Class 10 Science Chapter-2

Do you know why it is advised to clean your mouth after consuming food?

Acids are produced in the mouth due to the degradation of sugar and food particles by certain bacteria. As a result, the pH of mouth is lowered.

pH is the measure of the acidity or alkalinity of a solution. The pH scale varies from 0 to 14. A pH value from 0 to 6.9 represents an acidic solution, whereas a pH value from 7.1 to 14 represents a basic solution. A pH value of 7 represents neutral solutions.

Tooth enamel, which is made of calcium phosphate, is sensitive to pH.It does not dissolve in water (neutral pH), but gets corroded when pH in the mouth is below 5.5.
Hence, tooth decay starts when the pH of mouth is lower than 5.5.

Do You Know:

pH change as the cause of tooth decay:

Toothpastes used by us for cleaning our teeth are generally basic in nature. Hence, it can neutralize excess acid present in the mouth and prevent tooth decay.

Let us study the role of pH in our daily life.

All living organisms are pH sensitive and can survive only in a narrow range of pH.

If the pH of rain water is less than 5.6, it is called acid rain. Acid rain contains mainly nitric and sulphuric acids, which are formed by the dissolution of nitrogen oxides and sulphur dioxide (respectively) in the atmosphere. When water from acid rain flows into the rivers, it results in a decrease in the pH value of river water. This decrease in the pH makes the survival of aquatic life very difficult.

Thus, acid rain causes extensive harm to soil, water resources, forests, and human health.

pH in our digestive system:
We know that our stomach produces gastric juice during digestion. Gastric juice contains hydrochloric acid, which helps in the digestion of food without harming the stomach. When the stomach produces an excess of acid, it causes irritation and pain and results in indigestion.

Indigestion can be treated by taking antacids. Magnesium hydroxide called milk of magnesia is a mild base. It is used as an antacid, and can be used to neutralize the effect of excess acid in the stomach and cure indigestion.

Do You Know:

Sodium hydrogen carbonate, commonly known as baking soda, is also used as an antacid to neutralize stomach acidity. When sodium hydrogen carbonate is taken orally, it reacts with excess hydrochloric acid present in the stomach, and reduces its strength by neutralizing its effect.

Self defence by animals and plants through chemical warfare:
Baking soda is also used to neutralize the effect of a bee sting. A bee sting leaves methanoic acid inside our body, which causes pain and irritation. Thus, baking soda neutralizes the effect of methanoic acid.

Plants also require a specific value of pH for healthy growth. For example, there are a few plants that require a pH of 4.5 to 5.5. Such plants are called acid loving plants. Some evergreen shrubs and bushes are acid loving plants.

Plants are sensitive to pH changes:
Soil pH and plant growth:
pH of soil,which is required for healthy growth of plants can be easily measured. Do you know how?

To measure the pH of soil, take 2g of soil to be measured in a test tube. Add 5 mL water to it and shake the contents for some time. Now, filter the contents of the test tube and collect the filtrate in another test tube. You can measure the pH of the filtrate using a universal indicator.

Common salt is an important dietary substance and is essential for life. Common salt is chemically known as sodium chloride. An important method for obtaining common salt is by the evaporation of sea water. Seawater contains many salts dissolved in it. Sodium chloride is separated from this mixture of salts.

Salts can also be prepared by the process of neutralization reactions. For example, sodium chloride (NaCl) can be prepared by reacting hydrochloric acid (HCl) and sodium hydroxide (NaOH).

Potassium sulphate, sodium sulphate, calcium sulphate, magnesium sulphate, copper sulphate, sodium chloride, sodium nitrate, sodium carbonate, and ammonium chloride are other examples of salts.

Do you know which acids and bases are used to obtain these salts? First, let us try to write the chemical formulae of each of these salts. The given table shows the chemical formula of each salt with their acidic and basic components.

Salts having the same positive or negative radicals are said to be belonging to the same family. Some families are discussed below:

Family I (Sulphate salts):

This family contains group as a common group. Potassium sulphate, sodium sulphate, calcium sulphate, magnesium sulphate, and copper sulphate are grouped in this family.

Family II (Chloride salts):

This family contains Chloride group as the common group. Sodium chloride and ammonium chloride are grouped in this family.

Family III (Sodium salts):

This family contains Na⁺ group as a common group. Sodium sulphate, sodium chloride, and sodium nitrate are grouped in this family.

