuffs are called natural acids or organic acids. These acids being weak are called weak acids. Some acids are strong. They are caustic/inflammatory, e.g. sulphuric acid (H₂SO₄), hydrochloric acid (HCl), and nitric acid (HNO₃). These acids are also called ‘mineral acids’. The skin gets burnt when their concentrated solution falls on the skin, similarly, their vapors if inhaled can be harmful to health. Strong concentrated acids are converted into dilute acids by slowly adding them to water. Such dilute acids are less harmful than their corresponding strong acids.

If you taste a dilute solution of baking soda, you will find it astringent/bitter. Substances having an astringent/bitter taste and slippery to the touch e.g. Lime water [(CaOH)₂], baking soda (NaHCO₃) caustic soda (NaOH), and soap are called bases. Bases are completely different from acids. Chemically they have properties opposite to that of acids. They are also inflammatory to the skin in their concentrated form. We know that the distilled water is tasteless. Water is neither acidic nor basic.

Indicator:
The substances which are neither acidic nor basic and are chemically inert. Touching or tasting of acid or base is very harmful and hence typical indicators are used to recognise them. The substances which change their colors in the presence of acid or base are called ‘Indicators’.

Indicators in the Laboratory:
In the laboratory, litmus paper is mainly used to test alkalis or bases. This paper is made by an extract of licane plant. It is red or blue colored. Blue litmus paper turns red on dipping in acid and red litmus turns blue by alkali. Similarly, phenolphthalein, methyl orange, and methyl red are used in the laboratory in solution form. The methyl orange indicator becomes pink in acid and yellow in alkali. Phenolphthalein remains colorless in acid and becomes pink in alkali. A universal indicator that is in a liquid state changes its color in the presence of acid and base.

Indicators and their Colours in Acid and Base

Name of Indicator	Colour of the Indicator	Colour in Acid	Colour in Alkali
1. Litmus Paper	Blue	Red	Blue (Remain Same)
2. Litmus Paper	Red	Red (Remain Same)	Blue
3. Methyl Orange	Orange	Pink	Yellow
4. Phenolphthalein	Colourless	Colourless	Pink
5. Methyl Red	Red	Red	Yellow

Domestic Indicator:
On the non-availability of laboratory indicators, ‘natural indicators’ can be made by using several domestic substances. You must have seen yellow food stains turning red after washing with soap. This color change is a result of a chemical reaction between the turmeric and alkaline material of soap. Here turmeric acts as an indicator. Natural indicators can also be prepared from red cabbage, radish, tomato, and similarly from hibiscus and rose.

Making of Natural Indicators:
Apparatus: Hibiscus, rose, turmeric, red cabbage leaves, filter paper, etc.
Activity: Rub the red petals of the hibiscus flower on the white filter paper, This gives hibiscus indicator paper. Similarly rub, rose petals on the white filter paper. Cut strips of this paper, it is a rose indicator paper. Take turmeric powder, and add a little water to it. Dip filter paper or ordinary paper in the turmeric water for some time. After drying make strips of that paper. Prepare turmeric indicator paper in this way. Put leaves of red cabbage in a small quantity of water and heat it. Once the solution of cabbage leaves cool down, dip papers in it and dry it. Make strips of dried paper. In this way prepare red cabbage indicator paper.

Take baking powder. Add a little water to it. Add this solution on to lime juice, vinegar, orange juice, apple juice, etc., and note the findings. What do you observe about the addition of baking soda solution in the fruit juice? Whether bubbles formed or effervescence came out of fruit juice? From the above first activity, we came to know that turmeric indicator paper turns red yellow color in certain solutions. Similarly with the addition of baking soda solution in the acidic solution bubbles come out or effervescence is produced. By this simple and easy activity, we can identify acidic or alkaline substances.

From the above experiments, it is seen that blue litmus turns red in acids and red litmus turns blue in alkali. The orange color of methyl orange turns pink in acid while colorless phenolphthalein turns pink in an alkali solution.

