Chemical Reactions & Equations Part 1 | Class 10 Science Chapter 1 | CBSE 2026 | NCERT Explained

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[00:00:00]Hello students, welcome to Radhika Smart Classes. Let's start 10th standard with the first chapter of chemistry chemical reactions and equations.

[00:00:13]So chemical reactions and equation.

[00:00:16]First of all, do we know the difference between chemical change and physical change? Chemical change means change in the composition.

[00:00:25]Change in composition.

[00:00:28]For example, like burning. Here I'm showing the burning of the wood or grapes are fermented. When the grapes undergo the anorobic respiration, alcohol is made or food is cooked or digestion happens inside the body. These are all chemical changes means change in the composition. But when we talk about physical change only change in state change in state no change in compos composition no change in composition like melting of the ice melts form water but it remain H2O water heated boiled it becomes steam it still stays H2O. So our first question we are very clear what is the difference between physical change and chemical change. In chemical change new substance is formed. New substance is formed.

[00:01:28]Whereas in physical change no new substance. No new substance is formed is formed like chopping of the vegetables or um you change the shape of the dough you mold the clay. These are all physical changes.

[00:01:49]Now next is so when we see any chemical reaction how do we know reaction is taking place? For example when a paper is burnt how do we observe heat is coming out. So these are the m things uh observation which we look for change in color evolution of gas change in temperature formation of precipitate.

[00:02:14]For example, if we add zinc uh granle in the hydrochloric acid, you will see the bubbles. Bubbles represent the affirmations of hydrogen. So this is one observation which we look for and we say chemical reaction is taking place or evolution of um uh formation of precipitate. For example, if the berium chloride aqua solution of berium chloride react with the sodium sulfate, we see the white precipitate of berium sulfate.

[00:02:54]White precipitate plus 2 na.

[00:03:00]So this is one observation. Third thing is change in temperature. For example, some reactions takes place where your beaker or test tube becomes very hot means heat is raising. Heat is coming out that means chemical reaction is taking place. Whereas some reaction where the temperature decreases your beaker or testo becomes cold. It means heat energy is absorbed. So these are the things we look for the for the chemical reactions to takes place.

[00:03:32]change in state, change in color, evolution of the gas, change in the temperature or formation of the precipitate.

[00:03:42]Now third reaction is third question is how do we write a chemical equation?

[00:03:48]First of all, what is the difference between chemical reaction and equation?

[00:03:53]Chemical reaction mean when we say word hydrogen react with oxygen to form water. Equation means symbolic representation.

[00:04:04]When you write any equation in terms of the symbols like iron plus water give you Fe304 plus H2 or if I write in a symbol magnesium burns in air it forms the magnesium oxide. When you write any chemical reaction in terms of the symbols that is called the chemical equation left side represent the reactants and the right side represent the products. So now the most important thing about chemical reaction equation is why do we need to balance? Have you seen when we go to the market to buy vegetables weight and the weight of the vegetables both should be equal.

[00:04:53]Similarly when we have to balance the chemical reation number of atoms in the reactants and in the product should be same. For example, we are taking very simple example magnesium plus oxygen.

[00:05:13]I will erase it and I will show you.

[00:05:17]See magnesium plus oxygen. When magnesium burns in air, it form the magnesium oxide. So the rules to balance the chemical equation is first always balance the metals.

[00:05:33]Magnesium metal, magnesium metal both are one. Both are one means magnesium is balanced. Now look for the non-metals.

[00:05:41]Is there any nonmetals? No. Oxygen and hydrogens's are balanced in the last.

[00:05:47]For example, now here, how many oxygen are there? Two. But here one. So, how do we balance? Shall I put two in the foot or shall I put two in the front? What should I do? When you have to balance any chemical equation, we never put any number in the foot. Because if you put any number in the foot what will happen it will change the formula. So whenever you have to balance you have to remember one thing always put the numeral in front. Now this numeral belongs to all the elements we atoms in the compound.

[00:06:26]Now oxygen is also two but this magnesium has also become two. So I will put two here. Now see two magnesium 2 magnesium two oxygen two oxygen. There is one more reaction iron plus steam.

[00:06:44]Water is in a form of steam. It form Fe304 plus hydrogen.

