Physical and Chemical Changes (PDF)

Okay, here is the conversion of the image into a structured markdown format. ## Chapter 5: Physical and Chemical Changes Change is the law of nature. We come across many changes in our everyday life. Rising and setting of the sun, growth of trees, burning of wood, curding of milk, tearing of paper...

Okay, here is the conversion of the image into a structured markdown format. ## Chapter 5: Physical and Chemical Changes Change is the law of nature. We come across many changes in our everyday life. Rising and setting of the sun, growth of trees, burning of wood, curding of milk, tearing of paper, different shapes of the moon, falling leaves etc. are a few common examples of changes around us. Broadly all changes can be divided into two types - physical changes and chemical changes. In this chapter, we shall discuss about these changes. Before interpreting these changes scientifically, let us understand the terms like physical properties, chemical properties, reversible, irreversible processes etc. ### Physical and Chemical Properties #### Physical Properties Properties like shape, size, color, and state of a substance are called its physical properties. For example, when a sheet of paper is folded, its shape changes and this is an example of change in physical property. #### Chemical Properties The internal properties such as reactivity with other chemicals of a substance are known as chemical properties. For example, curd is the product of milk but the internal properties of milk and curd are completely different. #### Reversible Process Processes which can be reversed are called reversible. For example, a folded sheet of paper can be unfolded. Thus, folding a sheet of paper is a reversible process. #### Irreversible Process Processes which cannot be reversed are called irreversible. For example, when milk turns into curd; it cannot be changed back to milk and hence it is an. irreversible process. ### Physical Change A change, in which only the physical properties such as shape, size, colour or state of any substance are changed and no new substance is formed, is called a physical change. Most of the physical changes are reversible. Tearing a paper, stretching a rubber band, melting of ice are a few examples of physical change. **Activity: Tearing/folding paper** 1. Take a piece of paper and fold it. 2. Unfold and tear it into pieces. Each piece of paper is like the original sheet, even after being torn into very small pieces. No new substance is formed. Thus tearing/folding paper is a physical change. From the above activity, there is no change in the properties of paper, though it is not possible to get back the original sheet from its pieces. Some other examples of physical changes are described below: #### Dissolution of a Solid in a Liquid Sugar is sweet. It dissolves in water easily. A solution of sugar also tastes sweet .i.e., sugar on dissolution does not lose its basic properties. Solution of sugar when concentrated by heating and then cooled, gives back the crystals of sugar. These crystals also taste sweet. Thus, sugar (the original substance) is recovered from the solution by a simple method. Therefore, the dissolution of sugar in water is a physical and reversible change. ### Further Examples of Physical Change * **Melting of Wax:** In melting, only the state of the wax changes; from solid to liquid. Solid wax can be obtained from molten wax. This is an example of physical and reversible change. * **Melting of Ice into Water:** In melting, only the state of water changes. Water can be changed into ice and vice-versa. This is an example of physical and reversible change. * **Freezing of Water:** After freezing, water changes into ice. In this change; there is only the change in the state of water. Water can be obtained back from ice, by melting. Freezing of water is a physical change. * **Change of Water into Vapour (Vaporisation):** Vapour is another state of water. Water can be obtained after condensation of vapour. This is a physical and reversible change. * **Change of Vapour into Water (Condensation):** This is also a physical and reversible change as water can be changed into vapour again by the process of vapourization. * **Stretching of a Rubber Band:** In this change, only the size of the rubber band changes. The rubber band comes back in its original shape and size, once it is released. This is a physical and reversible change. #### Characteristics of a Physical Change * During a physical change only physical properties of the substance are changed. * During a physical change no new substance is formed. * Physical changes are temporary and reversible, a physical change can be reversed by a simple physical process. * None or very small amount of energy is either absorbed or evolved during a physical change. ### Chemical Change A change in which composition and chemical