Do you know salts can be acidic, basic, or neutral in nature? The salts of strong acids and strong bases are neutral with a pH value of 7. On the other hand, salts of strong acids and weak bases are acidic with a pH value of less than 7, and those of strong bases and weak acids are basic in nature with a pH value of more than 7.

Common salt is a neutral salt. It is formed by the combination of a strong acid (hydrochloric acid) and a strong base (sodium hydroxide). The following activity can be performed to know more about acidic, basic, and neutral salts.

Activity 1:

A small amount of sodium chloride, potassium nitrate, aluminium chloride, zinc sulphate, copper sulphate, sodium acetate, sodium carbonate, and sodium hydrogen carbonate are collected. A few crystals of each salt are separately dissolved in 2 mL distilled water. Then, the pH of each sample solution is checked by using a pH paper.

The given table lists the observations obtained in the activity.

Hence, it can be concluded that sodium chloride and potassium nitrate are neutral salts. On the other hand, aluminium chloride, zinc sulphate, and copper sulphate are acidic salts, while sodium acetate, sodium carbonate, and sodium hydrogen carbonate are basic salts.

Do you know that common salt is an important raw material for various materials of daily use? For example, common salt is used in the formation of sodium hydroxide, baking soda, bleaching powder, and washing soda.

Do You Know:

Sodium chloride was an important symbol in our struggle for freedom. The Salt Satyagrah or Mahatma Gandhi’s Dandi March was a non-violent protest against the British salt tax.

Sodium hydroxide:

It is produced from an aqueous solution of sodium chloride (called Brine). Electrolysis (passing of electricity through a solution) of brine  results in the decomposition of sodium chloride and formation of sodium hydroxide. This process is also called chlor-alkali process (chlor for chlorine and alkali for NaOH). The reaction for the above process can be written as follows:

Hydrogen and chlorine gas are released at the cathode and anode respectively.

Sodium hydroxide is a very useful substance. It is used in the manufacturing of paper, soaps, detergents, and synthetic fibres. It is also useful in the manufacturing of household bleaches and dyes.

Bleaching powder:

Chlorine gas obtained from mixing a hypochlorite(HClO) bleach with an acid(HCl), in the electrolysis it is used for the manufacture of bleaching powder. Chlorine gas on reacting with dry slaked lime [Ca(OH)₂]

 produces bleaching powder (CaOCl₂). The chemical equation involved in the reaction can be represented as:

Bleaching powder is used for disinfecting water to make it germ-free. It is used in the textile industry to produce cotton and linen materials. It is also used as an oxidizing agent in many chemical industries.

Baking soda:

We know that baking soda is most commonly used in the kitchen for making delicious pakoras. The chemical name of baking soda is sodium hydrogen carbonate (NaHCO₃). Baking soda is produced by using sodium chloride as one of the raw materials. The chemical equation can be represented as follows:

Do You Know:

Sodium hydrogen carbonate is used as an ingredient in antacids. It is alkaline in nature. Hence, it neutralizes excess acid in the stomach and provides relief.

Sodium hydrogen carbonate is also used in soda acid fire extinguishers. It is also used for cooking and sometimes, it is added for faster cooking. The following reaction takes place when it is heated during cooking:

Washing soda:

Washing soda is also produced from common salt. The chemical name of washing soda is hydrated sodium carbonate and its chemical formula is Na₂CO₃.10H₂O. It is obtained by heating sodium hydrogencarbonate.

The re-crystallization of sodium carbonate produces washing soda.

Sodium carbonate is used in the manufacturing of glass, soap, and paper. It is also used for the manufacturing of borax, for removing permanent hardness of water.

Calcium carbonate:

Calcium carbonate is formed when calcium hydroxide reacts with carbon dioxide.

Ca(OH)2+CO2→CaCO3+H2OCalciumCarbonCalciumhydroxidedioxidecarbonate$\mathrm{Ca}(\mathrm{OH}{)}_2+{\mathrm{CO}}_2\to {\mathrm{CaCO}}_3+{\mathrm{H}}_2\mathrm{O}$

It is used in the manufacture of cement and glasses. It is also used in the extraction of iron from its ore.

We know that washing soda is produced by mixing water and sodium carbonate. The molecular formula of washing soda is Na₂CO₃.10H₂O.

