Rajasthan Board RBSE Class8 Science Notes Chapter 2 Metals and Non-Metals
Metals and Non-Metals in Nature:
We see many things around us in our daily life such as wooden chair, coal, aluminium sheet, copper utensils, soil etc. All these things are made up of different elements. Some objects are hard, bright and solid while some other objects are bright less, soft and very soft, those element which are lustrous are generally metals while objects which are non lustrous are non metals. In the earth’s crust and above earth some element are found free while most of the metals and non metals are found in combined state such as – aluminium, iron, manganese, oxygen and phosphorous are found in combined state.
Physical Properties of Metals and Non-Metals:
Physical Properties of Metals:
- State: Most of the metals are found in solid at normal temperature except mercury (Hg) which is found’in liquid state at normal temperature.
- Colour: Mostly metals are golden and black in colour.
- lustrous: Most of the metals are lustrous hence they are capable of reflecting rays of light from their upper surface therefore, they are metallic lustrous such as gold, silver, copper, aluminium, are specially lustrous.
- Hardness: Most of the metals are hard. Hardness of different metals are different. Sodium and potassium are less hard compared to other metals.
- Sonorous: When metals are struck with other hard metals or objects, then a special sound is produced this property is called sonorous due to this property of metals they are used to manufacture bell and music instruments etc.
- Density: Generally metals have high density. Things made up of metals are heavy so when they are put in water they sink while those have less density they float in water such as sodium (Na) and potassium (K). etc.
- Melting point: That temperature at which solid changes into liquid state is called melting point of that object. Due to hardness of solid their melting point is high such as iron (Fe). The melting point of iron is 1593° C while galium is an exception it melts just by putting it on hand palm as its melting point is very low.
- Heat conductivity: When a rod of steel is heated after sometimes it becomes hot while a wooden rod when submerged in hot water does not get heated. Hence we can say that metals are good conductor of heat. Silver (Ag) is the best conductor while lead (Pb) is the least conductor. This is the reason that cooking utensils are made of metals and their handies are made of non-conductors like wood, plastic etc.
- Maleability: Metals spread on beating hence by the force of hammers they are converted into sheets.
- Ductility: When they are stretched convert into wires this property of metals is called ductility.
- Electricity Conductivity: Metals allow electricity to pass through them, this property is called conductivity of electricity and metals are called good conductor of electricity. Silver is the best conductor of electricity.Hence those objects which are hard, lustrous, sonorous, ductile, maleable, and good conductor of heat and electricity are called metals.
Physical Properties of Non-Metals:
- Physical state: They are found in three states i.e. solid, liquid, and gas at normal temperature.
such as – solid- carbon (C), sulphur (S), iodine (I); liquid – bromine (Br); gas- oxygen (02), nitrogen (N2), hydrogen (H2).
- Colour: Non-metals have variety of colours such as sulphur (yellow), chlorine gas (greenish yellow), phosphorous (red, white).
- Lustor: Non-metals are non lustrous they do not reflect light. Diamond and iodine are exceptions as these are lustrous.
- Hardness: They are soft. Diamond is exception it is the hardest non-metal found on earth. It is an isotope of carbon.
- Sonorous:They do not produce sound.
- Density: They have a less density.
- Melting point: Very low melting point except diamond and graphite they have very high melting point. They are isotopes of carbon.
- Heat and electric conductivity: They are bad conductor of heat and electricity except graphite. It is a good conductor of electricity.
- Brittle: When they are beaten with hammer, convert into power or pieces it is called brittleness.
Chemical Properties of Metals:
Metals react with air, water and acid and form many chemical substances.
- Reaction with air : Metals form their oxides when react with oxygen present in air.
Metals + Oxygen > Metal oxides
When ribbon of magnesium (Mg) is burn, it turns into white ash. When this ash is dissolved in a test tube with water and tested with a red litmus paper it converts or turns into blue. Hence, it is proved that this solution (oxide) is alkaline in nature. In the same way copper and aluminium make their oxides due to which utensils of metals look dirty and non lustrous.
- Reaction with Water : They form metal hydroxides and release hydrogen.
Sodium (Na) + Water (H2O) → Sodium hydroxide (NaOH) + Hydrogen (H2)
When a very small piece of (Na) sodium is dried in a filter paper and then put into a beaker filled with water. It seems that piece of (Na) moves fast as it forms sodium hydroxide and hydrogen reacting with water. As (Na) is very reactive hence it is kept dip in kerosene, if it is kept open it burns with oxygen and reacts with water.
- Reaction with acids : They produce hydrogen gas while reacting with acid.
Chemical Properties of Non-Metals:
- Reaction with air: Non metals form their oxides, when react with air (oxygen). These oxideare acidic in nature. Take some powder of sulphur in a metal lid of a bottle and heat it. Wind this metal lid with a metal wire as the sulphur burn. Took it into a gas jar of glass and cover it by a lid so the gas not to be let out. Now pour some water into the gas jar and shake it well. Test the solution with blue litmus paper it turns red, it proves that the solution is acidic.
Sulphur (S) + Oxygen(O2) → Sulphur dioxide (SO2)Sulphur powder reacts with oxygen and produce sulphur dioxide gas which is an acidic oxide of non-metal.
- Reaction with water: Generally non-metals do not react with water or steam hence phosphorous is kept in water.
- Reaction with acid: Most of the non-metals do not react with dilute acids but sulphur reacts with concentrated nitric acid and forms sulphur dioxide, nitrogen oxide and water.
Use of Metals and Non-Metals in Daily Life:
Use of metals:
- To make cooking wares.
- In making electric appliances, electric wires, fridge etc.
- Sheets of aluminium and iron for building materials.
- In manufacturing jewellery from gold, silver and coins from copper, aluminium etc.
- Mercury (Hg) is used in thermometer (clinic).
- Compounds formed from sodium such as sodium chloride (common salt), sodium carbonate (washing soda), sodium
- bicarbonate (baking soda) are used in daily life.
Use of non-metals:
- Sulphur is used as – acid, medicines, and gunpowder.
- Red phosphorous in match boxes, crackers and insecticides.
- Graphite is used in making electrodes.
- In pencils, graphite is used in place of lead as an alternative.
Inert or Fine Metals:
Such metals on which air, water, acid, alkalies do not effect are called pure or fine metals like gold and silver as they, are least reactive metals. Unit to test the purity of gold is carat. 24 carat of gold is the most purest. In 23 and 22 carat gold some amount of other metal is mixed.
Mixed Metal or Alloy:
When some amount of metal or non metal is mixed with main metal, then it is called mixed metal or alloy, for example-in stainless steel, chromium and nickel is mixed to protect it from rusting, brass is an alloy of zinc and copper. Mixing of one or more (metal or non-metal) in a fixed quantity of metal to get desired properties such proportionate mixture is called an alloy such as bronze, brass, and stainless steel.