Chemical Reactions and Equations Class 10 Science Chapter 1 Notes - NCERT CBSE

Chemical Reactions and Equations - Let's Start From the Beginning

So basically, every single day something is changing around us. Milk turns into curd. Iron left in the open turns reddish brown. A burning candle keeps getting smaller. Now see, all these are examples of chemical reactions. In this chapter, we are going to understand what really happens during these changes, how we write them in a short form, and why they happen the way they happen.

I always tell my students - this chapter is the foundation of the whole Science subject. Once you understand this properly, half of your Class 10 Chemistry becomes easy. So let's go step by step, no rushing.

Why This Chapter Matters

Observe that this chapter is not just for marks. It explains things you see in your kitchen, in your school lab, and even on your bicycle chain that gets rusty. Also, this chapter builds the base for many future topics like Acids, Bases and Salts, and Metals and Non-metals. That's why we need to be very clear here, because if this chapter is weak, the next chapters become harder to understand.

From the exam point of view also, this is one of the highest weightage chapters. Almost every year, questions come from balancing equations, types of reactions, and reasons behind colour changes.

Basic Foundation - What You Must Know Before We Start

Before jumping into reactions, we need to be clear about two words - Physical Change and Chemical Change.

A physical change is when only the form or state of a substance changes, but no new substance is formed. Example: Ice melting into water. Here, water is still water, just the state changed from solid to liquid.

A Chemical Reaction is a process in which one or more substances change into one or more new substances with completely different properties. So basically, it forms a new substance. The original substances are called reactants, and the new substances formed are called products.

Note: A common mistake students make is thinking that just mixing two things is a chemical reaction. That's not always true. Mixing sugar in water is a physical change, because sugar can be obtained back. But when sugar is heated strongly and turns black, that is a chemical change, because we cannot get sugar back.

Core Concepts - Characteristics of a Chemical Reaction

Now see, how do we know that a chemical reaction has actually happened? There are some clear signs we observe. I always ask my students - if you see any of these, mostly a chemical reaction is taking place.

1. Change in State

Sometimes the physical state of the substance changes. Example: When we burn a candle, solid wax converts into a gas during burning.

2. Change in Colour

This is one of the most asked observations in exams. Note that we must remember exact colour changes, because examiners love asking these.

3. Evolution of Gas

Sometimes bubbles form, which means a gas is being produced. Example: When zinc reacts with dilute sulphuric acid, hydrogen gas is produced and we observe bubbles.

Zn(s) + H2SO4(aq) -> ZnSO4(aq) + H2(g)

[Zinc] + [Sulphuric acid] -> [Zinc sulphate] + [Hydrogen gas]

4. Change in Temperature

Some reactions release heat, some absorb heat. We will study this in detail under Exothermic and Endothermic reactions ahead.

5. Formation of Precipitate

A precipitate is an insoluble substance formed during a reaction that settles at the bottom. Example: When silver nitrate solution is mixed with sodium chloride solution, a white precipitate of silver chloride is formed.

AgNO3(aq) + NaCl(aq) -> AgCl(s) + NaNO3(aq)

[Silver nitrate] + [Sodium chloride] -> [Silver chloride] + [Sodium nitrate]

Memory Trick: To remember the five characteristics, use the word SCEPT - State change, Colour change, Evolution of gas, Precipitate formation, Temperature change.

Chemical Equations - Writing the Short Form

Chemical Equation - the symbolic representation of a chemical reaction using formulas of reactants and products is called a chemical equation.

Example: When magnesium burns in air, it forms magnesium oxide.

Mg(s) + O2(g) -> MgO(s)

[Magnesium] + [Oxygen] -> [Magnesium oxide]

Note: Magnesium burns with a dazzling white light, that's why this reaction is also used in fireworks.

Balancing a Chemical Equation

Now see, why do we balance equations? Because of the Law of Conservation of Mass, which states that mass can neither be created nor destroyed in a chemical reaction. So basically, the number of atoms of each element must remain the same on both sides.

Let's take an unbalanced equation and balance it step by step.

Fe + H2O -> Fe3O4 + H2 (unbalanced)

After balancing:

3Fe(s) + 4H2O(g) -> Fe3O4(s) + 4H2(g)

[Iron] + [Water] -> [Iron(II,III) oxide] + [Hydrogen gas]

Common Mistake: Students often change the formula of the compound to balance it, like writing H2O2 instead of H2O. Note that we never change formulas. We only adjust the numbers in front (coefficients).

Types of Chemical Reactions

This is the most important part of the chapter. Mostly, all reactions can be grouped into a few major types. Let's go one by one, in the exact pattern - definition, example, equation, word translation, note.

1. Combination Reaction

Combination Reaction - the reaction in which two or more reactants combine to form a single product is called a combination reaction.

=> Formula pattern: A + B -> AB

Example: Quick lime reacts with water to produce slaked lime, releasing a lot of heat.

CaO(s) + H2O(l) -> Ca(OH)2(aq)

[Calcium oxide] + [Water] -> [Calcium hydroxide]

Note: Mostly combination reactions are exothermic reactions, which means heat is released.

