Acids, Bases and Salts - Science Chapter 1 PDF

## Acids, Bases and Salts ### What you have learnt - Acid-base indicators are dyes or mixtures of dyes which are used to indicate the presence of acids and bases. - Acidic nature of a substance is due to the formation of H+(aq) ions in solution. - Formation of OH-(aq) ions in solution is respons...

## Acids, Bases and Salts ### What you have learnt - Acid-base indicators are dyes or mixtures of dyes which are used to indicate the presence of acids and bases. - Acidic nature of a substance is due to the formation of H+(aq) ions in solution. - Formation of OH-(aq) ions in solution is responsible for the basic nature of a substance. - When an acid reacts with a metal, hydrogen gas is evolved and a corresponding salt is formed. - When a base reacts with a metal, along with the evolution of hydrogen gas a salt is formed which has a negative ion composed of the metal and oxygen. - When an acid reacts with a metal carbonate or metal hydrogencarbonate, it gives the corresponding salt, carbon dioxide gas and water. - Acidic and basic solutions in water conduct electricity because they produce hydrogen and hydroxide ions respectively. ### 2.4 More About Salts #### 2.4.1 Family of Salts - Write the chemical formulae of the salts given below: - Potassium sulphate - Sodium sulphate - Calcium sulphate - Magnesium sulphate - Copper sulphate - Sodium chloride - Sodium nitrate - Sodium carbonate - Ammonium chloride #### 2.4.2 pH of Salts - Collect the following salt samples: - Sodium chloride - Potassium nitrate - Aluminium chloride - Zinc sulphate - Copper sulphate - Sodium acetate - Sodium carbonate - Sodium hydrogencarbonate - Check the solubility in water (use distilled water only). - Check the action of these solutions on litmus and find the pH using a pH paper. - Which of the salts are acidic, basic or neutral? - Identify the acid or base used to form the salt. | Salt | pH | Acid used | Base used | | ----- | ----- | ----- | ----- | | | | | | | | | | | | | | | | #### 2.4.3 Chemicals from Common Salt - By now you have learnt that the salt formed by the combination of hydrochloric acid and sodium hydroxide solution is called sodium chloride. This is the salt that we use in food. - Seawater contains many salts dissolved in it. Sodium chloride is separated from these salts. Deposits of sold salt are also found in several parts of the world. These large crystals are often brown due to impurities. This is called rock salt. Beds of rock salt were formed when seas of bygone ages dried up. Rock salt is mined like coal. - You must have heard about Mahatma Gandhi's Dandi March. Did you know that sodium chloride was such an important symbol in our struggle for freedom? #### 2.4.4 Are the Crystals of Salts really Dry? - Heat a few crystals of copper sulphate in a dry boiling tube. - What is the colour of the copper sulphate after heating? - Do you notice water droplets in the boiling tube? Where have these come from? - Add 2-3 drops of water on the copper sulphate obtained after heating. What do you observe? Is the colour of copper sulphate restored? ### What does 10H₂O signify? Does it make Na₂CO₃ wet? - We will address this question in the next section. - Sodium carbonate and sodium hydrogencarbonate are used as chemicals for many industrial processes as well. #### Uses of washing soda - Sodium carbonate (washing soda) is used in paper industries. - It in used in the manufacturer of sodium compounds such as sodium bicarbonate. - Sodium carbonate can be used as a cleaning agent for domestic purposes. - It is used for removing permanent hardness of water. ### 2.5 HOW STRONG ARE ACID OR BASE SOLUTIONS? - We know how acid-base indicators can be used to distinguish between an acid and a base. We have also learnt in the previous section about dilution and decrease in concentration of H* or OH- ions in solutions. - Can we quantitatively find the amount of these ions present in a solution? - Can we judge how strong a given acid or base is? - We can do this by making use of a universal indicator, which is a mixture of several indicators. The universal indicator shows different colours at different concentrations of hydrogen ions in a solution. - A scale for measuring hydrogen ion concentration in a solution, called pH scale has been developed. The p in pH stands for 'potenz' in German, meaning 'power.' On the pH scale we can measure pH generally from 0 (very acidic) to 14 (very alkaline). pH should be thought of simply as a number which indicates the acidic or basic nature of a solution. Higher the hydronium ion concentration, lower is the pH value. - The pH of a neutral solution is 7. Values less than 7 on the pH scale represent an acidic solution. As the pH value increases from 7 to 14, it represents an increase in OH- ion concentration in the solution, that is, increase in the strength of alkali (Fig. 2.6). Generally paper impregnated with the universal indicator is used for measuring pH. ### 2.6 WHAT DO ALL ACIDS AND ALL BASES HAVE COMMON? - In section 2.1 we have seen that all acids have similar chemical properties. What leads to this similarity in properties? We saw in Activity 2.3 that all acids generate hydrogen gas on reacting with metals. Hydrogen seems to be common to all acids. Let us perform an activity to investigate whether all compounds containing hydrogen are acidic. ### 2.1 UNDERSTANDING THE CHEMICAL PROPERTIES OF ACIDS AND BASES #### 2.1.1 Acids and Bases in the Laboratory - Collect the following solutions from the science laboratory. - Hydrochloric acid (HCl) - Sulphuric acid (H₂SO₄) - Nitric acid (HNO₃) - Acetic acid (CH₃COOH) - Sodium hydroxide (NaOH) - Calcium hydroxide [Ca(OH)₂] - Potassium hydroxide (KOH) - Magnesium hydroxide [Mg(OH)₂] - Ammonium hydroxide (NH₄OH) - Put a drop of each of the