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# Acids Bases and Salts

Acids and bases affect chemistry and our day to day life as well. They can be identified by their taste; acids taste sour and bases taste bitter. Acids are found in many substances including food items but their presence in fruits is very prominent, e.g., citric acid is present in citrus fruits such as orange and lemon. Vinegar contains acetic acid. Apart from these, some acids are widely used in the laboratory, like hydrochloric acid, sulfuric acid and nitric acid.
The bases which are soluble in water are called alkalis. In general, bases are found in household cleaners to clean grease from windows and floors and also in soaps, toothpaste, egg whites, dish washing liquids and household ammonia.

Our body contains some very common acids like dilute hydrochloric acid in the stomach, which causes indigestion of food. If the contents of our stomach become too acidic, we get indigestion and a burning sensation in the stomach. Acids and bases also regulate metabolic activities in the human body through equilibrium processes. Bee stings are acidic in nature while wasp stings are alkaline in nature.

All acids generate hydrogen gas in reaction with metals. Hydrogen is common to all acids. In fact,all acids contain hydrogen ions (H+). A base is a substance, which on dissolving in water yields hydroxyl ions (OH-) as the only negative ions. A base may be an oxide or a hydroxide of a metal. If a base is soluble in water, it dissociates to form a metal ion and it is the only negative hydroxyl ion (OH-).
• Acids, bases and salts are three main categories of chemical compounds. They have certain definite properties which distinguish one class from the other.
• The sour taste of many fruits and vegetables, lemon for instance, is due to various types of acids present in them.
• The digestive fluids of most animals and humans also contain acids. The word 'acid' is derived from a Latin word, which means "sour".
• The acids we use in the laboratory are very strong acids like hydrochloric acid and sulfuric acid. Strong acids are corrosive and can burn your skin.
• Bases, on the other hand, are the chemical opposite of acids. They are bitter in taste and soapy to touch.
• Sea water and detergents are some examples of substances that are basic. Many bases are oxide or hydroxide compounds of metals.
• Strong bases can also burn one's skin. Tasting a substance is not the right way of finding out if it is an acid or a base! Acids and bases can be better distinguished with the help of indicators.
• Indicators are substances that undergo a change of color with a change of acidic, neutral or basic medium.
• Litmus, a purple dye extracted from the lichen plant, is commonly used as an indicator in laboratories.
• Acids change the color of litmus solution to red, and bases change the color of litmus solution to blue. Turmeric is another common household indicator.
• A stain of turmeric based food spill on a white cloth becomes reddish-brown when soap is scrubbed on it.
• Soap is basic in nature and changes the color of the turmeric stain. It turns yellow again when the cloth is washed with plenty of water.

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## Properties of Acids Bases and Salts

### Properties of Acids

• Acid is a compound which yields hydrogen ion (H+), when dissolved in water.
• Acid is sour to the taste and corrosive in nature. The pH value of acids is less than 7.
• Generally, all acids readily react with metal to release hydrogen gas. For example, metal zinc reacts with hydrochloric acid to form zinc chloride and hydrogen gas.
• Acids react with limestone (CaCO3) to produce carbon dioxide. For example, hydrochloric acid reacts with limestone to produce carbonic acid and calcium chloride.
• Acids can be classified into organic and inorganic acids. Acetic acid (CH3COOH) is the best example of organic acid, and acids produced from minerals are termed as inorganic acids like sulfuric acid (H2SO4), hydrochloric acid (HCl), etc.
• Acid converts blue litmus paper to red in color.
• Acids have a tendency to corrode metal surfaces quickly.
• Phenolphthalein solution is colorless in an acidic solution and turns methyl orange solution to red. Red cabbage juice which is purple in color changes to red in an acidic medium.
On the basis of number of hydrogen ion, acids can be classified as
1. Monoprotic acid – Such type of acids produce one mole of H+ ions per mole of acid, e.g., HCl , HNO3, etc.
2. Diprotic acid – They can produce two moles of H+ ions per mole of acid, e.g., H2SO4.
3. Triprotic acid – They produce three moles of H+ ions per mole of acid, e.g., H3PO4.
4. Polyprotic – They can produce more than three H+ ion per mole of acid.
On the basis of strengths or capacity to donate hydrogen ions, acids can be described as:
• Strong acids: Which are completely (100%) ionized in aqueous solutions. Hence at equilibrium, the concentration of acid molecules is very less and concentration of hydrogen ion reaches to maximum, e.g., HCl, HNO3, HClO4.
• Weak acids: They are only partially ionized in solution at equilibrium state. At equilibrium state, acid molecules are present and the concentration of hydrogen ion is less, e.g., HF, CH3COOH.

### Properties of bases

• Bases are compounds which yield hydroxide ion (OH-), when dissolved in water.
• Bases are bitter to taste and corrosive in nature. They feel slippery and soapy.
• Bases are good conductor of electricity and show a pH value of more than 7.
• Bases react with oils and grease to form soap molecules.
• Bases convert red litmus paper to blue in color.
• Bases also have the tendency to corrode metal surfaces.
• A reaction between a base and a metal is similar as for acid to form salt and release hydrogen gas. But this reaction can only occur when a metal is strong enough to displace another metal from its parent constituent.
2NaOH + Zn $\to$Na2ZnO2 + H2
• Phenolphthalein solution turns pink in color in a basic solution. Bases turn methyl orange to yellow. Red cabbage juice which is purple in color changes to yellow in a basic medium.