2. Decomposition Reaction

Decomposition Reaction - the reaction in which a single reactant breaks down into two or more simpler products is called a decomposition reaction. So basically, it is the exact opposite of combination reaction.

=> Formula pattern: AB -> A + B

=> Types of Decomposition based on energy used:

1. Thermal Decomposition - when heat is used to break the compound.

Example: Calcium carbonate, when heated strongly, decomposes to form calcium oxide and carbon dioxide.

CaCO3(s) -> CaO(s) + CO2(g)

[Calcium carbonate] -> [Calcium oxide] + [Carbon dioxide]

2. Electrolytic Decomposition - when electricity is used to break the compound.

Example: Water decomposes into hydrogen and oxygen on passing electricity through it.

2H2O(l) -> 2H2(g) + O2(g)

[Water] -> [Hydrogen] + [Oxygen]

3. Photolytic Decomposition - when light energy is used to break the compound.

Example: Silver chloride, on exposure to sunlight, decomposes into silver and chlorine.

2AgCl(s) -> 2Ag(s) + Cl2(g)

[Silver chloride] -> [Silver] + [Chlorine]

Note: Here we observe that white silver chloride turns grey when kept in sunlight, because silver metal is formed. This reaction is the basis of black and white photography.

Memory Trick for the three types: Remember TEP - Thermal, Electrolytic, Photolytic.

3. Displacement Reaction

Displacement Reaction - the reaction in which a more reactive element displaces a less reactive element from its compound is called a displacement reaction.

=> Formula pattern: A + BC -> AC + B

Example: Iron is more reactive than copper, so iron displaces copper from copper sulphate solution.

Fe(s) + CuSO4(aq) -> FeSO4(aq) + Cu(s)

[Iron] + [Copper sulphate] -> [Iron sulphate] + [Copper]

Note: We observe that the blue colour of copper sulphate solution fades, and a brownish coating of copper forms on the iron nail. This happens because iron is higher in the reactivity series than copper.

4. Double Displacement Reaction

Double Displacement Reaction - the reaction in which two compounds exchange their ions to form two new compounds is called a double displacement reaction. Mostly, these reactions form a precipitate.

=> Formula pattern: AB + CD -> AD + CB

Example: Sodium sulphate reacts with barium chloride to form a white precipitate of barium sulphate.

Na2SO4(aq) + BaCl2(aq) -> BaSO4(s) + 2NaCl(aq)

[Sodium sulphate] + [Barium chloride] -> [Barium sulphate] + [Sodium chloride]

Note: Reactions in which a precipitate is formed are also called precipitation reactions.

5. Oxidation and Reduction (Redox Reactions)

Oxidation - the process in which a substance gains oxygen or loses hydrogen is called oxidation.

Reduction - the process in which a substance loses oxygen or gains hydrogen is called reduction.

=> A reaction in which both oxidation and reduction happen together is called a Redox Reaction.

Example: When copper oxide is heated with hydrogen, copper oxide loses oxygen (reduction) and hydrogen gains oxygen (oxidation).

CuO(s) + H2(g) -> Cu(s) + H2O(l)

[Copper oxide] + [Hydrogen] -> [Copper] + [Water]

Note: Here CuO is reduced to Cu, and H2 is oxidised to H2O. So this is a redox reaction. We observe that the black colour of copper oxide changes to the brown colour of copper.

Deep Explanation - Exothermic and Endothermic Reactions

Now see, every reaction either gives out heat or takes in heat. Based on this, reactions are of two types.

1. Exothermic Reaction

The reaction in which heat is released along with the products is called an exothermic reaction. Example: Burning of natural gas.

CH4(g) + 2O2(g) -> CO2(g) + 2H2O(g) + heat

[Methane] + [Oxygen] -> [Carbon dioxide] + [Water] + Heat

2. Endothermic Reaction

The reaction in which heat is absorbed from the surroundings is called an endothermic reaction. Example: Decomposition of calcium carbonate requires heat continuously, that's why it is endothermic.

Note: Respiration in our body is also an exothermic process, because it releases energy that we use for daily activities.

Effects of Oxidation Reactions in Daily Life

=> There are two important effects we must know:

1. Corrosion

The process in which a metal is slowly eaten up by the action of air, moisture, or chemicals present in the surroundings is called corrosion. Example: Iron, when exposed to moist air for a long time, forms a reddish brown coating called rust.

4Fe(s) + 3O2(g) + 2xH2O -> 2Fe2O3.xH2O(s)

[Iron] + [Oxygen] + [Water] -> [Hydrated Iron(III) oxide / Rust]

Note: Silver articles turn black, and copper articles develop a green coating after some time, due to corrosion.

2. Rancidity

When fats and oils present in food are oxidised, they become smelly and their taste changes. This is called rancidity. Example: Chips packets are usually flushed with nitrogen gas to prevent rancidity, because nitrogen does not allow oxidation to happen.

Note: Antioxidants are added to foods, and food is also kept in airtight containers, to prevent rancidity.