Acid
Acid is such a substance that gives H⁺ ions in the solution state.
e.g. HCl dissociates in water solution.
HCl (aq) (Hydrochloric acid) → H⁺ (Hydrogen ion) + Cl^– (Chloride ion)

Examples of Some Acids:
Hydrochloric acid (HCl), Nitric acid (HNO₃), Sulphuric acid (H₂SO₄), Carbonic acid (H₂CO₃) (in cold aerated drinks), ascorbic acid, citric acid in lemon and other fruits, acetic acid in vinegar, etc. Our daily diet contains a few natural (organic) acids. They are mild and hence are not harmful like mineral acids.

Substance/Source	Acids (Natural/Organic)
1. Vinegar	Acetic acid
2. Orange	Ascorbic acid
3. Tamarind	Tartaric acid
4. Tomato	Oxalic acid
5. Curd	Lactic acid
6. Lemon	Citric acid

Properties of Acid:

• Acids are sour.
• Acid molecules contain hydrogen ions (H+) as a main constituent.
• Acid reacts with metal to form hydrogen gas.
• Acid reacts with carbonates and liberates CO₂ gas.
• Blue litmus turns red in acid.

Use of Acids:

• Acids are used in the production of chemical fertilizers.
• Acids are used in the production of explosives, oil purification, medicines, dyes, and paints.
• Hydrochloric acid is used for the preparation of different types of chloride salts.
• Dil. H₂SO₄ acid is used in the batteries. (electric cell)
• Dil. HCl is used for sterilization of water.
• Acid is used for making white paper from wood pulp.

Causticity of Concentrated Acid and Base:
Dissolution of conc. H₂SO₄ in water generates a large amount of heat. Therefore for dilution of concentrated acid, it is slowly added to water. Never add water to the concentrated sulphuric acid. It will produce enormous heat and cause an explosion. Bases like sodium hydroxide and potassium hydroxide are strong and caustic. Their concentrated solution burns the skin as it decomposes the proteins in the skin.

Base

The base is a substance whose water solution gives hydroxide (OH^–) ions.
e.g. NaOH (aq) (Sodium Hydroxide) → Na⁺ (aq) (Sodium ion) + OH^– (aq) (Hydroxide ion)

Formulae of Bases and their Uses

Name of Base	Formula	Use
1. Sodium Hydroxide/Caustic Soda	NaOH	Washing Soap
2. Potassium Hydroxide/Potash	KOH	Soap/Shampoo
3. Calcium Hydroxide/Lime Water	Ca(OH)2	Lime/White Washing
4. Magnesium Hydroxide/Milk of Magnesia	Mg(OH)2	Acidity resistant medicines (Antacids)
5. Ammonium Hydroxide	NH4OH	Used for the production of fertilizers.

Properties of Alkalies:

• Bases have a bitter taste.
• They are slippery.
• Bases contain hydroxide (OH^–) as a main constituent.
• Metal oxides are generally basic.

Neutralization
We have seen that acid contains (H⁺) hydrogen ions and base contains(OH^–) hydroxide ions. Salt and water are formed by the combination of acid and base.
Acid + Base → Salt + Water
HCl (Hydrochloric Acid) + NaOH (Sodium Hydroxide) → NaCl (Sodium Chloride) + H2O (Water)
This chemical reaction is called neutralization.

Hydrochloric acid is present in our stomach. It helps to ease the digestion process. However excessive acid leads to indigestion. Antacids are used to control this hyperacidity. This medicine contains milk of magnesia [Mg(OH)₂]. Such alkaline medicine neutralizes excess acid present in the stomach. The acidity of soil increases due to excessive use of chemical fertilizers. Under the guidance of agriculturists limestone or lime water is mixed in the acidic soil. Such alkaline substances neutralize excess acid present in the soil.

Detailed Maharashtra State Board Class 8 Science Notes Introduction to Acid and Base are particularly useful for answering essay questions.