# Chemical Reactions - Grade 10 (SEE) Science
## Complete Chapter Notes with All Exercises - NEB Curriculum
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## 🎯 Chapter Overview
**Welcome to the ultimate guide for the Class 10 Science chapter, "Chemical Reactions"!** These notes have been carefully designed to be your best friend for SEE preparation. They are super detailed, easy to understand, and packed with fun tricks and analogies to make learning chemistry a breeze.
**This chapter is a cornerstone of chemistry, teaching you the language of chemical changes.** We'll cover everything from what a chemical reaction is, how to write and balance equations, the different types of reactions, and the factors that speed them up or slow them down. **All exercises from your textbook are solved here**, including definitions, give reasons, differentiations, balancing equations, and experimental questions, ensuring you are 100% prepared.
**Weightage:** 8-10 marks in SEE examination
**Time Required:** 15-20 periods
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## 📚 Learning Objectives
After mastering this chapter, you will be able to:
- Define chemical reactions, reactants, and products.
- Differentiate between physical and chemical changes.
- Write and balance chemical equations like a pro.
- Identify and explain the four main types of chemical reactions with examples.
- Distinguish between exothermic and endothermic reactions.
- Explain the factors affecting the rate of a chemical reaction.
- Describe simple experiments to demonstrate these factors.
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## ⚛️ 1. WHAT IS A CHEMICAL REACTION?
Think of baking a cake. You start with **ingredients** like flour, sugar, and eggs. After mixing and heating, you get something completely new: a **cake**. You can't easily turn the cake back into flour, sugar, and eggs.

A **chemical reaction** is just like that! It's a process where one or more substances, called **reactants** (the ingredients), are converted into one or more different substances, called **products** (the cake).

**Reactants** $\rightarrow$ **Products**
Example: When magnesium ribbon burns in the air, it combines with oxygen to form a white powder, magnesium oxide.
**Magnesium + Oxygen $\rightarrow$ Magnesium Oxide**
($2Mg + O_2 \rightarrow 2MgO$)

### 1.1 Physical Change vs. Chemical Change
It's important to know the difference!

| Feature | Physical Change | Chemical Change |
|---|---|---|
| **What happens?** | Only the state, size, or shape changes. | A completely new substance is formed. |
| **Nature** | Temporary and easily reversible. | Permanent and irreversible. |
| **Composition** | The chemical makeup stays the same. | The chemical makeup changes. |
| **Example** | Melting ice into water ($H_2O$ is still $H_2O$). | Burning paper to ash (Ash is not paper). |

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## ✍️ 2. CHEMICAL EQUATIONS & BALANCING
A **chemical equation** is a shorthand way to represent a chemical reaction using symbols and formulas.

### Pro-Trick for Balancing Equations: The M-N-O-H Rule!
Balancing ensures that the number of atoms of each element is the same on both the reactant and product sides (Law of Conservation of Mass). Follow this simple order:
1.  **M** - Balance **Metals** first (e.g., K, Mg, Al, Fe).
2.  **N** - Balance **Non-metals** next (except O and H) (e.g., Cl, S, P).
3.  **O** - Balance **Oxygen** atoms.
4.  **H** - Balance **Hydrogen** atoms last.

**Example: Balance $Al + H_2SO_4 \rightarrow Al_2(SO_4)_3 + H_2$**
1.  **Metals (Al):** There is 1 Al on the left and 2 Al on the right. Put a **2** in front of Al.
    `2`Al + $H_2SO_4 \rightarrow Al_2(SO_4)_3 + H_2$
2.  **Non-metals (S):** There is 1 S on the left and 3 S on the right (inside the bracket). Put a **3** in front of $H_2SO_4$.
    2Al + `3`$H_2SO_4 \rightarrow Al_2(SO_4)_3 + H_2$
3.  **Oxygen (O):** Now there are 3x4=12 'O' on the left and 4x3=12 'O' on the right. Oxygen is balanced!
4.  **Hydrogen (H):** There are 3x2=6 'H' on the left and 2 'H' on the right. Put a **3** in front of $H_2$.
    2Al + 3$H_2SO_4 \rightarrow Al_2(SO_4)_3 + `3`H_2$
**The equation is now balanced! ✅**

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## 🔄 3. TYPES OF CHEMICAL REACTIONS
Let's learn the types with simple analogies!

