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Everyday we come across many changes in our surroundings. Some of them includes water becoming water vapour, burning of firewood,  rusting of iron, melting of wax, explosion of crackers etc.

Physical change is the change in the physical properties such as shape, size or state of an object. During the physical change only a change in the arrangement of molecules occur. No new substances are formed during the physical change. So it is a temporary change.

The chemical change is the process in which substances change into new substances by absorbing or releasing energy. The chemical change is a permanent change. During chemical change new substances are formed, that is new molecules are formed. So this chemical changes are irreversible in nature.

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Take a beaker. Fill three fourth of it with the water. Put a small piece of sodium into it. After reaction add 2-3 drops of phenolphthalein to it. After the reaction we can observe that the colour of the solution changes to pink.

The water is neutral that is it is neither an alkali nor an acid. When we we add sodium into the water the sodium reacts vigorously with water and form sodium hydroxide and hydrogen gas. Sodium hydroxide is alkaline in nature so when we add phenolphthalein due to the presence of sodium hydroxide it shows pink colour. Since new product is obtained after the reaction and it is irreversible in nature the reaction of sodium with water is a chemical change.

The balanced chemical equation for the reaction is given below.
2 Na + 2 H2O ➝  2NaOH + H ₂

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Reaction of sodium with water......

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The reactions which involve absorption or liberation of heat energy are known as thermochemical reactions. There are two types of thermochemical reactions.
1. Exothermic reaction
2. Endothermic reaction

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The chemical reactions which liberate heat are known as exothermic reaction.

Example : Add 5 ml dilute hydrochloric acid of magnesium taken in a test tube. Touch the bottom of the test tube. We feel heat as a result of the reaction. Because as a result of reaction of hydrochloric acid with magnesium the heat is evolved along with the products hydrogen and magnesium chloride. Since the heat is produced as a result of the reaction it is an exothermic reaction. Balanced  chemical equation for the reaction is given below.
Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

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https://youtu.be/0U-hiRv-v9Q
Click the  link to watch the reaction of magnesium ribbon with hydrochloric acid.

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Chemical reactions which absorb heat energy are called endothermic reaction.

Example : Take a few crystals of potassium permanganate in a dry test tube. Heat the test tube. Bring a glowing incense stick to the mouth of the test tube. At that time we can observe that the colour of the residue becomes black and the incense stick burn more brightly.

The incense stick burn more brightly due to the evolution of oxygen. Test tube become black due to the formation of manganese dioxide. Here heat  is required to accelerate the reaction. The balance the chemical equation for the reaction is given below.
2 KMnO₄ → K ₂ MnO ₄ + MnO ₂(s) + O

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Watch the thermal decomposition of Potassium permanganate as an example of Endothermic reaction.

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Thermal decomposition of potassium permanganate & positive test for oxygen

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1. Find out more examples of reactions from daily life in which involves the absorption or liberation of heat energy and record them in your science diary.
2. What kind of chemical reaction is the neutralization reaction between sodium hydroxide and hydrochloric acid?

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Chemical reactions which liberate or absorb light energy are known as photochemical reactions.

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Photosynthesis is a photochemical reaction. In the presence of chlorophyll, sunlight, water and carbon dioxide plants prepare their own food is called photosynthesis. The products formed during photosynthesis are glucose and oxygen. The glucose that formed is stored by plants in the form of starch.

The balanced chemical equation for photosynthesis is given by:
6CO ₂ + 6H ₂ O → C ₆ H ₁₂ O ₆ + 6O 2

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Come on..... Let us understand photosynthesis in detail....

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Photosynthesis | The Dr. Binocs Show | Learn Videos For Kids

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DISSOCIATION OF SILVER BROMIDE - A PHOTOCHEMICAL REACTION

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DISSOCIATION OF SILVER BROMIDE - A PHOTOCHEMICAL REACTION

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Take some silver bromide into dry watch glasses. Wrap one of them with black paper. Keep both of them in sunlight for sometime. After sometime we can see that the silver bromide kept open in sunlight become brown colour.

The dissociation of silver bromide is a photochemical reaction. When we expose silver bromide in sunlight it absorb light energy and dissociate into silver and bromine. Due to the deposition of silver it appears brown colour.

The balanced chemical equation for the dissociation of silver bromide is given below:
2AgBr(s)→ 2Ag(s) + Br2(g)

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Listen the audio to know about the secret behind the glow of Firefly.

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padlet audio file ecef6afc 0e5d 4944 a2be c22d32d4a185

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1. What could be the reason for storing certain medicines and chemicals in brown bottles?
2. Find more examples for photochemical reactions from daily life.

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The chemical reactions in which the electrical energy is consumed or produced are known as electrochemical reactions.

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The process in which a substance undergoes decomposition by the absorption of electrical energy is known as electrolysis. In electrolysis electrical energy is converted into chemical energy.