Sodium carbonate is obtained by heating sodium hydrogencarbonate.

The re-crystallization of sodium carbonate then produces washing soda.

Ten water molecules are present in the formula of washing soda. These water molecules are called water of crystallization.

Water of crystallization refers to a fixed number of water molecules present in one formula unit of salt.

The following experiment will help in understanding the concept of water of crystallization.

Aim: To prepare crystals of copper sulphate (CuSO₄)

Material required: Beaker, distilled water, copper sulphate (CuSO₄​) crystals, glass rod, thread, watch glass

Theory: Water of crystallization imparts the characteristic blue colour to copper sulphate (CuSO₄​) crystals. These crystals are obtained by the process of seeding. In this process crystallization is induced with the help of small crystal of pure hydrated copper sulphate (CuSO₄.5H₂O​) which is added in the saturated copper sulphate solution.

Procedure:

1. Take a beaker and prepare a saturated solution of copper sulphate (CuSO₄​) at 80^o C and filter the solution to remove any undissolved impurity.
2. Cover the filtrate with watch glass.
3. Cool down the filtrate and leave it undisturbed for 24 hours.
4. Some crystals of copper sulphate (CuSO₄.5H₂O​) will be formed at the bottom of the beaker. Collect a few of them.
5. Suspend one of the well formed small crystal in the saturated solution by tying to a glass rod using a thread.
6. Again cover the beaker with watch glass to avoid dust entering the solution.
7. Leave it undisturbed.

Observation: The suspended crystal grows in size with each passing day.
 

Water of crystallization is the fixed number of water molecules present in one formula unit of salt. It is in chemical combination with a crystal It is necessary for the maintenance of crystalline properties of the crystal It can be removed by sufficient heat

The following experiment shows the effect of heat on solids that do not contain water of crystallisation.

Aim: To show effect of heat on solids that do not contain water of crystallisation.

Material required: Test tube, burner, potassium nitrate (KNO₃)

Theory: Not all crystalline solids contain water of crystallisation. These solids when heated decompose to form new compounds.

Procedure:

1. Take some potassium nitrate crystals in a test tube.
2. Heat the tube gently.

Observation: The crystals form a colourless solution giving off a gas, that bursts the glowing splinter. It signifies that oxygen is being involved. In the end, a pale yellow residue is left in the test tube. 

2KNO3Potassiumnitrate→Δ2KNO2Potassiumnitrite+O2$\underset{}{2{\mathrm{KNO}}_3}$

Hydrated Substances 

Those substances which contain water of crystallization like hydrated copper sulphate (CuSO₄.5H₂O), are called hydrated substances. The water of crystallisation gives their crystals shape and in some cases colour.
 

Gypsum is another salt that possesses the water of crystallization. It has a chemical formula of CaSO₄.2H₂O. It is also known as hydrated calcium sulphate.

When hydrated calcium sulphate (CaSO₄.2H₂O) or gypsum is heated at 373K, it loses its water molecules and forms calcium sulphate hemihydrate. This hemihydrate form of calcium sulphate is known as Plaster of Paris. It is in the form of a white powder.

When the powder of Plaster of Paris is mixed with water, it becomes hard and solid gypsum. Plaster of Paris is generally used to support fractured bones in their correct positions.

In , only half a water molecule is shown as the water of crystallization because two formula unit of CaSO₄ share one molecule of water.

Plaster of Paris is used for making toys, materials for decoration, and for making smooth surfaces.

Determination of Water of Crystallization 

Heat a known weight of a hydrated substance to a temperature above 100^o C. Weigh the residue. Repeat these two steps, till the weight of the residue becomes constant.
Use the following formula to obtain the percentage of water of crystallization in a link:

Initial weight of the hydrated substance = x g
Final constant weight of the substance after heating = y g
%ofwaterofcrystallization=x−yx×100$\%\mathrm{of }\mathrm{water }\mathrm{of }\mathrm{crystallization}=\frac{x- y}{\mathrm{x }}\times 100$

Anhydrous Substances 

Those substances which do not contain any water of crystallization or the substances from which the water of crystallization have been removed like sodium chloride (NaCl) are called anhydrous substances.
The water of crystallization can be removed by using any of the following methods:

• Direct heating of the hydrated substance
• Heating the hydrated substance in dry and hot air
• Heating the hydrated substance under vacuum
• Using dehydrating/desiccating agents

Drying Agents 

The substances that absorb moisture from other substances without undergoing a chemical reaction with them are called drying agents or desiccants or desiccating agents. Examples of drying agents are anhydrous calcium chloride, anhydrous zinc chloride etc. Most of the hygroscopic substances are desiccating agents like concentrated sulphuric acid, silica gel etc.

