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Class 8th-Science: Chapter 4- Materials Metals and Non-Metals

Definition of Metals & Non-Metals:

Metals:

The materials which are hard, lustrous, malleable, ductile, sonorous and good conductors of heat and electricity are called metals.

For Example: iron, copper, aluminium, gold, silver, platinum, calcium, magnesium, etc.

Non-Metals:

The materials which are soft, dull in appearance, break down into powdery mass on tapping with hammer, not sonorous and poor conductors of heat and electricity are called non-metals.

For Example: sulphur, carbon, oxygen,

Examples of metals & Non-Metals

We see and use many materials in daily life which are made up of metals or non-metals.

MetalsIron, Aluminium, Gold, Silver, Copper, Platinum

Non-MetalsSulphur, Chlorine, Coal, Graphite

Q.  But how can we differentiate metals and non-metals?

Ans.  We can differentiate metals and non-metals on the basis of their physical and chemical properties.

Physical & Chemical Properties

Physical Properties:

 A physical property is a characteristic of a substance that can be observed or measured without changing its chemical identity or composition.

For Example: color, taste, mass, volume, density, temperature, melting point, boiling point, hardness, lustre, malleability, ductility, sonorous, conduction of heat, conduction of electricity.

Physical Properties of Metals:

1. Generally, metalsare hard

For Example: iron, copper, aluminium, silver, gold, etc. are hard.

Hardness:

Hardness of a substance refers to its resistance to bending, scratching, abrasion or cutting.

2. Metals are Lustrous.

For Example: Iron, copper, aluminium, silver, gold, etc. are lustrous.

Luster:

Luster is the property of surface of a substance to reflect light due to which the substance appears shiny.

3. Generally, metals are Malleable.

For example: Silver Foil on Sweets, Aluminium Foil, Gold Foil (gold is the most malleable metals).

Malleability:

The property of metals by which they can be beaten into thin sheets is called malleability.

4. Metals are generally Ductie.

For Example: Copper wires, Silver wires Aluminium wires, Gold wires (gold is the most ductile metals).

5. Metals are Sonorous.

Sonorous:

The things made of metals produce ringing sound when struck hard. this property of metal is called sonority and metals are called sonorous.

6. Metals are generally good conductors of heat

Q. Can you hold a hot metallic pan which is without a plastic or a wooden handle and not get hurt? Perhaps not! Why?

Ans. Yes, I will get hurt (with burn) if i hold a hot metallic pan which is without a plastic or a wooden handle.

Plastic and wood are non-conductor of heat. But the metallic pan is the conductor of heat. Without the handle, heat from hot pan will get conducted up to the handle, which in turn will become hot. And that can burn hand of pan holder.

7. Metals are good conductors of electricity

Q. How to check whether electricity can pass through an object or not?

Ans. By making an electric circuit, we can test whether electricity can pass through an object or not.

You can check electrical conductivity of various objects using this method like aluminium rod, gold ring, coal, sulphur etc.

Silver is the best conductor of electricity.

Physical Properties of Non-Metals:

1. Non-metals are generally soft. They are not hard.

For Example: Sulphur, coal, graphite etc. are soft.

2. Non-metals are mostly dull in appearance. They are non-lustrous.

For Example: Sulphur, coal etc. are dull in appearnce.

3. Non-metals are not malleable. They are brittle as they break down into powdery mass on tapping with hammer.

4. Non-metals are not ductile.

5. Non-metals are not sonorous.

6. Non-metals are generally poor conductors of heat.

7. Non-metals are generally poor conductors of electricity.

Chemical Properties:

A chemical property is a characteriatic or behavior of a substance that cn be observed when it undergoes a chemical change or reaction.

For Example: ability to rust, reactivity ect.

1. Reaction with Oxygen

metals

Metal + Oxygen —> Metal Oxide

Magnesium + Oxygen —> Magnesium Oxide

2Mg + Oxygen —> 2MgO

Iron + Oxygen —> Iron Oxide

2Fe + O2 —> 2FeO

Copper + Oxygen —> Copper Oxide

2Cu + O2 —> 2CuO

Q. Recall the reaction by which rust is formed.

Ans. Rusting takes place on iron when iron comes in contact with oxygen in presence of water vapour in the atmosphere.

Iron + Oxygen + water —> Hydrated Iron(III) Oxide (Rust)

4Fe + 3O2 + H2O —> 2Fe2O3H2O (Rust)

Q. What do you observe? Is the solution acidic or basic?

Ans. The red litmus turns blue from red in this solution which confirms that the solution is basic in nature.

=> Oxide of iron is basic in nature.

Q. Now recall the activity of burning magnesium ribbon. The ash obtained on burning magnesium ribbon is dissolved in water and tested for its acidic / basic nature. Is the solution acidic or basic? How do you ascertain this?

Ans. Magnesium + Oxygen —> Magnesium Oxide ( 2Mg + O2 —> 2MgO )

The solution is basic. This is ascertained by pitting red litmus paper in the solution, which turns blue which is the test of basic nature of the solution.