Example : Take a beaker and filled with copper sulphate solution. Dip two carbon rods into the copper sulphate solution. Connect these two carbon roads using a connecting wire battery and switch. The circuit diagram given above. When electricity is passed through the copper sulphate solution, the colour of the copper sulphate solution fades. The carbon rods appears brown colour.

When we pass electricity, the copper sulphate solution dissociates into copper ions and sulphate ions. These copper ions are deposited into carbon rods which appears brown colour. Copper ions are responsible for the blue colour of copper sulphate solution. When electricity is passed through the copper sulphate solution the number of copper ions in the solution decreases and this is the colour of solution decreases.

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The arrangements which produce electricity as a result of chemical reaction are known as electrochemical cells. In electrochemical cells chemical energy is converted into electrical energy.

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Take a beaker and fill it with the dilute sulphuric acid. Dip one rod of copper into it. Connect these rods to a bulb and switch. When we on the switch we can see that the bulb glows.

Here electricity is produced as a result of chemical reaction between acid and metal kept immersed in it. This arrangement produced electricity as a result of chemical reactions.

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Dilute acid, zinc and copper make an electric cell | Electricity | Physics

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The thin coating of the metal on the surface of another metal with the help of electricity is known as electroplating. In electroplating electrical energy is converted into chemical energy.

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Take a beaker and fill it with the copper sulphate solution. Connect the positive terminal of a battery with the copper road and the negative terminal of a battery with iron ring. When electricity is passed the copper atoms migrate to the copper sulphate solution from the copper rod. The copper ions from the copper sulphate solution get attached to the iron ring.

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ELECTRO PLATING TECHNOLOGY SIMPLEST WAY | M4 TECH |

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Take a few lemons and in each of them fix a copper rod and a zinc strip. Connect the single strip of one lemon to the copper rod of the next lemon. Connect to the copper road on the first lemon and the zinc strip on the last lemon to an LED using a conducting wire. The LED glows.

Because the whole arrangement act as an electrochemical cell. Here electricity is produced by the reaction of citric acid in the lemon with metals.

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How to Glow LED using Lemon -- Lemon Battery

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1. Construct an electrochemical cell using orange and explain its working.
2. How will you coat the silver on an iron scale. Explain with the help of a neat diagram.
3. List out the instruments that use electrochemical cells. What are the merits and demerits of using such cells.

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It is used in radios, cameras, clocks, toys etc...

These types of cells cannot rechargeable.

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Used in watches, calculators, electronic instruments etc.

These types of cells cannot be recharged and reused.

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These types of cells are used in rechargeable torches and cameras.

These types of cells can be recharged and reused.

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These types of cells are used in mobile phones and laptops.

These cells can be recharged and reused.

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The battery contain poisonous substances such as Mercury which causes environmental pollution and causes damages to living organism. So after the use of cells, the cells should be disposed wisely.

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1. What do we do the cells after their use?
2. Which are the cells can be recharged and reused?
3. Prepare a poster to aware the public about the environmental pollution caused by the battery.

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The learners can

Classify the changes happening in nature into physical changes and chemical changes.

Classify chemical changes into thermochemical changes, photochemical changes and electrochemical changes.

Find examples for exothermic and endothermic reactions.

Use electrochemical cells judiciously.

Electroplate different objects.

Engage in activities to prevent environmental pollution.

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1. Heap up some potassium permanganate on a tile. Keep a piece of dry cotton Wick on the top of it. Poor one or two drops of glycerine on the wick. Observe the changes.
2. Take a magnesium ribbon and clean it by scrubbing. Collect the product formed and dissolve it in water. Dip litmus papers and pH paper in this solution and observe. Find out the reason for the results of the observations.
3. Take sodium chloride solution in a beaker. Add to it a few drops of phenolphthalein. With the help of two carbon rods, let electricity pass through the solution. Record your observations. What is your inference?
4. Take a some silver nitrate solution in a test tube and add some sodium chloride solution to it. What is the colour of the precipitate formed? Filter the precipitate using filter paper, place it on a watch glass and keep it exposed to sunlightfilter the precipitate using filter paper place it on a watch glass and keep it exposed to sunlight. What do you observe?
Try to write down the equation of your observation with the help of your teacher.
Identify the change of energy involved in the 2nd chemical reaction and write down what type of chemical reaction it is.

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Scan the above QR code to read more about the chemical changes....

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Click the link given above to attend a quiz about the concepts in Chemical Changes...

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Let's evaluate our progress by our selves...

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DIFFERENCE BETWEEN PHYSICAL CHANGE AND CHEMICAL CHANGE

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DIFFERENCE BETWEEN PHYSICAL CHANGE AND CHEMICAL CHANGE

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Enjoy the video to explore more about Chemical changes and Physical changes.

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1. List out the physical and chemical changes around us.
2. Prepare a collage by collecting pictures from daily life about Chemical changes.