The following table illustrates the techniques used to dry certain substances.
 

Substance	Drying technique
Gases	By passing through concentrated sulphuric acid Used for drying acidic gases like HCl gas By passing through a drying tower or a U-tube containing anhydrous sodium sulphate By passing through a drying bulb containing anhydrous calcium chloride
Liquids	By keeping them over anhydrous sodium sulphate or calcium chloride for over a night After this, solid is removed by filtration.
Solids	By placing them in a desiccator (air-tight vessel with a drying agent like calcium chloride spread at the bottom)

Dehydrating Agents

The substances that can remove chemically bounded water from compounds are called dehydrating agents. Concentrated sulphuric acid is a strong dehydrating agent. It can remove water molecules from hydrated copper sulphate ​(CuSO₄.5H₂O). 

CuSO₄.5H₂O−→−−−−−Conc.H2SO4$\stackrel{Conc.H_{2}S{O}_4}{\to }$ CuSO₄ +  5H₂O
^{(blue)                              (white)}
The following table explains the differences between drying agents and dehydrating agents.
 

Drying Agents	Dehydrating Agents
Removes moisture from other substances	Removes chemically bounded water molecules from substances
Performs a physical change in the substance	Performs a chemical change in the substance

​You must have observed that in a laboratory, almost all acids are stored in a glass or plastic bottle. However, acids are never kept in metal containers. Do you know why?

This is because acids are corrosive in nature. They can react with metals and destroy them.

Do you know what happens when metals react with acids?

When an acid reacts with metals, salt and hydrogen gas are produced. Thus, metals react with acids and replace the available hydrogen(s) to form salt. This can be represented as follows:

For example, sulphuric acid (H₂SO₄) reacts with calcium (Ca) to produce hydrogen gas (H₂). Calcium sulphate (metal salt) is also produced in the reaction. The chemical equation involved is as follows:

DO YOU KNOW?

When acid-containing food items such as pickles, curd, and citrus fruits are kept in metallic utensils, a reaction between the metal and the acid (present in the food item) takes place. Iron, aluminium, and copper are more prone to acid attack. Sometimes, these reactions result in the formation of toxic substances.

The following activity can be performed to confirm that hydrogen gas is produced when metals react with acids.

Now, let us see what happens when metals react with bases.

Metals react with bases and replace the available hydrogen(s) to form salt as follows: For example, metal (Zn) displaces hydrogen from the base (NaOH) and produces hydrogen gas. The reaction between sodium hydroxide and zinc can be represented as:

The following activity can be performed to confirm that hydrogen gas is produced on reacting metals with bases.

Activity:

Place a few pieces of zinc granules in a test tube. Then, add 2 mL sodium hydroxide (NaOH) in it and heat the solution. A reaction between bases (such as NaOH) and metals (such as Zn) is initiated on heating.

Figure 2: Experiment showing the reaction of bases with metals

We will observe that bubbles are formed in the test tube as soon as dilute sodium hydroxide solution is added to the zinc granules. This proves that gas is evolved in the reaction. The gas evolved in the test tube is then passed to the soap solution through the delivery tube. Soap bubbles filled with hydrogen gas will be seen in the soap solution. The presence of hydrogen gas is confirmed by its burning with a ‘pop’ sound when a candle is brought near the bubbles.

Hence, hydrogen gas is formed and evolved in the reaction.

Now, let us study some other chemical properties of acids and bases. Do you know how metallic oxides react with acids and non–metallic oxides react with bases?

Acids react with metal oxides such as copper oxide, zinc oxide, etc. to produce metal salts. The reaction between a metal oxide and an acid can be written as:

DO YOU KNOW?

Metal oxides:

Many metals react with oxygen to produce metal oxides. For example, sodium (Na) reacts with oxygen (O₂) to produce sodium oxide (Na₂O). The chemical equation for the same is as follows:

2Na+O2→Na2O2$2\mathrm{Na}+{\mathrm{O}}_2\to {\mathrm{Na}}_2{\mathrm{O}}_2$

Metal oxides on reacting with strong acids produce salt and water (similar to the reaction of bases with acids). Hence, metal oxides are also known as basic oxides.

