Introduction
There are 118 known elements till now. We know, that these elements combine to form a large number of compounds. Compounds can be classified into three groups on the basis of their chemical properties and they are:
🔴 Acids
🔴 Bases and
🔴 Salts
In our daily life all of us use so many substances such as lemon, curd, Orange, tamarind, common salt, vinegar, Amla and baking soda. Have you ever thought about their taste? Do we know the taste of all the substances? Whether the substances mentioned above taste the same or do they taste different?
After tasting the above listed substances we find that curd, lemon, Orange, tamarind, vinegar and Amla taste sour. The taste of baking soda is bitter. We all know very well the taste of common salt chemically known as sodium chloride is salty to taste.
CAUTION:
🔴 Never taste anything unless asked to do so.
🔴 Do not touch anything in the laboratory without the permission of the teacher.
When we talk about taste, it is said that there are five basic tastes they are; sweet, sour, bitter, umami (Savory )and salty. Here umami means the taste of meats and other high protein foods. This taste is produced by the presence of glutamate (crystalline amino acid) and nucleotides (compound consisting of nucleoside linked to a phosphate).
As we know very well it is not only possible to taste all the substances but unadvisable because some acids and bases are very corrosive in nature.
That is the reason we use and indicator for testing acids and bases. Come let's let's know more about indicators.
INDICATORS
🔴 An indicator is a dye that changes colour when it is put into an acid or a base.
🔴 An indicator tells us whether a substance is acid or a base by change in its colour.
🔴 The three most common indicators for testing acids and bases are:
Litmus paper
Methyl orange and
Phenolphthalein
There are various types of indicators such as a natural indicators, synthetic indicators, olfactory indicators and acid-base indicators.
NATURAL INDICATORS
🔴 The most common indicator used for testing acid and base in the laboratory is litmus,which can be used in the form of litmus paper or litmus solution. Litmus paper are of two types :blue litmus and red litmus.
🔴 Litmus is a natural indicator. Litmus solution is a purple dye extracted from lichen. It is made into red and blue litmus, to detect the colour change when an acid or base added to it.
🔴 An acid turns blue litmus to red.
🔴 A base (or alkali )turns red litmus to blue.
🔴 When the litmus solution is neither acidic or basic its colour is purple.
Other examples of natural indicators are turmeric China rose, and red cabbage leaves.
Turmeric (haldi )contains a yellow dye. It is also a natural indicator. It turns red in basic solutions.You might have observed a yellow stain of curry on your clothes specially white coloured cloth. When we rub soap solutions on it to clean or remove the stain then we see that the yellow colour stain changes to red- brown. When we rinse the cloth again with water the stain turns to yellow again because the basic soap gets removed with water.
The red cabbage extract is also a natural indicator. The red cabbage remains red in acidic solutions and changes to green on adding to the basic solutions.
SYNTHETIC INDICATORS
The man made indicators are called synthetic indicators. Methyl orange and phenolphthalein are examples of it.
🔴 Methyl orange indicator gives red colour in in acid solution.
🔴 Methyl orange indicator gives yellow colour in basic solution.
🔴 The neutral colour of methyl orange is 'orange'.
🔴 The neutral colour of phenolphthalein is colourless.
🔴 Phenolphthalein indicator is colourless in acid solution.
🔴 Phenolphthalein indicator gives pink colour in basic solution.
OLFACTORY INDICATORS
The substances whose smell changes in acidic or basic solutions are called olfactory indicators. The meaning of the term 'olfactory' is 'relating to the sense of smell'. When an acid or a base is added to olfactory indicator then its mil cannot be detected. Examples of olfactory indicators are onion or onion extract and Vanilla extract.
Points to remember:
🔴 When a basic solution like sodium hydroxide is added to onions or onion extract and Vanilla extract we cannot detect the smell of onion and Vanilla extract.
🔴 When an acidic solution such as hydrochloric acid is added to onions or onion extract and Vanilla extract the smell cannot be destroyed. The smell of onion and Vanilla remains as it is.
👉 Another acid base indicator is the 'universal indicator'. The information about universal indicator is given below in the chapter.
Physical properties of acids
* Acids are sour in taste
* Acid turns blue litmus to red
* Examples are vinegar, hydrochloric acid, sulphuric acid, nitric acid and acetic acid.
Chemical properties of acids:
* Acids produce H+ ions when dissolved in water.
H+ ions cannot exist alone hence they combine with water molecule to form H3O+ ion or hydronium ions.
* It conducts electricity.
* It is highly exothermic.
Because it release large amount of heat.
If water is added to concentrated acid the heat generated may cause the mixture to splash out and cause burns. Hence to avoid burns always acid must be added dropwise into water with constant stirring. So ,the heat generated spreads in water.
Chemical reactions of acids:
*Reaction of acid with metals:
Most of the acids react with metals to form Salts and hydrogen gas. Formation of hydrogen gas is common to the reaction of all acids with metals.
We should not keep curd and other sour foodstuffs such as vinegar, lemon juice and orange juice in metal vessels like Copper vessels or brass vessels because curd and other sour foodstuffs contains acid which can react with the metal of the vessel to form poisonous metal compounds which can cause food poisoning and it affects our health.
Acid + metal ➡️Salt + hydrogen gas
2HCl+Mg➡️MgCl2+H2
* Reaction with metal carbonate
Acid +metal carbonate ➡️salt +CO2 +H2O
2HCl+Na2CO3➡️2NaCl+CO2+H2O
* Reaction with metal hydrogen carbonate
Acid + metal hydrogen carbonate ➡️ salt + CO2 + H2O
HCl+NaHCO3➡️NaCl+H2O+CO2
* Reaction with metallic oxides
Acid + metal oxide ➡️ salt + water
2HCl+CuO ➡️CuCl2+H2O
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