Chemical Changes versus Physical Changes. Study Materials for Foundation Course Class VII

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1 Chemical Changes versus Physical Changes Supratim Das.Chemquest. Durgapur Study Materials for Foundation Course Class VII It is important to understand the difference between chemical and physical changes. Some changes are obvious, but there are some basic ideas you should know. Physical changes are usually about physical states of matter. Chemical changes happen on a molecular level when you have two or more molecules that interact. Chemical changes happen when atomic bonds are broken or created during chemical reactions. Physical Change A physical change is one in which the shape, size, appearance or state, of a substance may alter, but its chemical composition remains same. No new substance is formed. It is usually a change, which is reversible. By reversing the process, the original substance can be obtained. A more common example is that of water, which can be converted into solid ice, liquid water, and gaseous water vapour. It can be reconverted to its previous state by various methods. Yet, in all the three forms, the chemical composition of water is not altered. No new substance with new chemical properties is formed. In all these, water consists of two atoms of hydrogen and one atom of oxygen having formula H2O. Examples of Physical Changes Dissolving of Sugar in Water Magnetising An Iron Rod Glowing of An Electric Bulb Dissolving of Sugar in Water When sugar is dissolved in water, it seems to disappear completely, and not even a very powerful microscope can show us the molecules of sugar in water. However, the solution obtained is evenly sweet. By evaporating the water, the sugar can be recovered in a crystalline form. Magnetising an Iron Rod An iron rod can be converted into a magnet by passing an electric current through an insulated coil wound around. When the current flows, the iron bar gets magnetised. One end becomes North Pole and the other the South Pole. If the direction of the current is reversed, the North Pole changes to South Pole and the South Pole changes to North Pole. Glowing of an Electric Bulb When an electric current is passed through the filament of a bulb, the filament starts glowing and the glow stops as soon as the current is cut off. A physical change is one in which the shape, size, appearance or state, of a substance may alter, but its chemical composition remains same. A change in which the composition of a substance is altered is called as chemical change. As a result, the original properties get changed and one or more new substances are formed.

2 Chemical Equations A chemical equation is a 'balanced account of a chemical transaction.'in any chemical transaction or reaction, the number of atoms of all the participating elements will remain proportionately constant before and after the reaction maintaining the stoichiometry. Types of Chemical Reactions When a chemical reaction occurs how are new substances produced? Do atoms of one element transform themselves into atoms of another element? Combination or Addition or Synthesis Reaction Combination or addition or synthesis is a chemical change in which atoms or molecules of two or more substance combine to form molecules of a single new substance. Decomposition Reaction A chemical reaction in which molecules of a substance break down to form simpler molecules of two or more new substances is recognized as a decomposition reaction. Displacement or Substitution Reaction A substitution or displacement reaction is a chemical change in which atoms of one element replace the atoms of another element from the molecules of a compound. Double Displacement Reaction Neutralisation is a type of double displacement reaction, in which, the reactants are a base and an acid, and the products are salt and water. Oxidation Reduction or Redox Oxidation is a reaction in which oxygen is added or hydrogen is removed from a substance. The Effects of Oxidation Reactions In Every Day Life Many metals are chemically active elements and get easily affected by substances like moisture, air, acids, etc. One must have observed iron articles that are shiny when new, but get coated with a reddish brown powder when left for some time. The Effects of Oxidation Reactions Corrosion:- Many metals are chemically active elements and get easily affected by substances like moisture, air, acids, etc. One must have observed iron articles that are shiny when new, but get coated with a reddish brown powder when left for some time. This process is commonly known as rusting of iron. The problem with iron (as well as many other metals) is that oxidation takes place and the oxide formed does not firmly adhere to the surface of the metal causing it to flake off easily. This eventually causes structural weakness and disintegration of the metal.