properties of a substance are changed is called a chemical change. During a chemical change one or more new substances are formed. The properties of the new substance formed are different from those of the original substance. Chemical changes occur only under proper conditions. Most of the chemical changes are irreversible. For example, when carbon is burnt, carbon dioxide is produced. Carbon dioxide has properties entirely different from those of carbon. So, the burning of carbon is a chemical change. Some examples of chemical changes are described below. * **Rusting of Iron:** When iron objects are exposed to air and moisture (humid air), they are covered with a brown powdery layer called rust. The formation of rust on the surface of iron is called rusting of iron. Here, the composition and properties of rust are different from those of iron. Rust cannot be converted back into iron by simple physical methods. So, rusting of iron is a chemical change. * **Burning of a Candle:** Candles are made from wax and a cotton thread (called wick). When the candle burns the molten wax rises through the cotton thread and undergoes combustion to produce carbon dioxide and water vapour. The cotton thread gets charred to a black mass. Heat and light energies are given out during the burning of candle. It is not possible to recover either the burnt wax or the thread in the original form. Therefore, the burning of candle is a chemical change. It is to be noted that melting of wax and burning of wax are different kinds of changes. Burning of wax is a chemical change, while melting of wax is a physical change. Burning of wax is irreversible, while melting of wax is reversible process. Thus burning of a candle exhibits both a physical and a chemical change at the same time. * **Reaction between Vinegar and Baking Soda:** Vinegar is an acid (acetic acid). The chemical name of baking soda is sodium hydrogen carbonate. When vinegar reacts with baking soda, it gives carbon dioxide. A hissing sound is produced because of production of carbon dioxide. $$ \text{Acetic Acid} + \text{Sodium hydrogen carbonate} \rightarrow \text{Sodium acetate} + \text{Carbon dioxide} + \text{Water} $$ $$ (CH_3COOH) + (NaHCO_3) \rightarrow (CH_3COONa) + (CO_2) + (H_2O) $$ * When carbon dioxide is passed through lime water (calcium hydroxide), it turns milky. This is due to the formation of calcium carbonate. Water is also formed in this reaction. $$ \text{Calcium hydroxide} + \text{Carbon dioxide} \rightarrow \text{Calcium carbonate} + \text{Water} $$ $$ Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O $$ These are 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. Both of these reactions are irreversible, as original substances cannot be retrieved from these by simple physical processes. Thus it is a chemical change. **Activity: Burning of Magnesium Ribbon** * Magnesium ribbon burns with a dazzling light in air and forms magnesium oxide. $$ \text{Magnesium ribbon} + \text{Oxygen (from air)} \rightarrow \text{Magnesium oxide (MgO)} $$ $$ Mg + O_2 \rightarrow MgO $$ * The ash of magnesium oxide gives magnesium hydroxide when mixed with water. the solution of magnesium hydroxide turns blue litmus paper red; this shows its basic characteristic. $$ \text{Magnesium oxide (MgO)} + \text{Water} (H_2O) \rightarrow \text{Magnesium hidroxide} (Mg(OH)_2) $$ These are chemical changes as new substances are formed after change. Original substances cannot be retrieved by simple physical processes. #### Activity-2 To study the burning of a magnesium ribbon. * Materials required: Magnesium ribbon, China dish, A pair of tongs, Sand paper, A burner or spirit lamp. * Procedure: * Take a small piece of magnesium ribbon and clean its one end with a fine sand paper. * Hold it with a pair of tongs and heat the cleaned end in a candle flame/spirit lamp/Bunsen burner flame. * What do you observe? * Observation: Magnesium ribbon burns rapidly with dazzling white light. Burning of paper, wood, cracker or anything is also an example of a chemical change. When something is burnt, many new substances, especially carbon dioxide, are formed. Once burnt, the ash of the new substance cannot be turned into the original substance. Hence burning of anything is a chemical and irreversible change. **Activity-3 Reaction Between Solution of Copper Sulphate and Iron** : Copper sulfate solution is also known as blue vitriol. When an iron nail or shaving blade is left in copper sulphate solution,. for some time, the solution becomes blue to pale green due to formation of iron sulphate. Layer of brown copper gets deposit over the iron. $$ \text{Copper sulphate} + \text{Iron} \rightarrow \text{Iron sulphate} + \text{Copper} $$ $$ CuSO_4 + Fe \rightarrow FeSO_4 + Cu $$ In the above reaction. we cannot get back copper sulphate by any physical method. Thus it is a chemical reaction. **Activity-3: Copper