Acids, Alkalies And Salts

Ionisation reaction involves the conversion of neutral molecules into their constituent ions (ions are charged atoms or molecules). In the above question, we can complete the ionisation reactions by breaking the given compounds into ions (cations and anions) as follows:

Note: cations are positively charged ions and anions are negatively charged ions.

•KCl → K⁺ + Cl^-

KCl dissociates into potassium ion and chloride ion.

•HNO₃→ H⁺ + NO₃^–

Nitric acid ionises to give a proton and nitrate ion.

•Mg(OH)₂→ Mg²⁺+ 2OH^–

Magnesium hydroxide ionises to give magnesium ion and two hydroxide ions.

•H₂SO₄→ 2H⁺ + SO₄^2–

Sulphuric acid dissociates to give two protons and a sulphate ion.

•NH₄Cl → NH⁺₄ + Cl^-

Ammonium chloride ionises to give ammonium ion and chloride ion.

•CaSO₄→ Ca²⁺ + SO₄^2-

Calcium sulphate ionises to give calcium ion and sulphate ion.

We can observe that all the ions given in the question are negatively charged. All of them are anions. Names of given anions can be written as follows:

•Name of salt is magnesium chloride [MgCl₂]

Magnesium hydroxide is a strong base [Mg(OH)₂] and hydrochloric acid [HCl] is a strong acid. When a base and an acid react, they mutually nullify their properties and gives rise to the formation of salt and water. Such reactions are called neutralisation reactions. Thus magnesium hydroxide and hydrochloric acid will react to give salt and water and name of this salt is magnesium chloride.

•Mg(OH)₂ + 2HCl → MgCl₂ + H₂O

The reaction between magnesium hydroxide and hydrochloric acid is a neutralisation reaction in which acid and base react to give salt and water. Thus one mole of magnesium hydroxide reacts with two moles of hydrochloric acid to give magnesium chloride (salt) and water.

•Sulphuric acid [H₂SO₄] is used for preparing magnesium sulphate.

The chemical formula for magnesium sulphate is [MgSO₄]. From its formula, we can conclude that for its preparation we require a base that can provide us with the magnesium ion and an acid which can give us sulphate ion. In the given problem, we are using magnesium hydroxide as a base which will give us magnesium ion. Now the acid which can give us sulphate ion is sulphuric acid. The preparation of [MgSO₄] can be represented as follows:

Mg(OH)₂ + H₂SO₄ → MgSO₄ + H₂O

Salts are neutral compounds which are ionic in nature and dissociate into positive and negative ions when dissolved in water. The positively charged ion is called cation and the negatively charged ion is called anion. Thus in the above question we need to dissociate the given salts into the respective positive and negative ions, which can be done as follows:

A. pH value of distilled water is 7.

The pH scale is the method to express the acidity/alkalinity of a solution. It is based on the amount of H⁺ ions present in a given solution.

Lesser the value of pH, more will be the acidity. For a neutral solution the pH value is 7. Distilled water is neutral in nature because equal number of H⁺ and OH^- are present in it. Thus distilled water has a pH value of 7.

B. The pH value will change when caustic soda or vinegar is added to the distilled water as follows:

a. pH value will increase and become much greater than 7 on adding caustic soda to water.

Caustic soda is general name for NaOH which is a strong base. So when we add caustic soda to water, it will make the water basic in nature. A basic solution has a pH greater than 7. So we can say that adding caustic soda will make the pH of water greater than 7.

b. pH value will decrease and become slightly less than 7 on adding vinegar to water.

Vinegar contains acetic acid which is a weak acid. So when we add vinegar to water, it will make the water acidic in nature. An acidic solution has a pH less than 7. So we can say that adding vinegar will make the pH of water less than 7.

The correct match between salts, their formulas, and their uses is as follows:

Some other uses of given salts can be summarised as follows:

Washing soda

•Used in glass, soap and paper industries.