[00:06:51]Now how will we balance? First rule always balance the metals. One iron and the three iron. So how many atoms of iron in the product? Three. So I will put three here. Now see oxygen. See now I will balance the hydrogen in the last because hydrogen is alone here. How many oxygen here? Four. Here one. Can I put in the foot? No. I should put in the front. Now oxy has become four but hydrogen has also become eight. But here how many? Only two. So how much shall I put? Four. So this is a balanced chemical equation.

[00:07:31]So these are the rules and you have to always remember first metals then non-metals then oxygen and hydrogen.

[00:07:42]So you don't need to show all the working in the exam you can do in a single step otherwise you can use two step. There are some question for practice. Tell me which one shall I do?

[00:07:53]Shall we do this second one? Let's start.

[00:07:57]So first sulfur here one here one but here only one so in the reactant here one here one total two but in the product it is one what should I do shall I put two here okay let's see oxygen how many two here how many one hydrogen is how many now two 2's are four put two here now again recheck it is very important in the last to check all the atoms two sulfur and one sulfur right so now it means how many sulfers are there in the reactant can somebody tell me yes two sulfur one sulfur so total three but here we have two can I put it here three yes two's are four hydrogen 2's are four hydrogen two oxygen two oxygen so it is balanced shall we try One last. Okay. How many chlorine are here? One metal, one metal.

[00:09:02]One aluminium, one. Now second step is non-metal. How many chlorine? Three.

[00:09:07]Here one. Shall we see here? Three. But how many hydrogen here? Two. But here three. So now see this is one small trick. When both are prime numbers take the LCM. 2 3's are six. So three 2's are six.

[00:09:26]So I have to make it 6 H. So what will I do? I will erase this and I will put 6 HCL.

[00:09:36]6 HCL. Right? So now how many chlorine?

[00:09:39]Three. Now here two. Now two. Sometimes we have to start from the metal but slowly slowly after balancing we have to come in a reverse order. Got it? rest of the reaction you can go for yourself.

[00:10:00]Now whenever you have to write a chemical equation remember some important points. Physical state is very important whether it is a solid liquid gas precipitate. Solid is written as a S. Liquid is written as a N. Gas you can write the first letter G or capital U upward arrow and for precipitates you can write PPT or downward arrow.

[00:10:28]Precipitates are nothing. This is a insoluble solid. Best example you seen in home when mama makes a paneer from the milk when she put a lemon juice or citric acid in the milk will get precipitated. that paneer act as a precipitate.

[00:10:47]And the second thing which is most important writing the chemical equation is if there any particular conditions any particular color or any particular smell order anything.

[00:11:01]Now here is a first blue box which is very very important. Why should magnesium ribbon be clean? We will do now. These are the balancing. Try yourself.

[00:11:19]Please do this question yourself. And these are the word equations. Very very important. This second equation is very important here. This equation is very important. Please do it by heart. If there is any doubt, you can DM me. Now question arises next. types of the chemical reaction. Understand we divide the chemical reaction on the various basis. First let's describe the chemical reaction on the basis of heat released and heat absorbed. When I talk about the heat released and heat absorbed what do you mean? The reaction in which heat energy is released they are called exofothermic reaction. What are they called? Exofothermic.

[00:12:05]Exo exit. Exit means out. Exo. Exo means exit. Exit means out. You go to the theaters. There is a written word exit.

[00:12:17]Exit means out. So heat energy is given out. Heat absorbed. The reaction in which heat absorbed endothermic in endothermic.

[00:12:33]So now my question to all of you is when the reaction takes place we can't see heat energy is coming or absorbed. How do you find whether a reaction is exothermic or endothermic? See there are two ways. When the reaction is a exothermic when the heat energy is given out your beaker or test tube will become hot and when it is endothermic reaction energy is absorbed.

[00:13:03]Energy is absorbed your beaker will become cold.

[00:13:09]Endo D letter absorbed the letter cold D letter. So remember this we will learn more reaction about exonendo as we will move further. Now types of chemical reaction.

[00:13:26]So we have a combination reaction, decomposition reaction, displacement reaction, double displacement and redux reaction.

[00:13:35]So first of all we will learn the first reaction combination reaction. We will learn the first reaction combination reaction. But before going to the combination reaction, I want to ask how does the reaction takes place? What is there? What happens in the chemical reaction? In chemical reaction, what happens?