### A. Combination Reaction (Synthesis)
**Analogy: Getting Together! 🤝**
Two or more simple reactants combine to form a single, more complex product.
**General Form:** A + B $\rightarrow$ AB
**Example:** Sodium (Na) metal reacts with Chlorine (Cl₂) gas to form Sodium Chloride (NaCl).
$2Na + Cl_2 \rightarrow 2NaCl$

### B. Decomposition Reaction
**Analogy: Breaking Up! 💔**
A single complex reactant breaks down into two or more simpler products. It's the opposite of combination.
**General Form:** AB $\rightarrow$ A + B
**Example:** When calcium carbonate ($CaCO_3$, limestone) is heated, it breaks down into calcium oxide (CaO) and carbon dioxide ($CO_2$).
$CaCO_3 \xrightarrow{Heat} CaO + CO_2$

### C. Single Displacement Reaction
**Analogy: Cutting In a Dance! 💃**
A more reactive element "cuts in" and displaces a less reactive element from its compound.
**General Form:** A + BC $\rightarrow$ AC + B
**Example:** When a zinc (Zn) strip is placed in copper sulphate ($CuSO_4$) solution, the more reactive zinc displaces copper.
$Zn + CuSO_4 \rightarrow ZnSO_4 + Cu$

### D. Double Displacement Reaction
**Analogy: Switching Partners! 🕺💃**
Two compounds react by exchanging their ions to form two new compounds.
**General Form:** AB + CD $\rightarrow$ AD + CB
**Example:** When silver nitrate ($AgNO_3$) solution is mixed with sodium chloride (NaCl) solution, they swap partners.
$AgNO_3 + NaCl \rightarrow AgCl \downarrow + NaNO_3$
(The down arrow `↓` means AgCl is a solid precipitate.)

**A special type of this is the Neutralization Reaction:**
An acid reacts with a base to form salt and water.
Acid + Base $\rightarrow$ Salt + Water
$HCl + NaOH \rightarrow NaCl + H_2O$

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## 🔥 4. EXOTHERMIC & ENDOTHERMIC REACTIONS

### Exothermic Reaction (Exo = Exit)
A reaction that **releases** heat energy into the surroundings. The surroundings get warmer.
**Think:** Burning wood, explosion of a firecracker.
**How to write it:** Heat is shown as a product.
C + $O_2 \rightarrow CO_2$ + **Heat**

### Endothermic Reaction (Endo = Enter)
A reaction that **absorbs** heat energy from the surroundings. The surroundings get colder.
**Think:** Melting ice, cooking food.
**How to write it:** Heat is shown as a reactant.
$N_2 + O_2$ + **Heat** $\rightarrow 2NO$

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## ⚡ 5. RATE OF REACTION & FACTORS AFFECTING IT
The **rate of a chemical reaction** is the speed at which reactants are converted into products. Some factors can change this speed:

1.  **Temperature:** 🔥 Increasing the temperature generally increases the reaction rate. (Molecules move faster and collide more often).
    * **Example:** Food cooks faster at a higher temperature.
2.  **Concentration:** 💧 Increasing the concentration of reactants increases the reaction rate. (More particles are packed into the same space, so they collide more).
    * **Example:** A concentrated acid reacts faster than a dilute one.
3.  **Surface Area:** पाउडर Increasing the surface area of a solid reactant increases the reaction rate. (More particles are exposed to react).
    * **Example:** Powdered sugar dissolves faster in water than a sugar cube.
4.  **Catalyst:** 🦸‍♂️ A substance that changes the rate of a reaction without being consumed itself.
    * **Positive Catalyst (Promoter):** Speeds up a reaction. E.g., Manganese dioxide ($MnO_2$) speeds up the decomposition of hydrogen peroxide.
    * **Negative Catalyst (Inhibitor):** Slows down a reaction. E.g., Glycerine slows down the decomposition of hydrogen peroxide.
5.  **Light:** ☀️ Some reactions are triggered or sped up by light (photochemical reactions).
    * **Example:** Photosynthesis in plants. $H_2 + Cl_2 \xrightarrow{Sunlight} 2HCl$.
6.  **Pressure:** 💨 For reactions involving gases, increasing the pressure increases the reaction rate. (Forces gas particles closer together, leading to more collisions).
    * **Example:** The Haber process for making ammonia: $N_2(g) + 3H_2(g) \xrightarrow{High \ Pressure} 2NH_3(g)$.

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## 📋 6. COMPLETE EXERCISE SOLUTIONS
### **1. Define the following:**
**a. Chemical Reaction:** A process that leads to the chemical transformation of one set of chemical substances (reactants) to another (products).
**b. Endothermic Reaction:** A chemical reaction that absorbs heat from its surroundings.
**c. Catalyst:** A substance that alters the rate of a chemical reaction without undergoing any permanent chemical change itself.
**d. Neutralization Reaction:** A type of double displacement reaction where an acid and a base react to form salt and water.

### **2. Answer the following questions:**
**a. What are the conditions required for a chemical reaction? Explain any three.**
**Answer:** The conditions required are heat, light, pressure, catalyst, concentration, and surface area.
1.  **Heat:** Many reactions need heat to start. For example, heating potassium chlorate ($KClO_3$) causes it to decompose. $2KClO_3 \xrightarrow{Heat} 2KCl + 3O_2$.
2.  **Light:** Some reactions are photochemical. For example, hydrogen and chlorine react in the presence of sunlight. $H_2 + Cl_2 \xrightarrow{Sunlight} 2HCl$.
3.  **Pressure:** This is crucial for gas reactions. In the Haber process, high pressure is used to produce ammonia. $N_2(g) + 3H_2(g) \xrightarrow{Pressure} 2NH_3(g)$.