Quick Comparison Table - Combination vs Decomposition

Students Often Ask

Sir, why does the colour of copper sulphate solution fade when we put an iron nail in it?
Because iron displaces copper from copper sulphate solution. The blue colour is due to copper sulphate, and once copper is removed, the solution turns light green because iron sulphate is formed.

What if I forget whether oxidation means gaining or losing oxygen?
Just remember, OIL RIG - Oxidation Is Loss, Reduction Is Gain (of hydrogen). For oxygen it is the opposite, Oxidation Is Gain of oxygen, Reduction Is Loss of oxygen.

Sir, why do we need to balance equations? Can't we just write the formulas?
Because of the Law of Conservation of Mass. Atoms cannot disappear or appear from nowhere. So the equation must show equal atoms on both sides.

What if a reaction shows more than one characteristic, like colour change and gas formation both?
That is completely normal. Many reactions show multiple changes together, and we should mention all the observations in our answer.

Real-Life Applications

=> Digestion of food in our body involves many chemical reactions.

=> Photosynthesis in plants is a chemical reaction where carbon dioxide and water form glucose and oxygen using sunlight.

=> Respiration converts glucose into energy, releasing carbon dioxide and water.

=> Rusting of iron objects like gates and railings is a slow oxidation reaction we see every day.

=> Cooking food itself involves several chemical changes, that's why cooked food cannot be converted back to raw form.

Exam-Oriented Section

=> Always write balanced equations with state symbols (s), (l), (g), (aq). Many students lose marks just for forgetting state symbols.

=> For 1-mark questions, focus on definitions and identifying the type of reaction.

=> For 3-mark and 5-mark questions, examiners often ask to write the equation, balance it, identify the type, and explain the observation (colour change, gas, precipitate).

=> A very common question is to differentiate between oxidation and reduction with examples - practice this thoroughly.

=> Corrosion and rancidity are short-answer favourites - remember real-life examples like rusting and food spoilage.

=> NCERT activities like the iron nail and copper sulphate experiment, and the magnesium ribbon burning experiment, are asked directly in board exams. Practice writing the observation in your own words.

Quick Revision Notes

• Chemical reaction forms new substances with new properties
• Five signs of a reaction: state change, colour change, gas evolution, temperature change, precipitate
• Chemical equation must always be balanced (Law of Conservation of Mass)
• Five types of reactions: Combination, Decomposition, Displacement, Double Displacement, Redox (Oxidation-Reduction)
• Decomposition has three types: Thermal, Electrolytic, Photolytic
• Oxidation = gain of oxygen / loss of hydrogen
• Reduction = loss of oxygen / gain of hydrogen
• Corrosion and rancidity are everyday effects of oxidation

Key Takeaways

• A chemical reaction always forms a new substance, unlike a physical change
• Balancing equations is based on the Law of Conservation of Mass
• Every type of reaction has a fixed pattern - learn the pattern, not just the example
• Colour changes and observations are exam favourites - memorise them with the substances involved
• Oxidation and reduction always happen together in a redox reaction
• Corrosion and rancidity connect this chapter directly to daily life

Conclusion

So basically, this chapter is all about understanding change - how substances transform, how we represent that change on paper, and why it happens the way it does. Once you are clear with the five types of reactions and can balance equations confidently, this chapter becomes one of your strongest scoring areas.

Now go back, read the Memorise Page below two or three times, write the equations by hand at least once, and then try answering the NCERT in-text and exercise questions on your own. That's how this chapter becomes permanent in your memory. All the best, keep practicing, and you will master this chapter completely.

Memorise Page

Colour Changes

• Blue copper sulphate -> Light green (iron displaces copper)
• White silver chloride -> Grey (in sunlight)
• Black copper oxide -> Brown copper (with hydrogen)
• Reddish brown copper -> Green (corrosion)
• Iron -> Reddish brown rust (corrosion)

Key Equations

• Zn + H2SO4 -> ZnSO4 + H2 (Displacement)
• CaO + H2O -> Ca(OH)2 (Combination)
• CaCO3 -> CaO + CO2 (Thermal Decomposition)
• 2H2O -> 2H2 + O2 (Electrolytic Decomposition)
• 2AgCl -> 2Ag + Cl2 (Photolytic Decomposition)
• Fe + CuSO4 -> FeSO4 + Cu (Displacement)
• Na2SO4 + BaCl2 -> BaSO4 + 2NaCl (Double Displacement)
• CuO + H2 -> Cu + H2O (Redox)

Types of Reactions

• Combination - A + B -> AB
• Decomposition - AB -> A + B
• Displacement - A + BC -> AC + B
• Double Displacement - AB + CD -> AD + CB
• Redox - Oxidation + Reduction together

Important Terms

• Oxidation = Gain of oxygen / Loss of hydrogen
• Reduction = Loss of oxygen / Gain of hydrogen
• Corrosion = Slow oxidation of metal surface
• Rancidity = Oxidation of fats and oils
• Precipitate = Insoluble solid formed in a reaction

Mnemonics

• SCEPT - State, Colour, Evolution of gas, Precipitate, Temperature
• TEP - Thermal, Electrolytic, Photolytic decomposition
• OIL RIG - Oxidation Is Loss, Reduction Is Gain