Class 10 Science Unit 2: Acids, Bases and Salts
The chapter Acids, Bases and Salts is one of the most practical and interesting topics in Class 10 Science because it connects chemistry with everyday life. We use acids, bases, and salts in food, medicine, cleaning, agriculture, laboratories, and industry. From the sour taste of lemon to the cleaning action of soap, from antacids used for acidity to the salt we eat every day, this chapter explains many familiar things in a scientific way. It also gives students the basic language needed to understand chemical properties, indicators, neutralization, pH, common laboratory preparations, and the use of important salts.
This chapter is important because it is not only about memorizing definitions. It is about understanding how substances behave in water, how acids and bases differ from each other, how indicators help identify them, how reactions produce salts and water, and how pH affects daily life, health, soil, and the environment. Once these ideas are clear, the chapter becomes logical and easy to revise. Many board questions come from this unit because it contains direct theory, observations from experiments, chemical equations, and application-based concepts.
What Are Acids?
Acids are substances that taste sour and turn blue litmus red. In chemistry, acids are substances that produce hydrogen ions or hydronium ions in aqueous solution. A substance behaves as an acid only when it dissolves in water and releases ions. This is why water plays a very important role in acid-base chemistry. A dry acid does not show its acidic behaviour until it is mixed with water.
Common examples of acids found in daily life include lemon juice, curd, vinegar, tamarind, orange juice, and tomatoes. These substances contain acids such as citric acid, lactic acid, acetic acid, and ascorbic acid. Although these are weak acids, they still show acidic properties.
Acids are important in both nature and industry. Some acids are strong and dangerous, such as sulphuric acid, nitric acid, and hydrochloric acid. These are used in laboratories and industries with caution. Strong acids can be corrosive and can damage skin, clothing, and metal surfaces.
What Are Bases?
Bases are substances that taste bitter and feel soapy to touch. They turn red litmus blue. In chemistry, bases are substances that produce hydroxide ions in aqueous solution. Like acids, bases show their properties only in the presence of water.
Common examples of bases include lime water, soap solution, sodium hydroxide, potassium hydroxide, and magnesium hydroxide. Some bases dissolve in water completely and are called alkalis. So every alkali is a base, but not every base is an alkali. This distinction is important.
Bases are used in soaps, detergents, antacids, cleaning agents, and certain industrial processes. Some bases are strong and dangerous, especially sodium hydroxide and potassium hydroxide. These must be handled carefully because they can cause burns.
What Are Salts?
Salts are substances formed when the hydrogen ion of an acid is replaced by a metal or an ammonium ion. In a broader sense, salts are ionic compounds produced by the reaction between acids and bases. Many salts are familiar in daily life, the most common being common salt, or sodium chloride.
Salts may be neutral, acidic, or basic depending on the strength of the acid and base from which they are formed. Some salts are used in cooking, some in medicine, and some in industry. Salts are very important in chemistry because they are produced in many reactions and have many uses.
Natural and Synthetic Indicators
Since acids and bases often look similar, we use indicators to identify them. Indicators are substances that change colour in acidic and basic solutions. They help us test whether a solution is acidic or basic.
Litmus
Litmus is a natural indicator obtained from lichens. It is commonly available as blue litmus and red litmus paper. Blue litmus turns red in acid, while red litmus turns blue in base. Litmus is one of the simplest and most commonly used indicators.
Phenolphthalein
Phenolphthalein is a synthetic indicator. It remains colourless in acidic solution and turns pink in basic solution. It is useful in laboratory experiments, especially titration.
Methyl Orange
Methyl orange is another synthetic indicator. It turns red in acid and yellow in base. It is also widely used in laboratory testing.
Turmeric, China Rose, and Other Natural Indicators
Turmeric turns reddish-brown in basic medium and remains yellow in acid. China rose indicator turns dark pink in acid and green in base. These natural indicators are useful because they can be obtained from plants and are easy to understand in school experiments.
Chemical Nature of Acids and Bases in Water
Acids and bases show their behaviour in water because water enables the release of ions. When hydrochloric acid dissolves in water, it produces hydrogen ions, which combine with water molecules to form hydronium ions. Similarly, sodium hydroxide dissociates in water to produce hydroxide ions.
This is why dry HCl gas does not change dry litmus paper, but hydrochloric acid solution does. The presence of water is essential for ion formation and therefore for acidic and basic behaviour. This concept is one of the central ideas of the chapter.
It is also important to note that acids should always be added to water slowly and carefully, not water to acid. This is because the process releases heat, and adding water to concentrated acid can cause splashing and danger.