[00:14:00]Some bonds break down. Some new bonds are made. For example, I tell you very simple thing. I have a word C A T cat.

[00:14:09]Can you make any other word with these same letters? Yes, I can make act tac.

[00:14:17]Right? Similarly, in a chemical reaction, there is no new atoms come only there is a rearrangement of the atoms to form the new products. Some bonds are broken, some new bonds are made. So chemical reactions involve breaking and making of the bonds between the atoms to produce the new substances.

[00:14:40]So let's learn first combination reaction. Now when we talk about the combination reaction, what does it mean? Please mark this.

[00:14:51]This is the definition. Combination reaction means when two or more substances now what do you mean by word here substances? It can be elements or it can be compounds.

[00:15:06]It can be elements or it can be compounds to form a single product. When two or more atoms or compounds combine in a fixed ratio to form a single product, that reaction is called the combination reaction. For example, for example, hydrogen plus oxygen, what does it make? Water.

[00:15:40]Now, this is a gas. This is a gas. But this is liquid. But here is it a balanced? No. Whenever we write a chemical reaction, it should be balanced.

[00:15:53]Right? Second reaction carbon plus oxygen, carbon dioxide, nitrogen plus hydrogen.

[00:16:05]These are all combination reaction, right? But now these are all atoms are combining. Can we write one reaction where compounds combining? Yes.

[00:16:17]Calcium oxide. Do you know what is calcium oxide? Tuna. When it is dissolved in the water, when it is in dissolved in the water, it forms slate line calcium hydro oxide. And this reaction here calcium oxide is dissolved in water very vigorously. This reaction happens very vigorously and it gives out the heat. So when the heat is given out, what is the example? it will come under the if you will see heat is given out it will come under the exothermic reaction.

[00:16:53]So here we cover the pot with a clo because our pot or a beaker becomes very hot and it is a self-balanced reaction.

[00:17:05]See here are the example. This is a very important reaction. Calcium oxide react vigorously. Please remember vigorously is a star word. Calcium oxide reacts normally vigorously. What is the right word? Which is the right statement?

[00:17:23]Vigorously with water to produce slate line. What is another word for slate line? Calcium hydroxide. Releasing.

[00:17:31]Releasing means it is a exothermic reaction. So this reaction belongs to combination. This reaction belongs to exothermic reaction. And it is a selfbalanced reaction. Calcium oxide combines with the water to form single product calcium hydroxide.

[00:17:50]These are the other example carbon plus oxygen, hydrogen plus water. Now I told you we will learn more about the exothermic reaction. Please understand exo and combination reactions are linked. It's not that only combination reactions are exothermic. Even displacement reactions are exothermic.

[00:18:10]So um respiration where our food is broken down with oxygen to give you energy in the form of ATP that is also a exofothermic reaction. This is also very very important question. Respirination is an exothermic reaction. Explain the carbohydrate. Which carbohydrate?

[00:18:35]Glucose is broken down with the oxygen to give you carbon dioxide, water and energy is given out out. So respiration is an exothermic reaction. In the exam, it is like a two marks question. If they ask you respiration is an exothermic reaction. Justify or explain. You have to write this two lines. 4 is broken down with oxygen to give you heat in the form of ATP. Which energy? ATP. Second most important thing, writing the reaction is very very important. Writing the reaction is very very important. If you will not write the reaction, if you will not show, we will not score marks.

[00:19:21]Let's move ahead.

[00:19:24]So this was all about the combination reaction. Next is the decomposition reaction. As it is clear from the name decomposition anybody decomposition reaction means breakdown.

[00:19:40]Decompose we have to break down the compound. Single compound is broken down into two or many products.

[00:19:51]But how can we break down the compound?

[00:19:53]We can break down with the heat. We can break down the compound with electricity. we can break down the compound with the sunlight. So first reaction which is most most important it is a thermal decomposition.

[00:20:09]Thermal decomposition reaction.

[00:20:14]Thermal means heat. Everybody's clear with the word thermal decomposition reaction in which compound is broken down.

[00:20:28]broken down with heat. Compound is broken down with heat. So what is the symbol of heat? We put a triangle. So this is the activity where ferrus sulfate crystals.

[00:20:45]What is the formula of ferrra sulfate?

[00:20:48]Fe04.