The following activity shows the reaction between an acid and a metallic oxide.

Activity:

Take 50 mg copper oxide in a beaker and slowly pour dilute hydrochloric acid on it, while stirring constantly. Observe the colour of the solution.

 Reaction of metallic oxides with acids.

HCl reacts with copper oxide (CuO) to produce a metal salt i.e., copper chloride and water. It will be observed that copper chloride dissolves in water and forms a solution that is blue-green in colour.

Hence, this shows that acids react with metal oxides to produce salt and water.

Similarly, non-metallic oxides react with bases to produce salt and water. The reaction between non-metallic oxides and bases is represented as follows:

DO YOU KNOW?

Non-metal oxides:

Non-metals react with oxygen to produce non-metallic oxides. For example, sulphur (S) reacts with oxygen (O₂) to produce sulphur dioxide (SO₂) as follows:

Non-metallic oxides react with strong bases to produce salt and water (similar to the reaction of acids with bases). Hence, non-metallic oxides are also known as acidic oxides.

For example, a reaction between a non-metallic oxide such as carbon dioxide and a base such as calcium hydroxide produces salt and water. This reaction can be written as:

Hence, non-metallic oxides react with bases to produce salt and water.

You have already read about acids and bases. Now, let us learn more about them by exploring their chemical properties.

You must have observed that when baking soda (sodium hydrogen carbonate) is treated with vinegar (acetic acid), a brisk effervescence is observed. Similarly, when chalk (calcium carbonate) is treated with sulphuric acid, effervescence is observed. Do you know what happens in these reactions? Which gas is evolved during these reactions?

All metal carbonates and hydrogen carbonates react with acids to give a corresponding salt, carbon dioxide, and water. Thus, the reaction can be summarized as:

For example, zinc carbonate on reacting with sulphuric acid produces zinc sulphate, water, and carbon dioxide as follows:

DO YOU KNOW?

Acid rain contains mainly nitric acid and sulphuric acid, which are formed by the dissolution of nitrogen oxides and sulphur dioxide in air. Acid rain corrodes buildings and statues made with marble and stone. Marble and stone generally contain carbonates of metals. Hence, they react with acid rain. The Taj Mahal (situated in Agra), which is a heritage build

Similarly, zinc hydrogen carbonate on reacting with sulphuric acid produces zinc sulphate, water, and carbon dioxide as follows 
  
Zn(HCO3)2(s)+H2SO4(aq)→ZnSO4(aq)+2CO2(g)+2H2O(l)Zinc hydrogen   carbonate Sulphuric   acid Zinc sulphate   Carbon dioxide Water

The following activity can be performed to confirm that carbon dioxide gas is produced when acids react with metal carbonates.

The reaction of the hydrogen carbonates of metals with acid can be tested in a similar manner.

The reaction for the same is

Carbon dioxide reacts with calcium hydroxide (lime water) to form calcium carbonate (white precipitate). The reaction of carbon dioxide gas with limewater is as follows:

Sodium chloride salt produced in the reaction dissolves in water.

Hence, this activity shows that acids react with metal carbonates and metal hydrogen carbonates to produce salts, carbon dioxide gas, and water.

Metal carbonates and metal hydrogen carbonates do not react with bases.

You must have observed that antacid tablets are used during indigestion. An antacid contains magnesium hydroxide, which is a mild base. Magnesium hydroxide neutralizes the effect of excess hydrochloric acid produced in the stomach during indigestion. Hence, it helps relieve the pain. Do you know what actually happens when acids and bases react with each other?

When acids are mixed with bases, they neutralize or cancel each other’s effect. The products formed on mixing acids with bases are salt and water.

The process of treating an acid with a base to form salt and water is called neutralization. The general reaction for neutralization can be written as follows:

A lot of heat is produced during the reaction. Hence, it is an exothermic process. Thus, we have

+ Heat

Do you know that all acids generate hydrogen (H⁺) and all bases generate hydroxyl (OH^-) ions in their aqueous solutions?

H⁺ ions cannot exist independently; rather they combine with water molecules (H₂O) to form hydronium ions (H₃O⁺).