•Used for removing hardness of water.

•Can be used as cleaning agent for domestic purposes.

Gypsum

•Used in cement industry.

•Used for making toys, decorative items.

•Used to support fractured bones.

Blue vitriol

•Being a fungicide, used to control molds and fungus from the places like lawns, gardens and some other wet areas where fungi grows.

Baking soda

•Used in antacids (which neutralise the excess acidity in our stomachs)

•Used in fire extinguishers.

a. Blood is slightly alkaline in nature.

A neutral solution has a pH of exactly 7, while an acidic solution has a pH less than 7 and a basic solution has a pH more than 7. Since the blood has a pH value of 7.36, it is alkaline in nature but we can say it only slightly alkaline.

b. The pH value decreases when milk changes to curd.

The process of conversion of milk into curd involves the formation of lactic acid. This lactic acid makes the curd more acidic than milk. On pH scale the acidity increases with the decrease in pH value. Thus pH value decreases from 6.3 to 4.5 when milk changes into curd.

c. i) Lime water is strongly alkaline.

The pH scale varies from 0 to 14. A neutral solution has a pH of exactly 7. Acidic solution has a pH less than 7 and basic solution has a pH greater than 7.

General rule: Lesser the pH more will be acidity.

OR

More the pH more will be alkalinity.

Since lime water has a pH of 10.5 (which is much greater than 7), so it is strongly alkaline.

ii) Milk has the weak acidic nature.

Both milk and vinegar has a pH less than 7 so both are acidic in nature. But milk has a pH slightly less than 7 (6.3) while vinegar has a pH which is much lower than 7 (4.2). Therefore, milk has weak acidic nature as compared to vinegar.

Number of acids are present in the fruits/vegetables and other eatables that we use. Such sources are listed below with the acids associated with them:

Plants require a specific pH range for their healthy growth. So it is very necessary to analyse the pH of soil. The pH value of soil can be determined as follows:

•Take about 2g of soil in a test tube and add 5 ml distilled water to it.

•Shake the contents and then filter. Collect the filtrate in a test tube.

•Measure the pH using universal indicator and match the colour from following chart:

The pH value of soil of different places along with the suitable crops is summarised as follows:

Those acids which contain more than one H⁺ ion, they dissociate in stages. Phosphoric acid is also an example of such acid.

Dissociation of phosphoric acid takes place in following stages:

H₃PO₄→ H⁺ + H₂PO₄^–

(Dihydrogen phosphate ion)

H₂PO₄^–→ H⁺ + HPO₄^2–

(Hydrogen phosphate ion)

HPO₄^2–→ H⁺ + PO₄^3–

(Phosphate ion)

Three types of salt.

We can see that phosphoric acid dissociates to give three types of anions which can combine with any metal cation to form a salt. So, three anions can result into only three types of salts.

The chemical names corresponding to given salts are as follows:

Litmus paper acts as an indicator to distinguish between acid and base. Blue litmus paper turns red in acidic medium and red litmus paper turns blue in basic medium.

i)

ii) The following acids and alkalies react to give the corresponding salts.

Acids and alkalies react to give salt and water.

•When a strong acid and a strong base react they give a neutral salt. Example: KCl

•When a strong acid and a weak base react they give an acidic salt. Example: (NH₄)₂SO₄

•When a weak acid and a strong base react they give a basic salt. Example: Na₂CO₃

iii) Litmus paper shows different colours in acidic and basic medium.

•When a strong acid and a strong alkali react, they give a neutral salt. Thus the litmus paper does not change its colour. Blue litmus paper remains blue and red litmus paper remains red. Example: KCl

•When a strong acid and a weak alkali react, they give an acidic salt. Thus blue litmus paper turns red and red litmus paper remains red. Example: (NH₄)₂SO₄

•When a weak acid and a strong alkali react, they give a basic salt. Thus red litmus paper turns blue and blue litmus paper remains blue. Example: Na₂CO₃