[00:20:52]Fe04.

[00:20:53]It is also called the green withdrawal.

[00:20:57]What is it called? Green vitrol. NCERTT has not put a green vitrol. They have started with anhydras. Please we will start with the green vitrol. Ferrer sulfate 7 water. It is called the green vitrol.

[00:21:13]It is called the green vitrol. Right? So when it is heated what happens? First these seven water molecules evaporate.

[00:21:23]It is a green color. It forms ferrus sulfate and hydras without water. And hydras means without water. Now it has become colorless. It has become colorless. Again it is further heated up. It will give you Fe23 plus SO2 gas upward arrow plus SO3 gas.

[00:21:51]So SO2 now what is the characteristics of this reaction? First point we have to balance put here 2 fpso4.

[00:22:00]Now next is what is this is solid? It is reddish brown. What is this reddish brown solid? Please remember the characteristic because sometimes they will ask you the question what happens when green vitrol is heated? Write any two observations. First observation reddish brown solid. Second observation pungent smell.

[00:22:22]Which compound give you pungent smell?

[00:22:24]SO2.

[00:22:26]SO2. Pungent smell. So this is one of very important question.

[00:22:32]So it is written here in the detail. I have just explained in the one line. Now second example of thermal decomposition is calcium carbonate.

[00:22:44]Do you know where do we find the calcium carbonate? Eggshell, marble, chalk. If you will heat it, it form calcium oxide plus carbon dioxide.

[00:22:57]So carbon dioxide is our gas. This is solid. This is solid.

[00:23:06]Third, what is the third example?

[00:23:11]Lead nitrate crystal. This example, this example, activity 1.6 is very very very important. Either activity 1.5 will come in the exam or either activity 1.6 will come in the exam. Without these two activities, your question paper is incomplete.

[00:23:30]So we take a lead nitrate crystal. Lead nitrate crystals are colorless.

[00:23:37]Please remember the color colorless. So you heat it. So when you heat it, what will it form? PBO solid plus NO2 brown fumes brown fumes plus oxygen plus oxygen shall we shall I tell you the trick to remember the balancing see first metal one lead one lead how many nitrogen here two but here only one here six oxygen here four oxy oxygen so and one here five total in the product. So best way to remember this, remember one coefficient. Please remember one coefficient thermal decomposition of lead nitrate. Remember one coefficient four NO2. Never learn the NO2. Always remember four NO2. Brown fumes. Now how many nitrogen? Four. But here how many?

[00:24:41]Two. Can I put two in the front? Yes.

[00:24:45]Now how many lead? Two. But how many here? Two. one. So it is balanced. 3 2's are 6 6 2's are 12. Total oxygen 12.

[00:24:56]Here how many? 4 2's are 8. 8 + 2 10 10 + 2 12. So this reaction looks tough but it is very easy. If you remember one small trick 4 N O2 brown fumes. And what is the second observation?

[00:25:12]This is a yellow solid. This is a yellow solid. Very easy. Now I am giving you two questions.

[00:25:22]You have to do yourself. What will happen if copper nitrate crystals are heated and second magnesium nitrate crystals are heated.

[00:25:36]It is same like a lead nitrate but I have changed the metal copper nitrate and magnesium nitrate. Please complete.

[00:25:44]So this is all about the thermal decomposition.

[00:25:48]Green vitrol, calcium carbonate, lead nitrate, copper nitrate, magnesium nitrate.

[00:25:54]Second type of the decomposition is electrolytic decomposition where compound is broken down with the electricity.

[00:26:04]So what is the example? Water.

[00:26:08]Water. So when the electricity is passed through the water, it will break down into hydrogen and oxygen. It is called electrolytic decomposition.

[00:26:22]When you break down any compound with electricity, we have two electrodes cathode and anode. See this figure 1.6 in your NCRT electrolysis of water which is very important. they will show you the figure and they will create a one one mark question A B C D. So first of all we take a distilled water. We never take a tap water because the impurities will interfere in the reaction. But as we know distilled water does not conduct electricity. We add few drops of the acid. There I've written a add few drops of the acid. Why? To conduct the electricity.

[00:27:02]Second, now we have made electrode and these electrodes are made up of graphite. Why do we take a graphite?

[00:27:11]Because graphite is a non matter. It does not react with water.

