Class X - NCERT (CBSE) Science (Chemistry)
Chapter 2, Acids, Bases and Salts
Chemistry (Science) Notes
Some Points to Remember
=> Olfactory indicators are some substances whose odour changes in acidic or basic media.
=> Acid + Metal --> Salt + H2
=> Metals react with bases and replace the hydrogen to form salt.
Base + Metal ---> Salt + H2 (however, all metals do not react with bases).
=> Acids react with metal oxides to produce metal salts.
Metal oxide + Acid ---> Salt + Water. (Since, this reaction is similar to the reaction of bases with acids so, metal oxides are also known as basic oxides).
=> Non-metallic oxides react with bases to produce salt and water. Since, this reaction is similar to the reaction of acids with bases so, non-metal oxides are also known as acidic oxides.
=> pH is the measure of Acidity or Alkalinity of a solution. The term pH stands for “Potential of hydrogen”.
=> If the pH rain water drops below 5.6, it is called acid rain.
=> Acid rain corrodes buildings made with marble such as, Taj Mahal. Marble (CaCO3) and stone generally contains metal carbonates which react with various acids like - Nitric Acid, Sulphuric Acid etc. present in the acid rain.
=> Neutralization Reaction: The process of treating an acid with a base to form salt and water is called neutralization. a neutralization reaction can be written as,
Acid + Base ---> Salt + Water + Heat
Hence, it is an exothermic process.
=> Alkalies: The bases that dissolve in water are known as alkalies. All bases are not alkalies, but all alkalies are bases.
=> Most reactions in human bodies take place in a pH range of 7.0 - 7.8 Living organisms can survive only in a narrow range of pH change.
Q1: Define (i) Acid (ii) Bases (iii) Salt
Answer: Acid: A substance which liberates hydrogen ions in aqueous solution, sour in taste, and turns blue litmus to red. Metals react with acids to form salts and liberate hydrogen gas. Many acids are corrosive such as, HNO3, H2SO4, HF, etc.
Base: A substance which reacts with an acid to form a salt and water only. If dissolved in water, they give hydroxyl ions (OH–) ions. Bases are bitter in taste and change the colour of red litmus to blue.
Salt: a chemical compound formed when the hydrogen of an acid has been replaced by a metal. a salt is produced together with water, when an acid reacts with a base. Salts are named according to the acid and the metal from which the salt is derived. For example, ‘sodium sulphate’ is a salt derived from sodium metal and sulphuric acid.
Q2: What is Universal Indicator?
Answer: Universal Indicator is a mixture of several indicators. The universal indicator shows different colours at different concentration of hydrogen ions in a solution. Thus, with the help of a universal indicator we can quantitatively judge how strong a given acid or base is.
Q3: Why is it advised to clean mouth after consuming food? or
pH change is a cause for toot decay. Explain.
Answer: Acids are produced in the mouth due to the degradation of sugar and food particles which remain in mouth after eating, by certain bacteria. As a result the pH of mouth is lowered. Tooth enamel made up of calcium phosphate is corroded when the pH in the mouth is lower than 5.5 So, tooth decay starts when the pH of the decreases below 5.5
Therefore, it is advised to clean the mouth after eating food with toothpastes which are generally basic.
Q4: How sodium hydroxide is produced? or
What is ‘Chlor-alkali’ process? or
What do we obtain by electrolysis of brine?
Answer: Sodium hydroxide is produced by electrolysis of aqueous solution of sodium chloride (NaCl) or Brine. electrolysis of brine results in the decomposition of NaCl and formation of NaOH. This process is called ‘chlor-alkali’ process because the products formed - chlor for chlorine and alkali for sodium hydroxide.
2NaCl (aq) + 2H2O(liq) --> 2NaOH (aq) + Cl2(g) + H2(g)
Chlorine is given off at the anode while hydrogen at cathode.
Q5: Give some uses of sodium hydroxide.