Thus, in an aqueous solution of acids:

Similarly, bases also dissociate in aqueous solutions. However, not all bases dissolve in water. The bases that dissolve in water are known as alkalis. Thus, it can be said that ‘all bases are not alkalis, but all alkalis are bases’.

Alkalis dissolve in water and produce  OH- ions.

These hydroxide ions (OH- ions) can exist freely in water or in an aqueous solution.

These hydrogen (H⁺) and hydroxyl (OH^-) ions react with each other in neutralization reactions to form water.

Hence, neutralization reactions, in terms of hydrogen and hydroxide ions can be represented as:

or

DO YOU KNOW?

A bee sting leaves methanoic acid on the skin, which causes burning, pain, and irritation. Methanoic acid is a weak acid. If a mild base such as sodium hydrogen carbonate or baking soda is applied to the stung area, then it gives relief. This is because baking soda neutralizes the effect of methanoic acid.

Let us perform the following activity to understand how acids and bases react with each other.

This activity shows that an acid nullifies the effect of the base in the solution and vice-versa. Acids react with bases to produce salt and water. The chemical equation involved in the given activity can be represented as:

Adding water to acidic or alkaline solutions decreases the concentration of H+ and OH-  ions per unit volume. This process of decreasing the concentration of H+ and ions per unit volume. This process of decreasing the concentration of H+ and   ions per unit volume is known as dilution.     The process of dissolving an acid or a base in water is an exothermic process. If water is added directly to the concentrated acid, then the heat generated may cause burning. Hence, acids should always be added slowly to water with constant stirring.

Characteristics Features of Metal
 

1. Alkali Metals

• Group 1 elements: lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr)
• Called alkali metals because their hydroxides are alkaline in nature
• Hydrogen is placed in this group but it is not an alkali metal

Occurance

• Do not occur in free state due to high reactivity

Atomic and Ionic Radii

• Increase with increase in atomic number (i.e., on moving down the group)

Bonding

• All metal salts other than some salts of Li are ionic

Physical Properties

• Silvery white, soft, and light metals

• Density increases down the group. (exception: Na is denser than K)

• Low melting and boiling points − Reason: weak metallic bond due to the presence of only one electron in the outermost shell

• They and their salts impart characteristic colour to an oxidising flame. Reason: after getting heat from the flame, the valence electrons get excited to higher energy level. Then they come back to ground state by emitting radiation in the visible region.
  • ​Crimson red − Lithium (Li)
  • Golden yellow − Sodium (Na)
  • Pale violet − Potassium (K)

Chemical Properties

• Highly reactive

• Reactivity increases down the group

• Reaction with air:

  • Tarnish in dry air
  • Reason: formation of oxides and then hydroxides with moisture

• Kept in kerosene oil − Reason: highly reactive towards air and water

• Reaction with water is voilent with release of hydrogen gas     

(M = alkali metal)

• Reaction with dihydrogen:

• Alkali metal hydrides are ionic solids with high melting point.
• ​Reaction with acids: React voilently with dilute acids​

2M+2HCl→2MCl+H2$2\mathrm{M}+2\mathrm{HCl}\stackrel{}{\to }2\mathrm{MCl}+{\mathrm{H}}_2$

• Reaction with halogen:

  • Reacts vigorously to form halides which are ionic in nature (exception: LiX is some what covalent − Reason: High polarizing capability of Li⁺ ion)
  • LiI is the most covalent.

• Reducing nature

  • Strong reducing agent
  • Form unipositive ions easily, M(g)→M+(g)+e−$M\left(\mathrm{g}\right)\stackrel{}{\to }\mathit{}{M}^{+}\left(\mathrm{g}\right)+{\mathrm{e}}^{-}$
  • Li is the strongest and Na is the weakest.
• Ionisation energy:  Low
• Show silver white mattalic luster on fresh cutting but soon turn dull

Uses

• In making useful alloys. For example, it is used with lead in making ‘white metal’ bearings for motor engines

• Li is used in making electrochemical cells.

• Na is used in making Na/Pb alloy, which are used to make organolead compounds − PbEt₄ and PbMe₄. These compounds were earlier used as anti-knock additives to petrol.

• Liquid Na is used as a coolant in fast breeder nuclear reactors.

• K plays an important role in biological systems

• KCl − used as a fertilizer

• KOH − in the manufacture of soft soap and as an absorbent of CO₂
• Cs is used in devising photoelectric cells.