=> Oxide of magnesium is also basic in nature.

In general, metallic oxides like Fe2O3, MgO, CuO etc. are basic in nature.

Q. Does copper also get rusted? I have seen a greenish deposit on the surface of copper vessels.

Ans. Yes copper also get rusted. When a copper vessel is exposed to moist air for long, it acquires a dull green coating. The green material is a mixture of copper hydroxide (Cu(OH)2) and copper carbonate (CuCO3). The following is the reaction 2Cu+H2O+CO2+O2 —> Cu (OH)2 + CuCO3

Non-metals

Non-metal + Oxygen —> Non-metal Oxide

Sulphur + Oxygen —> Sulphur Dioxide

S + O2 —> SO2

Carbon + Oxygen —> Carbon Dioxide

C + O2 —> CO2

Q(a). Specify what is formed as an end product

(i) when sulphur is heated in air?

(ii) when gas obtained from burning sulphur is dissolved in water?

Ans. (i) When sulphur is heated in air, sulphur dioxide is formed.

S + O2 —> OS2

(ii) When gas obtained from burning sulphur is dissolved in water, sulphurous acid is formed.

SO2 + H2O —> H2SO3

Q(b). What does litmus test reveals, when tested for this aqeous solution?

Ans. Blue litmus turns red in the aqeous solution, signifying acidic nature of sulphurous acid.

Q. Write the name of some laboratory acids and bases in Table 4.4. Identify the metal or non-metal present in them which forms oxides with oxygen.

Table 4.4 : Metals and Non-metals in Acids and Bases

2. Reaction with Water

Metals

Metal + Water —> Metal Oxide + Hydrogen Gas

Metal Oxide + Water —> Metal Hydroxide

Different metal reacts with water at rates. For example, sodium reacts vigorously with water, but other metals like iron reacts with water slowly.

Sodium metal is very reactive. It reacts vigorously with oxygen and water. A lot of heat is generated in the reaction. It is, therefore, stored in kerosene.

Q. When sodium is made to react with water, what happens to the beaker in which the reaction takes place. Also verify that the solution is acidic or basic?

Ans. The beaker becomes very hot because sodium reacts vigorously with water.

The solution thus obtained is basic in nature, as it turns red litmus blue.

Non-metals

Generally, non-metals do not react with water though they may be very reactive in air. Such non-metals are stored in water.

Q. Why is phosphorous stored in water?

Ans. Phosphorus is a very reactive non-metal. It catches fire if exposed to air. To prevent the contact of phosphorus with atmospheric oxygen, it is stored in water.

3. Reactions with Acids

Metal + Acid —> Metal Salt + Hydrogen Gas

Magnesium + Hydrochloric Acid —> Magnesium Chloride + Hydrogen Gas

Mg + HCl —> MgCl2 + H2

Metals react with acids and produce hydrogen gas that burns with a ‘pop’ sound but non-metals generally do not react with acids.

Q1. Is there a difference in the way metals and non-metals react with acids?

Ans. Yes, non-metals generally do not react with acids but metals react with acids and produce hydrogen gas that burns with a ‘pop’ sound.

Q2. What does the ‘pop’ sound indicate when a burning match stick is brought near the mouth of the test tube?

Ans. ‘Pop’ sound is produced when a burning matchstick is brought near the mouth of the test tube indicating the presence of hydrogen gas. It is the hydrogen gas that burns with a pop sound.

Q3. Name the metal which does not react with dilute hydrochloric acid even on heating but it reacts with sulphuric acid.

Ans. Copper does not react with dilute hydrochloric acid even on heating but it reacts with sulphuric acid.

4. Reactions with Bases

Some metals react with bases like (sodium hydroxide) and produce hydrogen gas that burns with a ‘pop’ sound. Reactions of non-metals with bases are complex.

5. Displacement Reactions

Displacement reaction is a chemical reaction in which a more reactive element displaces/ replaces/ removes a less reactive element from its compound.

For Example: Iron nails in copper sulphate solution.

Copper Sulphate + Iron —> Ferrous Sulphate + Copper

CuSO4 + Fe —> FeSO4 + Cu

Iron can displace copper from its compound

Q. Can copper displace iron from its compound? Can this reaction FeSO4 + Cu —> CuSO4 + Fe take place?

Ans. No, copper cannot displace iron from its compound because copper is less reactive than iron. Only more reactive metal can displace a less reactive metal from its compound. So the above reaction cannot take place.

Q. What is reactivity?

Ans. Reactivity is a measure of how readily a substance undergoes a chemical reaction. The reaction can involve the substance on its own or with other atoms or compounds, generally accompanied by a release of energy.

Q. What is the sequence of metals from more reactive to less reaction among zinc, iron and copper?

Ans. Zn > Fe > Cu

Q. What is the basic rule of displacement in a chemical reaction?

Ans. The rule is:

A more reactive metal can replace a less reactive metal, but a less reactive one cannot replace a more reactive metal.

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