[00:27:17]It does not react with water.

[00:27:23]It is inert.

[00:27:26]Now current is passed. So the terminal which is attached to the positive terminal that is called anode which is attached to the negative terminal that is called cathode. So when the current is passed through the water what happens water will break down into hydrogen and oxygen. Now question arises will will the hydrogen go to anode or cathode or oxygen will go to anode or cathode. How do we remember? Always remember metals go towards cathode and non-metals anode.

[00:28:02]Nonmetals are negative so they will go towards the positive and metals are positive they will go towards the cathode. Oxygen is non-metal so it will collect at the anode whereas hydrogen will collect at the cathode. Okay. So first part is done.

[00:28:22]Second part is this reaction is balanced? No. Let's balance.

[00:28:29]So what is the volume ratio? So second question is what is the volume ratio?

[00:28:35]Hydrogen is double as that of oxygen. So if you will see here there is a lot of space empty here. Here is a less space.

[00:28:45]Why? Because volume of the hydrogen is more volume of hydrogen is double. So hydrogen is two parts whereas oxygen is one part. So hydrogen is more oxygen is less. Next mass ratio. What is the mass ratio? Everybody learned in a n standard in hydrogen and oxygen in water 1 ratio 8.

[00:29:15]Now last question, how will you identify the gases? Because hydrogen and oxygen both are colorless. Next is hydrogen.

[00:29:26]Hydrogen is a very flammable. It catches a fire quickly and it burns with a pop sound. Where oxygen does not burn itself. It will support the commerion.

[00:29:36]So if you will bring the burning candle near the hydrogen, it will burn with a pop sound like a popcorns and oxygen candle will burn brightly.

[00:29:45]So this is still here we have done the electrolytic decomposition.

[00:29:51]So we have finished till here electrolytic decomposition of water. One thing I want to tell everyone here please these activity question be thorough with them and they have asked in these activity questions some questions these activities what happens in each case when you bring the candle near the oxygen and hydrogen which gas is present in each test tube. Please do these question.

[00:30:17]If still you are not clear you can subscribe and ask me in the comment. Now third type of the decomposition is the photoolytic decomposition. Photo means when you break down the compound with the sunlight. When you break down the compound with the sunlight. See we have done the decomposition with the heat thermal. We have done the decomposition with the electricity electrolytic. When you break down the compound with the sunlight it is called the photoolytic decomposition.

[00:30:49]So this activity 1.8 please note down.

[00:30:52]It is a photoic decomposition.

[00:30:56]Right? So in this case we have the two example.

[00:31:01]What are the two example? Silver salts Ag and AgBr. Silver chloride and silver bromide both are solid.

[00:31:13]If you expose the silver chloride in the sunlight it will it is itself is a colorless.

[00:31:22]It will turn gray. It will turn gray.

[00:31:27]So it means there is a change in color means reaction is taking place. Here AgBr is a silver broomemide is yellow in color. So again it will turn gray in color and chlorine gas will come out.

[00:31:45]So these reactions are called photoolytic decomposition reaction. So silver salts are always stored in a dark color amber colored bottles. Why?

[00:31:58]Because these salts are very sensitive to the sunlight and these reactions are used in the black and white photography.

[00:32:07]Please remember what is the use of these reaction? Black and white photography.

[00:32:11]Best example is in the olden time we have a camera rolls they have a coating of a silver. So whenever you open that roll in front of the sunlight photo used to be ruined. So why this is the only reason because they have a silver coating and silver salts are very sensitive to the sunlight. So decomposition reaction require energy.

[00:32:36]Require means absorb energy. Require means absorbs energy in the form of heat, light, electricity for breaking down the reactants. So these reactions are also known as endothermic reaction.

[00:32:53]Either it is a ther electrolytic or photoolytic. They are all endothermic reactions. But remember one decomposition reaction which is exo also.

[00:33:06]It is written here.

[00:33:14]Please remember this decomposition reaction on the page where we have learned that this decomposition reaction where vegetable matter change into the compost is a endothermic re exothermic reaction. All these decomposition photoolytic thermal thermal decomposition electrolytic they all are and endo but breaking down of the vegetable matter into compost that is an exothermic reaction. So guys we will stop it here today for the next part please follow me and we will continue the chapter in the next class.

[00:33:55]Thank you.