2. Alkali Earth Metals

• Group 2 elements: ­­ Beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba) and radium (Ra).

• They are called alkaline earth metals because their oxides and hydroxides are alkaline in nature, and their oxides are found in earth’s crust.

Occurance

• Do not occur in free state due to high reactivity (lesser than alkali metals)

Atomic and ionic radii

• Smaller than the corresponding alkali metals

Reason: Greater nuclear charge

• Increase with the increase in atomic number (i.e., on moving down the group)

Ionisation Energy

• Low

• Decreases on moving down the group

Physical Properties

• Silvery white, lustrous and soft (harder than alkali metals) (Exception − Beryllium and Magnesium are greyish)

• Melting and boiling points of these metals are higher than those of the corresponding alkali metals.

Reason: Smaller size than alkali metals

• Strongly electropositive in nature. Reason: Low ionisation enthalpies

• Flame test:

  • Calcium − Brick red
  • Strontium − Crimson
  • Barium − Apple green
  • Mg and Be do not impart colour because the electrons are too strongly bound to be excited.

• Good conductors of heat and electricity

Chemical Properties

• Less reactive than alkali metals

• Reactivity increases down the group

• Reaction with air and water:

  • Be and Mg are kinetically inert to oxygen and water. Reason − formation of oxide film on their surface
  • However, when Mg and powdered Be burn, they give MgO and Mg₃N₂, and BeO and Be₃N₂ respectively.
  • Ca, Sr and Ba react readily with air to form oxides and nitrides; also react with water (even cold), to form hydroxides

• Reaction with halogens:

  • At high temperature

M + X₂ → MX₂ (Halides)

(X = F, Cl, Br, I)

• BeF₂ is prepared by the thermal decomposition of (NH₄)₂BeF₄
• BeCl₂ is prepared from BeO by heating it with carbon and chlorine to about 600-900 K

BeO + C + Cl₂  BeCl₂ + CO

• Reaction with hydrogen:

  • React with hydrogen to form hydrides (Exception − Beryllium)
  • Preparation of BeH₂ from BeCl₂ and LiAlH₄ −

2BeCl₂ + LiAlH₄ → 2BeH₂ + LiCl + AlCl₃

• Reaction with acids:

M + 2HCl → MCl₂ + H₂

(M = Alkali earth metal)

• Reducing nature:

  • Strong reducing agent (weaker than corresponding alkali metals)
  • Be is a weaker reducing agent than other alkaline earth metals.

Uses

• Beryllium:

  • For making alloys such as Cu−Be alloys which are used for making high strength springs
  • Metallic Be, for making windows of X-ray tubes

• Magnesium:

  • For making alloys with Al, Zn, Mn and Sn
  • Mg−Al alloys are used in the construction of aircraft
  • Magnesium is used in flash powders and bulbs, incendiary bombs and signals
  • Milk of magnesia (suspension of (MgOH)₂ in water) is used as an antacid
  • MgCO₃ is a component in toothpaste

• Calcium:

  • In the process of extraction of metals from oxides which are difficult to reduce with carbon
  • For removing air from vacuum tubes. Reason − reacts with oxygen and nitrogen at high temperature

• Barium:

  • For removing air from vacuum tubes. Reason − reacts with oxygen and nitrogen at high temperature

• Radium:

  • In radiotherapy (e.g., in the treatment of cancer)

 

3. Transition Metals

• Elements of group 3 to group 12
• Called transition elements
• Zn, Cd, and Hg do not show properties of transition elements.
• All are metals.
• They form coloured ions, show attraction towards magnet, exhibit variable valencies, and are used as catalysts.

4. Inner-Transition Metals

• Lanthanoids → Ce (Z = 58) to Lu (Z = 71)
• Actinoids → Th (Z = 90) to Lr (Z = 103)
• They are called inner-transition elements.
• All are metals showing variable valencies
• Actinoid elements are radioactive.
• Elements after uranium are called Transuranium elements.

5. Metals, Non-metals, and Metalloids

• Metals → Appear on the left side of the periodic table

• Non-metals → Located at the top right hand side of the periodic table

• Elements change from metallic to non-metallic from left to right.

• Elements such as Si, Ge, As, Sb, Te show the characteristic properties of both metals and non-metals. They are called semi-metals or metalloids.