Acids, bases and salts

Introduction

Almost every substance that we use in our day-to-day life fall under the category of either acidic or alkaline. This depends mainly upon the pH level, and some other factors such as taste, colour, reaction with other substances. Why do substances vary in their taste, colour, and nature? Tamarind and lemon taste similar whereas salt tastes different, why? This is because of the differences in the percentage of hydrogen ion concentration which makes a substance into either an acid,  base or salt.  In our daily life, many consumer products such as milk, juices,  and snacks rely on the principle of hydrogen ion concentration which gives each of them a variety in terms of taste, appearance, reaction etc. In the past, acid, base and salt were defined based on their taste, gradually some experimental definitions have been developed to define them based. The experimental definition takes into consideration of not just the taste, but the complete chemical profile of a given substance.

Definitions

 “An acid is defined as a substance whose water solution tastes sour and when blue litmus is dipped, it turns into the red”.  Conversely,  “A base or an alkali is designated by its aqueous solution tasting bitter and turns red litmus into blue, adding to this, it has the ability to neutralize acids to give salt and water”. Lastly, , “A salt is a neutral substance whose aqueous solution does not affect the litmus”. Let us discuss each one in detail.

Acids

The word acid means sour; derived from a Latin word acidus or acere. They are identified mainly by their sour taste. When it is in the aqueous solution, an acid renders ionizable hydronium ion (H3O+)  therefore, it turns blue litmus paper into the red. An acid has  pH < 7, reacts with a metal to form hydrogen gas. It increases the H+ concentration in water by donating it. 

Examples:

Dissociation of Acids

Acids can be classified in many ways, however, depending upon their occurrence, they are classified as natural and mineral acids. Natural acids are obtained from natural sources like plant and animal products, and they are not synthetic in nature. For e.g. lactic, citric, and tartaric acid etc. On the other hand, mineral acids are those prepared from minerals. For example, Sulphuric Acid (H2SO4), Hydrochloric acid (HCl), and nitric acid (HNO3).

Bases

Bases are characterized by their bitter taste and soapy feel. A base releases a hydroxyl ion(OH–) in its aqueous solution and it turn the red litmus paper into blue when dipped.

 

 

Some naturally occurring acids and bases

1. Acetic acid (CH3CO2H)is produced through bacterial fermentation used in the production of vinegar and also as a solvent.

2. The citric acid (C6H8O7) is the component of citrus fruit used in food preservation and flavouring. Some of them are,  raw mango and lemon. 

3. Formic acid (HCOOH) can be found in the stings of ants and bees. It is used for a variety of purposes such as esterification and food preservation.

4. Lactic acid (C3H6O3)  is released by muscles during exercise. It is also an ingredient of milk products such as yoghurt and cheeses.

5. Ammonia (NH3), certain organisms produce ammonia from atmospheric nitrogen and it is used as a household cleaner, and also as a fertilizer.

6. Potassium hydroxide (KOH) is found while leaching some wood ashes. It was known historically to produce potash and potassium hydroxide is also used to produce soap and fertilizers.

7. Sodium bicarbonate (NaHCO3) is produced naturally from a mineral known as nahcolite. It is useful for various pest control purposes and antacids. It helps the dough or batter to expand.

Arrhenius acids and bases

In 1884, Chemist Svante Arrhenius has characterised the properties of acidity to hydrogen ions (H+) or protons. According to him, when an Arrhenius acid is added to water, the concentration of H+ ions increases. This is the reason chemists usually write H+(aq) indicating the importance of hydrogen ion when describing acid-base reactions. However, the free hydrogen nucleus fails to exist alone in the water but it exists as the hydronium ion, H3O+. Therefore, an Arrhenius acid increases the concentration of hydronium ions when added to water. Some examples include HCl and acetic acid. Conversely, an Arrhenius base increases the concentration of hydroxide (OH) ions when dissolved in water. Consequently,  the concentration of hydronium of the given solution increases.

H3O(aq) + OH(aq) ⇌ H2O(l) + H2O(l)

As shown in the reaction above, if we increase the concentration of hydronium, the concentration of hydroxide is spontaneously reduced and vice-versa.

Differences between acids and bases

Acids

Bases

Acids taste sour

Bases are bitter

An acid turns blue litmus into red colour.

A base turns red litmus into blue colour.

An ideal example of an acid is Hydrochloric acid HCl.

The most commonly found base is  Sodium hydroxide NaOH.

The strength of an acid Relies on the concentration of the hydrogen ions.

The strength of the base depends upon the concentration of the hydroxide ions( OH).

An acid is influenced by the temperature making them look-like either solid, liquid or in the form of gas depending upon the degree of heat.

These substances may be slippery and solid in nature with an exception being ammonia as it is gaseous.

Acids would release hydrogen ions (h+) when mixed with water.

The dissociation in the base is characterized by the release of hydroxide ions(OH-) when mixed with water.

pH of acid is less than 7

pH of a base is more than 7

Acids, when dissolved in water, doesn’t cause any change in the colour of water

When bases are dissolved in water, the water turns pink.

Common examples:  Hydrochloric acid (HCl), Sulphuric acid (H2SO4), Nitric acid (HNO3), Carbonic acid (H2CO3).       

Examples: Ammonium hydroxide (NH4OH), Calcium hydroxide (Ca(OH)2), Sodium hydroxide (NaOH).

Acid-Base Indicators

Acid-Base indicators are also known as pH indicators.  Acid-base indicators are weak acids or bases, which when dissolved in water dissociate slightly to form ions.

Name of the Indicator

If you mix acid

If you mix base

Blue litmus solution

Turns to red

No change

Red litmus solution

No change

Turns to blue

Turmeric

No change

Turns to red

Methyl orange

Turns to red

Turns to yellow

Phenolphthalein (colourless)

No change

Turns to pink

Salts

The chemical formula of common salt is NaCl ( Sodium Chloride). The pH of salt is 7, hence it is neither acidic nor alkaline).  It contains both the cations(+) and anions(-) in an equal proportion. In its natural form, it can exist as either crystalline or amorphous( powdery). Salts contain two groups of ions with a positively charged  (H+) and negatively charged (OH-). In our daily life, we use a lot of salts in many different forms they are:

  1. Salt used to cook food.

  2. Salt is used as a preservative while curing meat and fish.

  3. It is used in the manufacture of soap.

  4. In winter countries, salts are used to melt the ice.

  5. It is used for manufacturing washing Soda, baking soda etc.

 

More readings in Chemistry 

  1. Acids, Bases and Salts     

  2. pH in daily life 

  3. S-Block elements

  4. Equilibrium in Chemistry

 

 



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