Maharashtra Board Class 8 Science Chapter 12 Introduction to Acid and Base PDF Download

Introduction To Acid & Base

Can You Tell?

We consume many food items in our daily life. For example, lemon, tamarind, tomato, sugar, vinegar, salt etc. Do all items taste similar?

Write the taste of lemon, sugar, curd, lime, baking soda, amla, tamarind, raw mango, pomegranate, water. The tastes are: sour, astringent, sweet, bitter, tasteless.

Acid

You will notice that some substances have sweet taste, some are bitter, 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 sour taste are called acids. Acids are soluble in water and they are corrosive in nature. Animals and plants also possess acids in them.

Acids present in food stuffs are called natural acids or organic acids. These acids being weak in nature are called weak acids. Some acids are strong in nature. They are caustic or inflammatory. For example, 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 skin. Similarly their vapours if inhaled can be harmful to health. Strong concentrated acids are converted into their dilute acids by slowly adding them into water. Such dilute acids are less harmful than their corresponding strong acids.

If you taste dilute solution of baking soda, you will find it astringent or bitter. Substances having astringent or bitter taste and slippery to touch. For example, Lime water [Ca(OH)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 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 in nature. Touching or tasting of acid or base is very harmful and hence typical indicators are used to recognise them. The substances which change their colours in presence of acid or base are called Indicators.

Indicators In Laboratory

In the laboratory, litmus paper is mainly used to test alkali or base. This paper is made by an extract of licane plant. It is red or blue coloured. 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 laboratory in solution form. Methyl orange indicator becomes pink in acid and yellow in alkali. Phenolphthalein remains colourless in acid and becomes pink in alkali. Universal indicator which are in liquid state change their colour in the presence of acid and base.

Teacher's Note

Acids and bases are all around us in our daily life. For example, lemon juice is acidic and soap is a base. We use these things every day without knowing their chemical nature.

Exam Trick

Remember: Blue litmus turns red in acid. Red litmus turns blue in base. Think of it like traffic lights - red means stop (acid) and blue means go (base).

Points to Remember

Acids have sour taste and bases have bitter taste.
Acids are corrosive in nature.
Bases are slippery to touch.
Blue litmus turns red in acid.
Red litmus turns blue in base.

Laboratory Indicators

Teacher's Note

Different indicators change colour in different ways. In Indian schools, we use litmus paper because it is cheap and easy to find. Students can see the colour change clearly.

Exam Trick

Remember all indicator colours by making a simple table. Write blue and red litmus changes on paper. Practise writing them daily. This will help you remember in the exam.

Points to Remember

Litmus paper is the most common indicator in school labs.
Blue litmus turns red in acid only.
Red litmus turns blue in base only.
Methyl orange turns pink in acid.
Phenolphthalein turns pink in base.

Domestic Indicator

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

Making Of Natural Indicator

Apparatus: Hibiscus, rose, turmeric, red cabbage leaves, filter paper etc.

Activity: Rub red petals of 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. Add little water in 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 small quantity of water and heat it. Once 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.

Put some drops of following substances on the indicator papers prepared by the above method and write the effect in the following table.

Take baking powder. Add 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 on 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's turns red yellow colour in certain solutions. Similarly on addition of baking soda solution in the acidic solution bubbles come out or effervescence is produced.

By these simple and easy activity we can identify acidic or alkaline substance.

Teacher's Note

Students can make natural indicators at home using things from the kitchen. Turmeric is available in every Indian kitchen. This makes science fun and practical for children.

Exam Trick

Remember that turmeric turns red in base and turns yellow in acid. Think of turmeric as a yellow spice that becomes red when we add a base like soda.

Points to Remember

Turmeric is a natural indicator that turns red in base.
Red cabbage juice also works as a natural indicator.
Hibiscus flowers can show colour change in acids and bases.
Natural indicators are easy to make at home.
These indicators work just like lab indicators.

This is a preview of the first 3 pages. To get the complete book, click below.