Acids, Bases and Salts
Science Class Seven
Types of substances
Substances can be divided into three categories based on some of their characters. These categories are acid, base and salt.
Acid
Many substances contain acid. The acid present in such substances makes their taste sour.
Example: Lemon, Curd, Tamarind, tomato, pickles, etc.
In other words, substances which taste sour contain acid.
Substances, which taste sour are called acidic or acidic in nature.
The word "acid" came from the Latin word "acere" which means "sour". There is another word in Latin for "acid" which is "acidus".
Base
Many substances contain base. The taste of base is bitter. This means substances which contain base, taste bitter.
A base feels soapy when it is touched or rubbed between fingers.
Thus, substances that taste bitter and feel soapy are base or contain base.
Example: Baking soda, caustic soda, baking powder, soap, etc.
Bases are known as "alkali".
Thus, substances which are bitter in taste and feels soapy in touch are called basic in nature.
Indicator
It is not possible to taste every substance to find its chemical nature, i.e. acidic or basic nature. So, another types of substances are used to test the acidic or basic nature of a substance, these substances are called indicators.
Indicator is a type of substance which changes its colour with acid and base. Because these substances indicate the acidic or basic nature of a substance, that is why these are called Indicators.
Some examples of indicators: Litmus, turmeric, China rose, phenolphthalein, methyl orange, etc.
Litmus: A Natural Indicator
Litmus it the most common indicator which is used to test acidic or basic nature of a substance.
Litmus is a dye extracted from a plant called "Lichens". The natural colour of litmus is purple. Litmus was discovered in Netherlands by a Spanish Physician Arnaldus de Villa Nova.
The "Litmus" is a word used by the Norse which they used to denote "dye or colour".
The litmus turns into red colour when it is added to an acidic solution.
The litmus turns into blue when it is added a basic solution.
This means litmus gives red colour with acid and blue colour with base.
Litmus Paper
Litmus is available in the form of paper strip. It is available generally in two colours, blue and red.
A blue litmus paper turns red when it is dipped into acidic solution.
A red litmus paper turns blue when it is dipped into a basic solution.
Turmeric is another natural indicator
Turmeric acts as an indicator.
Turmeric solution changes into brown red with a basic solution.
Turmeric paper can be made by soaking blotting paper into turmeric solution.
Many time when vegetable curry or anything containing turmeric falls on cloths, the cloths get a yellow stain over it. When soap is applied to this cloth having turmeric stain, the stain turns reddish brown.
This happens because soap is basic, and turmeric changes its yellow colour into to reddish brown with base.
Making of Turmeric Paper
* Take a sheet of blotting paper.
* Cut about 1 cm wide and 3 cm long strips from it.
* Take some turmeric powder.
* Make a thin paste of turmeric powder.
* Apply the turmeric paste over the thin strips of blotting paper on both sides.
* Dry the thin strips of paper in sunlight.
Test of solution using turmeric paper
* Take some water in a tea glass of cup.
* Dissolve some detergent powder in the water kept in cup/glass.
* Take some water in other glass.
* Add one lemon juice in the water in it.
* Now, take two strips of turmeric paper.
* Dip one turmeric paper in detergent solution and dip another turmeric paper in lemon juice solution.
* Observe the change in colour of turmeric papers.
You will observe that turmeric paper which was dipped in detergent solution turns into reddish brown. And there is no change in colour take place for turmeric paper dipped in the lemon juice solution.
The change of colour from yellow to reddish brown of turmeric paper shows the presence of base in the detergent solution.
Turmeric does not change its colour with acid.
China Rose: another natural indicator
The extract of China rose also acts an indicator.
The extract of China rose is light purple in colour originally.
The extract of China rose turns into dark pink colour when added to acid or acidic solution. And the extract of China rose turns into light green colour when added to base or basic solution.
Light purple colour of China rose ⇒ Turns into dark pink (magenta) colour with acid.
Light purple colour of China rose ⇒ Turns into light green colour with base.
Light purple colour of China rose &rArr: No change with distilled water, which is neither acidic nor basic.
Phenolphthalein : an indicator
Phenolphthalein is an indicator which is used to test the acidic and basic character of a solution.
Phenolphthalein is a colourless solution.
Phenolphthalein remains colourless in acidic solution but it changes to pink when added to a basic solution.
Neutral Solution
Solution which is neither acidic nor basic is called a Neutral Solution.
Distilled water is neither acidic nor basic, so it is neutral.
Neutralisation
Acidic solution + Basic solution ⇒ Neutral Solution
Acid and base neutralises each other.
This means, when an acidic solution is mixed with a basic solution in proper proportion, it neutralises the basic solution.
Similarly, when a basic solution is added to an acidic solution in a proper proportion, it neutralises the acidic solution.
In precise words, acidic and basic solutions neutralise each other.
The reaction between acidic and basic solution is called neutralisation reaction.
This process of neutralising of acid by adding of base and base by adding of acid is called the neutralisation.
When acidic solution is mixed with a basic solution, salt and water are formed.
Acid + Base ⇒ Salt + Water
Neutralisation in Everyday Life
Indigestion
Stomach produces acid in order to kill the bacteria and help in digestion of food. The name of acid produced by our stomach is Hydrochloric Acid.
But sometimes because of overeating or any other problematic conditions our stomach produces more acid than required. Production of more acid by our stomach creates a problem called indigestion. This condition of indigestion is called Acidity or Hyper acidity. This makes one uneasy. Sometimes because of acidity we feel heart burn and uneasiness.
Doctors prescribe antacid to get rid of problem of acidity.
Anti + Acid = Antacid.
Clearly, antacid is a substance which acts against acid.
Since base neutralises the acid, so antacids are nothing but are mild base.
Milk of magnesia, Eno, Digene, etc. are the name of some medicines which is prescribed by doctor to get rid of the condition of acidity. These medicines contain base. An antacid is taken orally.
The base taken as medicine neutralises the extra acid produced by our stomach, and we get relieved.
Ant bite
Ant bite contains a type of acid called Formic acid. Ant has this gift from nature to keep save herself. While biting, ant injects formic acid into the skin. Because of injecting of formic acid, person who get bitten by ant, feels pain at the place of bite on his body.
By rubbing of a pinch of baking soda, or calamine solution the acid gets neutralise and victim of ant bite gets relief.
Soli Treatment
Many times soils get acidic or basic because of use of excessive fertilisers, resulting in less yield or no yield because of decrease or losing of fertility of soil.
This problem is treated with either adding of acidic or basic substance in the soil depends on changed nature of soil.
If soil becomes acidic, then either quick lime (calcium oxide) or slaked lime (calcium hydroxide) which are base, is added to the soil. This neutralises the acidic nature of soil and replenishes the fertility.
Similarly, if soil would become basic, then natural organic matter is added which releases the acid. This acid neutralises the basic nature of soil which replenishes the fertility.
Factory Waste
Most of the factories' wastes contain acid. If these acidic wastes are flushed to the river or water bodies without any treatment, these turn the water acidic. Aquatic animals and plants cannot survive in acidic water which is resulted in the form of degradation of water quality and water pollution. This harms the environment.
So, wastes coming of factories are treated with base in order to neutralise the acid present in them before flushing out in water or disposal.
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