### Strength of bases

• Strong bases: They are completely ionized in water to produce hydroxide ions, e.g, sodium hydroxide: NaOH(s) $\rightleftharpoons$ Na+(aq) + OH-(aq)
• Weak bases: Partially ionize and equilibrium lies mostly towards reactants side, e.g., ammonia in water: NH3(aq) + H2O(l) $\rightleftharpoons$ NH4+(aq) + OH-(aq)
On the basis of the number of hydroxide groups present in a base, they can be classified as monobasic (one OH-), dibasic (two OH-), tribasic (three OH-) bases and so on.

## Arrhenius Concept of Acid and bases

Arrhenius concept is the oldest concept to explain acids and bases. According to this concept.
• An acid is a hydrogen-containing compound which can give hydrogen ion (H+) in aqueous solution. For example, Hydrochloric acid (HCl) gives H+ ion in its aqueous solution.
HCl(g) $\overset{H_2O}{\rightarrow}$ H+(aq) + Cl-1(aq)

Such types of acids are called Arrhenius acids, like nitric acid (HNO3), acetic acid (CH3COOH), etc.
• Base is also a hydrogen containing compound, which can give hydroxide ion (OH-) in an aqueous solution. For example, Sodium hydroxide in water form s sodium ion (Na+ ) and hydroxide ion (OH-). Such type of bases are known as Arrhenius bases. Other example of Arrhenius base are ammonium hydroxide (NH4OH), aluminium hydroxide (Al(OH)3) and magnesium hydroxide(Mg(OH)2).

NaOH(s) $\overset{H_2O}{\rightarrow}$ Na+(aq) + Cl-1(aq)

## Properties of Salts

• Salts form by the combination of acid and base through neutralization reaction.
• The acidic and basic nature of salts depends on the acid and base combined in neutralization reaction.
 Acid Base Salt Example Strong Strong Neutral NaOH + HCl $\rightarrow$ NaCl + H2O Strong Weak Acidic HCl + NH4OH $\rightarrow$ NH4Cl + H2O Weak Strong Basic CH3COOH + NaOH $\rightarrow$ CH3COONa + H2O Weak Weak Neutral CH3COOH + NH4OH $\rightarrow$ CH3COONH4 + H2O

The most common salt is sodium chloride or table salt which forms by the combination of sodium hydroxide (base) and hydrochloric acid. Other examples include Epsom salts(MgSO4) used in bath salts, ammonium nitrate (NH4NO3) used as fertilizer, and baking soda (NaHCO3) used in cooking.
The pH of a salt solution depends on the strength of acids and bases combined in the neutralization reaction.
 Acid Base Salt pH strong strong pH=7 weak strong pH>7 strong weak pH<7 weak weak depends on which is stronger

## Is Salt An Acid or A Base?

Acids form hydrogen ions (H+) in aqueous solution. In place of hydrogen ions, it’s better to write hydronium ions (H3O+) which form due to reaction of H+ with water. For example, Hydrochloric acid (HCl) in water.
HCl + H2O $\to$ H3O+ + Cl-
At the same time bases are the substances that produce hydroxide ion OH- in water. Acids and bases react with one another to yield two products: water, and an ionic compound known as salt. The reaction is called as neutralization reaction.
NaOH + HCl $\to$ H2O + Na+Cl
Neutralization reactions are always exothermic in nature, which means they liberate the same amount of heat. This amount is fixed for any acid and base that is 57.7kJ per mole of H+. For any acid and base combination the net reaction is
H+(aq) + OH- (aq) $\to$ H2O
Because salt produced in neutralization reaction is ionic in nature and exists in the form of ions just before the reaction. ­Hence salts are the main products of a neutralization reaction.

## Formation of Ammonium Ion

When ammonia is dissolved in water it forms ammonium ions (NH4+) and hydroxyl (OH-) ions by the formation of a co-ordinate covalent bond.

This can be shown as follows.

Bases and acids neutralize each other, therefore, another way to define a base is, "a compound which reacts with an acid to give salt and water only." Like acids, alkalis can be strong or weak. The more hydroxide ions they produce, the stronger the alkali is.
The acidic property of an acid is due to the presence of hydrogen ions (H+) while that of a base or alkali is due to the presence of hydroxyl (OH-) ions in them. When strong acids and bases (alkali) combine, the positively charged hydrogen ion of the acid combines with the negatively charged hydroxyl ion of the base to form a molecule of water. Hence, the water molecule formed does not have any charge because the positive and negative charges of the hydrogen ions and hydroxyl ions get neutralized.
H+(aq) + OH-(aq) $\to$ H2O(l)
Neutralization can be viewed as a reaction in which an acid combines with a base, neutralizing the positively charged hydrogen ion and the negatively charged hydroxyl ion, to form a molecule of water and the respective salt.

Addition of Acids or Bases to Water

The process of dissolving an acid or a base in water is a highly exothermic one. As this reaction generates a lot of heat, care must be taken while mixing concentrated acids with water, specially nitric acid or sulfuric acid with water.

As a rule: The acid must be added slowly to the water with constant stirring. If one mixes the other way by adding water to a concentrated acid, the heat generated causes the mixture to splash out and cause burns.

The glass container may also break due to excessive local heating and cause damages! Mixing an acid or base with water results in dilution. It decreases the concentration of ions (H3O+/OH-) per unit volume thereby dissipating the heat effect easily. The list of acids and bases are shown below. Naming of acids and bases is done on the acid base scale.