Answer: Sodium hydroxide is a very useful substance. It is used in the manufacturing of paper, soaps, detergents, and synthetic fibers. It is also useful in the manufacturing of house-hold bleaches and dyes.
Q6: How bleaching powder is formed? Give some uses of bleaching powder.
Answer: Chlorine gas obtained by the electrolysis of brine is used for the manufacturing of bleaching powder. Bleaching powder is produced by the action of chlorine on dry slaked lime [Ca(OH)2] as per the following equation -
Ca(OH)2 + Cl2 --> CaOCl2 + H2O
Uses of Bleaching Powder
1. Disinfecting drinking water to make it free from germs.
2. As an oxidizing agent in many chemical industries.
3. As a bleaching agent in textile industry, paper industry, etc.
Q8: Give the chemical equation for the production of baking soda.
Answer: The chemical name of baking soda is Sodium Hydrogen Carbonate (NaHCO3). It is produced using sodium chloride as one of the raw materials as per the given equation -
NaCl + H2O + CO2 + NH3 --> NH4Cl + NaHCO3
Q9: What is baking powder?
Answer: Baking Powder is a mixture of baking soda and a mild edible acid such as tartaric acid. When baking powder is heated or mixed in water, the following reaction takes place -
NaHCO3 + H+ --> CO2 + H2O + sodium salt of acid.
Carbon dioxide produced during the reaction causes bread or cake to rise making them soft and spongy.
Q10: What happens when we heat sodium hydrogen carbonate (baking soda)?
Answer: By heating sodium hydrogen carbonate we get sodium carbonate (Na2CO3).
The sodium carbonate after recrystallization produces washing soda.
Na2CO3 + 10H2O --> Na2CO3.10H2O
Q11: What is ‘water of crystallization’?
Answer: Water of recrystallization is a fixed number of water molecules present in one formula unit of salt. For example, in washing soda (Na2CO3.10H2O) there are 10 water molecules present as water of crystallization.
The water of crystallization remain in chemical combination with crystal. It is necessary for the maintenance of crystalline properties of the crystal. It can be removed by sufficient heat.
Q12: What are hydrated salts? Give an example.
Answer: Salts containing a fixed number of water molecules in their crystal structure are called hydrated salts.
A molecule of sodium carbonate (Na2CO3.10H2O) contains ten molecules of water. This is known as hydrated salt of sodium carbonate.
Q13: Why does an indicator change its colour in an acidic or basic solution?
Answer: An indicator is itself a weak acid or base. When added to an acidic or basic solution, it accepts or loses protons de[pending on the conditions, and this rearranges the distribution of electrons of its molecules. The colour of the indicator changes because of this redistribution.
Q14: Name some chemicals obtained by using sodium chloride (common salt) as a raw material.
Answer: Sodium hydroxide (NaOH), Baking soda (NaHCO3), Washing soda (Na2CO3.10H2O), Bleaching powder (CaOCl2), etc.
Q15: What are strong and weak acids?
Answer: acids that give rise to more H+ ions are said to be strong acids and acids that give less H+ ions are said to be weak acids. a strong acid has pH value closer to zero while acids with higher pH values or closer to 7 are weak acids.
Q16: What is ‘Plaster of Paris’?
Answer: On heating gypsum at 3730 K, it loses water molecules and becomes calcium sulphate hemihydrates (CaSO4.1/2 H2O). This is called ‘Plaster of Paris’.
Further Study on Chapter 2, Acids, Bases and Salts
- 10th CBSE Science (Chemistry) | Chapter 2, Acids, Bases and Salts | NCERT Science Textbook Exercise Solution [Read]
- CBSE Xth Science (Chemistry) | Acids, Bases and Salts | NCERT Solutions for In-Text Questions [Read]
- CBSE Class 10 NCERT Science (Chemistry) - Multiple Choice Questions (mcq) | Chapter 2, Acids, Bases and Salts [Read]