3 When a metal is attacked by substances around it, it is said to corrode and this process is called corrosion. Corrosion causes deterioration of essential properties in a material. Question 1: What is the difference between Redox and Decomposition Reaction? Give two examples of Redox Reactions. Question 2: What is the electronic concept of Redox? Give two examples of thermal decomposition. Rancidity Have you ever tasted or smelt the fat/oil containing food materials left for a long time? This unpleasant change in the flavour and odour of a food is called rancidity. The most important cause of rancidity is the deterioration in fats and fatty foods because of oxidation process. When an oxygen atom replace hydrogen atom in the fatty acid molecule it destabilizes the molecule. Factors which accelerate fat oxidation include, salt, light, water, bacteria, moulds trace metals (iron, zinc, etc.). Usually substances which retard fat oxidation or rancidity are called antioxidants (such as BHT, BHA, vitamin E, and vitamin C). These are added to foods containing fats and oil to prevent such spoiling. Keeping food in air tight containers or air tight wrapping also helps to slow down oxidation. Physical Change means No Change to Molecules When you step on a can and crush it, you have forced a physical change. However, you only changed the shape of the can. It wasn't a change in the state of matter because the energy in the can did not change. Also, since this was a physical change, the molecules in the can are still the same molecules. No chemical bonds were created or broken. When you melt an ice cube (H2O), you have a physical change because you add energy. You added enough energy to create a phase change from solid to liquid. Physical actions, such as changing temperature or pressure, can cause physical changes. No chemical changes took place when you melted the ice. The water molecules are still water molecules. Chemical Change means Changing the Molecules Chemical changes happen on a much smaller scale. While some experiments show obvious chemical changes, such as a color change, most chemical changes are not visible. The chemical change as hydrogen peroxide (H2O2) becomes water cannot be seen since both liquids are clear. However, behind the scenes, billions of chemical bonds are being created and destroyed. In this example, you may see bubbles of oxygen (O2) gas. Those bubbles are evidence of the chemical changes. Melting a sugar cube is a physical change because the substance is still sugar. Burning a sugar cube is a chemical change. Fire activates a chemical reaction between sugar and oxygen. The oxygen in the air reacts with the sugar and the chemical bonds are broken. Iron (Fe) rusts when it is exposed to oxygen gas in the air. You can watch the process happen over a long period of time. The molecules change their structure as the iron is oxidized, eventually becoming iron oxide (Fe2O3). Rusty pipes in abandoned buildings are real world examples of the oxidation process.

4 First of all, when physical change occurs in a substance, you will not be able to create a new substance. The substance will remain in its original state. In contrast, when chemical change occurs in the substance, you will be able to produce a different kind of substance. This means you will lose the original substance and a new one will form. Based on this premise, any physical change that occurs in matter or substance is completely reversible. But when a chemical change happens, you will not be able to reverse or undo the transformation. For example, water can freeze so liquid can turn to solid but the substance is still water. You can unfreeze ice to revert to the liquid state of water. But if you burn paper, you will get a new substance called ash. You can not unburn ash to transform it back to paper. A Big Difference A big difference between physical and chemical change is the speed of transformation. Physical change occurs faster and sometimes instantaneously. Most chemical changes, on the other hand, take longer time to become discernible. You can crumple a tin can and you will immediately see physical changes. But corrosion of tin can occurs very slowly; it will take a long time before you see the appearance of rust on the can. Examples of Chemical Changes A new compound with new chemical composition results from a chemical change as the atoms rearrange themselves to form new chemical bonds. Burning wood Souring milk Mixing acid and base Digesting food Cooking an egg Heating sugar to form caramel Baking a cake Rusting of iron Examples of Physical Changes No new chemical species forms in a physical change. Changing the state of a pure substance between solid, liquid, and gas phases of matter are all physical changes since the identity of the matter does not change. Crumpling a sheet of aluminum foil Melting an ice cube Casting silver in a mold Breaking a bottle Boiling water Evaporating alcohol Shredding paper Sublimation of dry ice into carbon dioxide vapour Question 3: What is the difference between Redox and Decomposition Reaction? Give two examples of Redox Reactions. Question 4: What is the electronic concept of Redox? Give two examples of thermal decomposition. Question 5: What is the Anti-Oxidants? Give two examples of anti-oxidants to prevent rancidity.

5 Question 6: What is the Disproportionation Reaction? Give examples of photolytic decomposition.