Reaction of Acids and Bases with Metals
Acids react with many metals to produce a salt and hydrogen gas. This is a characteristic property of acids. For example, zinc reacts with dilute hydrochloric acid to form zinc chloride and hydrogen gas.
Similarly, some bases also react with certain metals like zinc or aluminium to produce salt and hydrogen gas. Such reactions are important in metallurgy and chemical testing.
In laboratory experiments, the gas evolved can be tested using a burning splint. If hydrogen is present, it burns with a pop sound. This is one of the identifying tests for hydrogen gas.
Reaction of Acids and Bases with Carbonates and Bicarbonates
Acids react with carbonates and bicarbonates to produce salt, water, and carbon dioxide gas. This reaction is very important because it helps identify carbonates and explains why effervescence occurs when acid is added to baking soda or marble chips.
Example:
NaHCO3 + HCl → NaCl + H2O + CO2
Carbon dioxide turns lime water milky, so this gas can be identified by passing it through lime water. This is a common laboratory test. The reaction of acids with carbonates is one of the most frequently tested concepts in the chapter.
Neutralization
When an acid reacts with a base, the two substances cancel each other’s acidic and basic properties and produce salt and water. This process is called neutralization. It is one of the most important reactions in everyday chemistry.
Example:
HCl + NaOH → NaCl + H2O
In this reaction, hydrochloric acid and sodium hydroxide neutralize each other. Neutralization is a double displacement reaction. It releases heat and is therefore exothermic. This concept is used in stomach antacids, soil treatment, and industrial processes.
Neutralization has many applications. If soil becomes too acidic, farmers add basic substances to improve it. If bee stings or ant stings make the skin acidic or painful, mild bases may help. If the stomach produces too much acid, antacids help neutralize it. This makes the concept practical and relevant.
pH Scale
The pH scale is used to measure how acidic or basic a substance is. It is an important tool in chemistry, agriculture, medicine, and environmental science. The pH scale usually ranges from 0 to 14. A pH less than 7 indicates acidity, a pH of 7 indicates neutrality, and a pH greater than 7 indicates basicity.
The lower the pH, the stronger the acid. The higher the pH above 7, the stronger the base. A pH value gives more precise information than a litmus test because it tells the degree of acidity or basicity.
pH can be measured using pH paper, universal indicator, or electronic pH meters. Universal indicator gives different colours for different pH values, making it useful for school-level testing.
Importance of pH in Daily Life
The pH scale is not only a laboratory concept. It affects many aspects of everyday life. Human blood has a narrow pH range, and even small changes can be dangerous. The pH of the stomach must be acidic for digestion. The pH of the soil affects plant growth. The pH of water bodies affects fish and aquatic life. Thus, pH is a practical scientific tool.
pH in the Human Body
Blood has a slightly basic pH around 7.4. If it changes too much, the body cannot function properly. The stomach contains hydrochloric acid, which helps digest food. Sometimes excess acid causes acidity, which can be relieved by antacids. Tooth enamel can be damaged if the mouth becomes too acidic. That is why oral hygiene is important.
pH in Soil
Plant growth depends on the pH of soil. Some plants grow best in slightly acidic soil, while others prefer neutral or slightly alkaline soil. Farmers test soil pH to improve crop yield. If the soil is too acidic, lime is added. If it is too basic, organic matter or other corrective substances may be used.
pH of the Environment
Acid rain is one of the major environmental applications of pH. When rainwater becomes too acidic due to dissolved pollutants like sulphur dioxide and nitrogen oxides, it can damage plants, buildings, and aquatic life. pH also matters in lakes, rivers, and oceans because living organisms are sensitive to changes in acidity.
Common Salt and Its Uses
Common salt, or sodium chloride, is one of the most familiar salts in daily life. It is obtained from seawater and natural salt deposits. It is not only used as table salt but also as a starting material for many important chemicals. Common salt is called the “mother substance” of many salts because several useful compounds are prepared from it.
Common salt is used in food, food preservation, industry, and laboratory preparation of chemicals. It is the raw material for washing soda, baking soda, bleaching powder, and caustic soda. This makes it extremely important in both domestic and industrial chemistry.
Important Salts and Their Preparation
1. Sodium Hydroxide
Sodium hydroxide is prepared by electrolysis of brine, which is a solution of common salt in water. During electrolysis, chlorine gas is released at the anode, hydrogen gas at the cathode, and sodium hydroxide remains in solution.
2NaCl + 2H2O → 2NaOH + Cl2 + H2
Sodium hydroxide is also called caustic soda. It is used in soap making, paper industry, textiles, and detergents. It is a strong base and must be handled carefully.