Sulphate and Iron Nails** * Materials required: Boiling tube, Iron nail, Copper sulphate crystals, Test tube stand. $$ \text{Change in colour of the copper sulphate solution due to reaction with iron} $$ * Procedure: * Take about 10 g of powdered copper sulphate crystals in a boiling tube. * Add about 25 mL of water and 5 drops of dilute sulphuric acid to the boiling tube. * Stir the solution to dissolve copper until you get a clear blue coloured solution. * Place a shining iron nail in the solution of copper sulphate. * Leave the boiling tube undisturbed on a test tube stand. * Take out the nail after about 1-2 hour and note for the following: * Any change in colour of the solution? * Any change in the appearance of the nail? * Observation:- The colour copper sulphate solution changes unto pure green when oberved ### Importance of Chemical Changes * During a chemical change, new substances with different properties are formed. Chemical changes are permanent. Thus a chemical change cannot be reversed by simple physical methods. * Relatively a larger amount of energy is either absorbed or evolved during a chemical change. * Relatively a larger amount of energy is either absorbed or evolved during a chemical change. These are the base of every type of reaction in our daily life. Breathing and oxidation of food digestion, circulation, excretion, photosynthesis etc. all life processes are the examples of chemical changes. Physical processes of cooking, transportation and dairy products all are resulted trom chemical changes. Indeed, most of the products or every new material from metals to medicines is discovered by studying chemical changes. **Differences between Physical and Chemical Changes** | Physical Change | Chemical Change | | :------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | :------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | In a physical change, only physical properties such as colour, physical state, shape etc, are changed. Chemical properties remain unchanged. | 1. In a chemical change, the chemical composition and chemical properties or the reacting substances undergo change. | | 2. No new substance is formed. | 2. One or more new substances are formed. | | 3. The original form of the substance can be obtained easily by simple physical methods. | 3. The original substance cannot be obtained by simple physical methods. | | 4. Physical change is a temporary change. It can be easily reversed by reversing the conditions. | 4. Chemical change is a permanent change. It cannot be reversed by just reversing the conditions. | | | 5. Energy is neither evolved nor absorbed. | 5. Energy is either evolved or absorbed. | ### Rusting of Iron Example * When an iron object is left in an open space for some time, it is covered with a brown layer. Deposition of the 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, rusting is a chemical and irreversible change. Reaction involved in rusting can be written an follows: $$ \text{Iron} + \text{Water} + \text{Oxygen} \rightarrow \text{Iron oxide (Rust)} $$ $$ Fe + H_2O + O_2 \rightarrow Fe_2O_3 $$ * For rusting, both water and oxygen should come in contact with iron. If arry one 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 ayer 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 galvanisation; 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. Water pipes made of iron are galvanised to prevent rusting. * Alloying:- Corrosion of iron can be prevented by alloying it with other metals such as nickel, chromium etc. For example, stainless steel, which does not rust, is an alloy of iron with nickel, chromium and manganese. Similarly, alnico used in permanent magnets is an alloy of aluminium, nickel and cobalt,. ### CRYSTALLISATION We know that common salt is obtained by the evaporation 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, a big crystal of common salt is obtained. Formation of big and pure crystals of a substance from its saturated solution is called crystallisation. Crystallisation is an example of physical change. **Multidisciplinary Approach ACTIVITY-4** To obtain the crystals of copper sulphate (or blue vitriol) from its saturated solution. Material:- Beaker, Dilute sulphuric acid, Filter paper, Funnel, Glass rod, Spirit lamp. Take some water in a beaker and very carefully add a few drops of dilute sulphuric acid in it. Take help from your teacher. * Heat the beaker. When water starts boiling add copper sulphate powder in it stirring continuously. * Continue add copper sulphate powder till no more powder can be dissolved. * Filter the solution and allow it to cool without disturbing. * Observe the solution after some time. Observation crystals of copper sulphate are seen at the bottom of the beaker. In the above activity we have obtained the crystals of copper sulphate. We can also get crystals of salt, alum, urea etc, in the same way.