Class 7 Science

Chemical Change

Changes in which chemical properties of substance change and a new substance is formed are called chemical changes. Some of the chemical changes are irreversible. Even in case of reversible changes in which chemical properties of substance change and a new substance is formed are called chemical changes. Some of the chemical changes are irreversible.

Examples:

Burning of paper, wood, fuel or anything:

When something is burnt, many new substances are formed; especially carbon dioxide is formed in most of the cases. Once something is burnt, the ash or carbon dioxide cannot be turned into the original substance. Hence, burning of anything is chemical and irreversible change.

Melting of wax and burning of wax are different kinds of change. Burning of wax is chemical change while melting of wax is a physical change. Burning of wax is irreversible while melting of wax is reversible.


Reaction between vinegar and Baking Soda:

Vinegar is an acid (Acetic acid). The chemical name of baking soda is sodium hydrogen carbonate or sodium bicarbonate.

When vinegar reacts with baking soda, it gives carbon dioxide. A hissing sound is produced when baking soda is added to vinegar. This happens because of production of carbon dioxide. When carbon dioxide is passed through lime water (calcium hydroxide), it turns the lime water milky. This happens because of formation of calcium carbonate. Water is also formed; along with calcium carbonate. The reaction involved in this can be written as follows:

CH3COOH + NaHCO3 → CH3COONa + CO2 + H2O

CO2 + Ca(OH)2 → CaCO3 + H2O

These are the examples of chemical changes. In the first case, a new substance; carbon dioxide is formed. In the second case, a new substance; calcium carbonate is formed. The lime water turns milky because of formation of calcium carbonate.

These reactions are irreversible, as original substance cannot be retrieved from these by simple physical processes.



Burning of magnesium ribbon:

Magnesium ribbon burns with dazzling light in air and forms magnesium oxide. The ash of magnesium oxide gives magnesium hydroxide when mixed with water. Solution of magnesium hydroxide turns blue litmus paper red; this shows its basic characteristic. Reaction involved in it can be written as follows:

2Mg + O2 → 2MgO

MgO + H2O → Mg(OH)2)

These are chemical changes as new substances are formed after change. Original substances cannot be retrieved by simple physical processes.

Reaction between solution of copper sulphate and iron: Copper sulphate solution is blue in colour. Copper sulphate is also known as blue vitriol. When an iron nail or shaving blade is left in copper sulphate solution; for some time; the colour of solution changes from blue to pale green. This happens because of the formation of iron sulphate. Additionally, a layer of brown copper gets deposited over the iron nail or blade. The reaction can be written as follows:

Fe + CuSO4 → Cu + FeSO4

Rusting of Iron: Deposition of a brown layer on iron is called rusting. In rusting, a new substance is formed. The chemical structures of iron and rust are completely different. Rust is iron oxide. Iron is a grey-black material while rust is reddish brown. Thus, this is a chemical and irreversible change. Reaction in rusting can be written as follows:

Fe + H2O + O2 → Fe2O3

When articles made of iron come in contact with moisture present in air, they get rusted. Iron is converted into iron oxide, i.e. rust. The iron article becomes weak in due course as all the iron slowly turns into rust. This is called corrosion of iron. Rusting gives a huge monetary loss to the people and nation.



Prevention of rusting: For rusting, both water and oxygen should come in contact with iron. If anyone of these is prevented to come in contact with iron, rusting can be prevented. So, rusting is prevented using following methods:

Painting: articles such as; iron gates, grills, etc. are painted at regular intervals of time.

Applying of layer of grease: Applying a layer of grease prevents the iron articles from coming in contact with moist air. This prevents rusting. That is why grease is applied over the chain of bicycle and also over many machine parts.

Galvanisation: In the process of galvanization; a layer of non-reactive metal, such as zinc is deposited over iron articles. The layer of non-reactive metal prevents the iron articles from coming in contact with moisture. Thus it prevents rusting.

Crystallisation: Common salt is obtained by the vapourisation of sea water, but crystals of common salt are very small. When a small crystal of common salt is left dipped in the saturated solution of common salt for some time, big crystal of common salt is obtained. Formation of big and pure crystal of a substance from the saturated solution is called CRYSTALLISATION.

Characteristics of Physical change:

  • Only physical properties of substance change.
  • No new substance is formed in this change.
  • Most of the Physical changes are reversible.

Characteristics of Chemical change:

  • Chemical properties of substance change.
  • New substance is formed.
  • Some of the Chemical changes are irreversible.
  • Chemical change cannot be reversed by simple physical means.

Signs of Chemical Change:

  • Evolution of heat.
  • Production of sound.
  • Change in colour.
  • Change in smell.
  • Evolution of gas.