**b. Describe an experiment to show that the rate of reaction increases with an increase in temperature.**
**Answer:**
**Materials:** Two beakers, oxalic acid, dilute sulphuric acid, potassium permanganate ($KMnO_4$) solution, burner.
**Procedure:**
1.  Take equal amounts of oxalic acid, sulphuric acid, and pink-colored $KMnO_4$ solution in two beakers, 'A' and 'B'.
2.  Gently heat beaker 'A' to about 60°C. Do not heat beaker 'B'.
3.  Observe the time it takes for the pink color to disappear in both beakers.
**Observation:** The pink color in the heated beaker 'A' disappears much faster than in the cold beaker 'B'.
**Conclusion:** This proves that increasing the temperature increases the rate of a chemical reaction.

### **3. Differentiate Between:**
**a. Combination Reaction and Decomposition Reaction**
| Combination Reaction | Decomposition Reaction |
|---|---|
| Two or more reactants form one product. | One reactant breaks into two or more products.|
| General Form: A + B $\rightarrow$ AB | General Form: AB $\rightarrow$ A + B |
| It is like 'synthesis' or 'building up'.| It is like 'breaking down'. |
| E.g., $2Mg + O_2 \rightarrow 2MgO$ | E.g., $CaCO_3 \rightarrow CaO + CO_2$ |

**b. Positive Catalyst and Negative Catalyst**
| Positive Catalyst (Promoter) | Negative Catalyst (Inhibitor) |
|---|---|
| Increases the rate of a chemical reaction.| Decreases the rate of a chemical reaction. |
| It provides an alternative path with lower activation energy. | It increases the activation energy. |
| Example: $MnO_2$ in the decomposition of $H_2O_2$.| Example: Glycerine in the decomposition of $H_2O_2$.|

### **4. Give reasons:**
**a. An acid-base reaction is called a neutralization reaction.**
**Answer:** Because in this reaction, the acidic properties of the acid and the basic properties of the base cancel or "neutralize" each other, resulting in the formation of a neutral salt and water.

**b. Paper burning to ash is a chemical change.**
**Answer:** Because a new substance (ash, carbon dioxide, etc.) with entirely different properties is formed. This change is permanent and irreversible; you cannot turn the ash back into paper.

**c. Powdered marble reacts faster with acid than marble chips.**
**Answer:** Powdered marble has a much larger surface area than marble chips. This allows more particles of the acid to come into contact and react with the marble simultaneously, leading to a faster reaction rate.

### **5. Change into balanced chemical equations:**
**a. Hydrogen + Chlorine $\rightarrow$ Hydrogen chloride**
**Ans:** $H_2 + Cl_2 \rightarrow 2HCl$
**b. Potassium chlorate $\rightarrow$ Potassium chloride + Oxygen**
**Ans:** $2KClO_3 \rightarrow 2KCl + 3O_2$
**c. Aluminium + Sulphuric acid $\rightarrow$ Aluminium sulphate + Hydrogen**
**Ans:** $2Al + 3H_2SO_4 \rightarrow Al_2(SO_4)_3 + 3H_2$
**d. Iron + Oxygen $\rightarrow$ Iron(III) oxide (rust)**
**Ans:** $4Fe + 3O_2 \rightarrow 2Fe_2O_3$
**e. Water + Carbon dioxide $\rightarrow$ Glucose + Oxygen (Photosynthesis)**
**Ans:** $6H_2O + 6CO_2 \rightarrow C_6H_{12}O_6 + 6O_2$

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## ✅ 7. CHAPTER CHECKLIST & EXAM TIPS
**Exam Hotspots 🔥:**
-   **Balancing Equations:** This is a guaranteed question. Practice the M-N-O-H rule.
-   **Types of Reactions:** Be ready to define and give one example for each of the four types. Single and Double displacement are very common.
-   **Factors Affecting Rate:** The "Give Reason" questions often come from this topic (e.g., why powdered salt dissolves faster).

**Your Pre-Exam Checklist:**
- [ ] I can define a chemical reaction.
- [ ] I can balance any given chemical equation.
- [ ] I can identify the four main types of reactions.
- [ ] I can explain the difference between exothermic and endothermic reactions.
- [ ] I can list and explain at least three factors affecting the reaction rate.
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## 🎊 CONCLUSION
And that's a wrap on Chemical Reactions! You've learned the secret language of chemists. Remember, chemistry is all about patterns. Once you understand the basic types and rules, you can predict how thousands of different substances will interact. Use these notes, practice the balancing trick, and you'll be all set to ace this chapter in your SEE exams.
**Your Formula for Success:** Practice + Understanding Analogies + Revision = Top Marks!