2. Bleaching Powder
Bleaching powder is prepared by the reaction of chlorine gas with dry slaked lime.
Ca(OH)2 + Cl2 → CaOCl2 + H2O
It is used for bleaching cotton and linen, disinfecting drinking water, and as an oxidizing agent. It releases chlorine slowly and is therefore useful in cleaning and sanitation.
3. Baking Soda
Baking soda is sodium hydrogen carbonate, NaHCO3. It is prepared from brine.
NaCl + H2O + CO2 + NH3 → NaHCO3 + NH4Cl
Baking soda is used in baking cakes and bread because it releases carbon dioxide and makes dough soft and spongy. It is also used in antacid formulations and soda-acid fire extinguishers.
4. Washing Soda
Washing soda is sodium carbonate decahydrate, Na2CO3·10H2O. It is prepared from sodium hydrogen carbonate.
2NaHCO3 → Na2CO3 + CO2 + H2O
Washing soda is used in cleaning, softening hard water, and manufacturing glass, soap, and paper. It is an important industrial salt.
5. Plaster of Paris
Plaster of Paris is calcium sulphate hemihydrate, CaSO4·1/2H2O. It is obtained by heating gypsum.
CaSO4·2H2O → CaSO4·1/2H2O + 3/2H2O
It is used for setting fractured bones, making moulds, and decorative items. When mixed with water, it hardens again into gypsum. This property makes it very useful.
Plaster of Paris and Gypsum
Gypsum is a naturally occurring hydrated salt. When heated, it loses part of its water and becomes Plaster of Paris. When water is added back, it sets into a hard mass again. This reversible hydration-dehydration process is very useful in medical and construction work.
Acid-Base Reactions in Industry and Home
The chapter is full of real-life examples. Acids are used in cleaning and food processing. Bases are used in soaps, detergents, and antacids. Salts are used in baking, water treatment, bleaching, glass making, textiles, and medicine. Understanding this chapter therefore helps students understand the chemistry of daily life.
Precautions in Handling Acids and Bases
Strong acids and bases can be harmful. They can cause burns and damage eyes, skin, and clothing. They should always be handled carefully in the laboratory. Acid must be added to water slowly, not water to acid. Goggles, gloves, and lab safety rules are very important.
Students should remember that concentrated acids and bases behave more strongly than dilute solutions. Safety is as important as knowledge in chemistry.
Important Definitions
- Acid: A substance that produces hydrogen ions in water and turns blue litmus red.
- Base: A substance that produces hydroxide ions in water and turns red litmus blue.
- Salt: A compound formed when the hydrogen of an acid is replaced by a metal or ammonium ion.
- Indicator: A substance that changes colour in acidic or basic medium.
- Neutralization: The reaction between an acid and a base to form salt and water.
- pH: A measure of the acidity or basicity of a solution.
- Alkali: A base that dissolves in water.
- Corrosion: Slow damage to materials due to reaction with air or moisture.
- Rancidity: Spoilage of fat and oil containing food due to oxidation.
Class 10 Science Unit 2 Notes PDF
📄 Download PDFExam-Oriented Revision Points
Students should learn the meaning of acids, bases, and salts clearly. They should know the colours of litmus, phenolphthalein, methyl orange, and natural indicators. They should remember common equations for neutralization, reactions with metals, reactions with carbonates, and preparation of important salts such as baking soda, washing soda, bleaching powder, and Plaster of Paris. The pH scale and its uses are also very important.
In exam answers, it is helpful to write the equation, observation, and type of reaction wherever relevant. For example, if asked about neutralization, mention the acid, base, salt, water, and heat. If asked about pH, explain its range and application in body, soil, and environment. If asked about salts, mention their preparation and uses.
This chapter is scoring because many answers are direct and fact-based. But it also requires understanding. A student who can connect the chemistry with daily life will find the chapter easier and more interesting.
Conclusion
Acids, Bases and Salts is one of the most useful chapters in Class 10 Science because it explains substances that we use every day. It teaches how to identify acids and bases, how reactions take place, how salts are formed, and how pH affects life and environment. It also explains the importance of neutralization and the industrial use of many common salts. The chapter combines theory, practical observations, and applications in a way that makes chemistry meaningful.
Once students understand this unit properly, many later topics become easier. It builds confidence in chemical formulas, laboratory reactions, and real-life applications. More importantly, it shows that chemistry is not separate from life. It is present in the food we eat, the medicines we take, the soil we grow crops in, and the products we use every day. That is what makes this chapter essential and memorable.
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