Physical and Chemical Changes
Science Class Seven
physical and chemical changes-II
The Reaction of Iron with copper sulphate solution
When an iron nail of a blade made of iron is dipped into the solution of copper sulphate for some time the green solution of copper sulphate turns bluish due to formation of iron sulphate and the copper gets deposited over the iron nail or blade.In this reaction, the solution of copper sulphate turns into a solution of iron sulphate, and copper gets deposited over the iron nail or blade made of iron.
The reaction involves in this can be represented as follows:
Copper Sulphate Solution (green) + Iron ⇒ Iron Sulphate (blue) + Copper (brown)
Copper sulphate solution (CuSO4) + Iron (Fe) ⇒ Iron sulphate (FeSO4) + Copper (Cu)
Since in this reaction two new substances are formed, thus, getting deposited of copper over iron nail and change of copper sulphate solution into iron sulphate solution are chemical changes.
The Reaction between Vinegar and Baking Soda
Vinegar on reaction with baking soda gives carbon dioxide, water, and sodium acetate.
When baking soda is added with vinegar, bubbles of carbon dioxide start coming out from the solution. The evolution of carbon dioxide in this reaction produces a hissing sound also.
Diluted acetic acid is called Vinegar, and it is used as a tastemaker in households. The chemical formula of acetic acid is CH3COOH.
The chemical name of baking soda is sodium hydrogen carbonate or sodium bicarbonate. The chemical formula of baking soda is NaHCO3.
The chemical formula of sodium acetate is CH3COONa or it is written as Na CH3COO also.
The reaction between vinegar and baking soda can be written as follows:
Vinegar (CH3COOH) + Baking soda (NaHCO3) ⇒ Carbon dioxide (CO2) + H2O + Sodium acetate (CH3COONa).
The Reaction between Carbon dioxide and Lime water
The evolution of carbon dioxide in the reaction between vinegar and baking soda can be confirmed by passing the gas evolved in this reaction into lime water.
Lime water turns milky when carbon dioxide is passed into it.
Lime water turns milky because of the formation of calcium carbonate.
The chemical name of lime water is calcium hydroxide and its chemical formula is Ca(OH)2. The chemical formula of calcium carbonate is CaCO3 and its general name is limestone.
The reaction between carbon dioxide and water can be written as follows. In the reaction between carbon dioxide and lime water, calcium carbonate and water are formed.
Lime water (Ca(OH)2) + Carbon dioxide (CO2) ⇒ Calcium carbonate (CaCO3) + Water (H2O)
The turning of lime water milky is the confirmatory test for the evolution of carbon dioxide.
Since in the reaction between carbon dioxide and lime water two new substances water and calcium carbonate are formed, thus it is a chemical change.
In fact, all the chemical reactions are examples of chemical changes.
Rusting of Iron
When articles made of iron is left for a long time in the open, iron reacts with oxygen and moisture present in the air, and a brown coloured substance gets deposited over it. This brown coloured substance is called rust. And this phenomenon, i.e. depositing of brown coloured substance over articles made of iron is called Rusting of Iron.
The chemical name of rust is Iron Oxide. The other names of Iron oxide are Ferric Oxide, Iron (III) Oxide.
The rusting of iron is a chemical change. In the rusting of iron, rust is formed over the articles made of iron. Rust is a new substance called Iron Oxide. We cannot get iron back by rust using general physical methods.
The reaction involved in the rusting of iron can be shown as follows:
Iron (Fe) + Water (H2O) + Oxygen (O2) ⇒ Iron oxide (Fe2O3)
Once rust gets deposited over the articles made of the iron gradually the whole article turns into rust. Rust gradually destroys the whole article made of iron, if not prevented in time. Every year many things made of iron get rusted and give a huge loss to public and personal property. For example iron gates, iron grills, bridges made of iron, cycle rim, ships, etc. Ships and bridges are always in contact with water, so, there has always been a great risk of getting rusted.
For rusting, water and oxygen are necessary to come in contact with iron. If anyone or both are prevented to come in contact with articles made of iron, rusting can be prevented.
Rust can be prevented by covering a layer of paint over the articles made of iron or by applying a layer of grease over them. By painting, applying a layer of grease, galvanisation, etc. over the articles made of iron we prevent them to come in contact with moisture and oxygen present in the atmosphere and this prevent rusting.
Galvanisation
The depositing of a layer of zinc or chromium over the articles made of iron is called the Galvanisation.
The galvanisation is done over the water pipes which are used to supply drinking water in our homes in order to prevent them from rusting. Galvanisation gives pipes a long life. If galvanised pipe are not used in supply of water, they will get rusted easily.
Crystallisation
The formation of solid salts by the process of evaporation of their solution is called crystallisation. The Formation of ice is also a phenomenon of crystallisation.
Seawater contains many salts dissolved in it. The common salt, which we use in our kitchen is one of them. The common salt is obtained by the vaporisation of seawater.
Crystals have a certain shape, it may be rounded, conical, square, or any random shape.
The crystals of sugar and salt can be seen easily.
The crystallisation is a physical change, as by crystallising only physical properties of salt change, and no new product is formed.
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