above solutions on a watch glass one by one and test with a drop of the indicators shown in Table 2.1. What change in colour did you observe with red litmus, bue litmus, phenolphthalein and methyl orange solutions for each of the solutions taken? | Sample solution | Red litmus solution | Blue litmus solution | Phenolphthalein solution | Methyl orange solution | | ----- | ----- | ----- | ----- | ----- | | | | | | | | | | | | | | | | | | | - These indicators tell us whether a substance is acidic or basic by change in colour. There are some substances whose odour changes in acidic or basic media. These are called olfactory indicators. Let us try out some of these indicators. #### 2.1.2 How do Acids and Bases React with Metals? - CAUTION: This activity needs the teacher's assistance. - Set the apparatus as shown in Fig. 2.1. - Take about 5 mL of dilute sulphuric acid in a test tube and add a few pieces of zinc granules to it. - What do you observe on the surface of zinc granules? - Pass the gas being evolved through the soap solution. Why are bubbles formed in the soap solution? - Take a burning candle near a gas filled bubble. What do you observe? - Repeat this Activity with some more acids like HCl, HNO₃, and CH₃COOH. Are the observations in all the cases the same or different? #### 2.1.3 How do Metal Carbonates and Metal Hydrogencarbonates React with Acids? - Take two test tubes, label them as A and B. - Take about 0.5 g of sodium carbonate (Na₂CO₃) in test tube A and about 0.5 g of sodium hydrogencarbonate (NaHCO₃) in test tube B. - Add about 2 mL of dilute HCl to both the test tubes. What do you observe? - Pass the gas produced in each case through lime water (calcium hydroxide solution) as shown in Fig. 2.2 and record your observations. - The reactions occurring in the above Activity are written as: - Test tube A: Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g) - Test tube B: NaHCO₃(s) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g) - On passing the carbon dioxide gas evolved through lime water: - Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l) #### 2.1.4 How do Acids and Bases React with each other? - Take about 2 mL of dilute NaOH solution in a test tube and add two drops of phenolphthalein solution. What is the colour of the solution? - Add dilute HCl solution to the above solution drop by drop. Is there any colour change for the reaction mixture? Why did the colour of phenolphthalein change after the addition of an acid? - Now add a few drops of NaOH to the above mixture. Does the pink colour of phenolphthalein reappear? Why do you think this has happened? - In the above Activity, we have observed that the effect of a base is nullified by an acid and vice-versa. The reaction taking place is written as: NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l) - The reaction between an acid and a base to give a salt and water is known as a neutralisation reaction. In general, a neutralisation reaction can be written as: - Base + Acid → Salt + Water #### 2.1.5 Reaction of Metallic Oxides with Acids - Take a small amount of copper oxide in a beaker and add dilute hydrochloric acid slowly while stirring. Note the colour of the solution. What has happened to the copper oxide? - You will notice that the colour of the solution becomes blue-green and the copper oxide dissolves. The blue-green colour of the solution is due to the formation of copper(II) chloride in the reaction. The general reaction between a metal oxide and an acid can be written as: - Metal oxide + Acid → Salt + Water #### 2.1.6 Reaction of a Non-metallic Oxide with Base - You saw the reaction between carbon dioxide and calcium hydroxide (lime water) in Activity 2.5. Calcium hydroxide, which is a base, reacts with carbon dioxide to produce a salt and water. Since this is the reaction between a base and an acid, we can conclude that non-metallic oxides are acidic in nature. ### 2.2 WHAT DO ALL ACIDS AND ALL BASES HAVE COMMON? - In section 2.1 we have seen that all acids have similar chemical properties. What leads to this similarity in properties? We saw in Activity 2.3 that all acids generate hydrogen gas on reacting with metals. Hydrogen seems to be common to all acids. Let us perform an activity to investigate whether all compounds containing hydrogen are acidic. #### 2.2.1 What Happens to an Acid or a Base in a Water Solution? - Do acids produce ions only in aqueous solution? Let us test this. #### 2.3 HOW STRONG ARE ACID OR BASE SOLUTIONS? - We know how acid-base indicators can be used to distinguish between an acid and a base. We have also learnt in the previous section about dilution and decrease in concentration of H* or OH- ions in solutions. - Can we quantitatively find the amount of these ions present in a solution? - Can we judge how strong a given acid or base is? - We can do this by making use of a universal indicator, which is a mixture of several indicators. The universal indicator shows different colours at different concentrations of hydrogen ions in a solution. - A scale for measuring hydrogen ion concentration in a solution, called pH scale has been developed. The p in pH stands for 'potenz' in German, meaning 'power.' On the pH scale we can measure pH generally from 0 (very acidic) to 14 (very alkaline). pH should be thought of simply as a number which indicates the acidic or basic nature of a solution. Higher the hydronium ion concentration, lower is the pH value. - The pH of a neutral solution is 7. Values less than 7 on the pH scale represent an acidic solution. As the pH value increases from 7 to 14, it represents an increase in OH- ion concentration in the solution, that is, increase in the strength of alkali (Fig. 2.6). Generally paper impregnated with the universal indicator is used for measuring pH. ### 2.5 COMMON SALT - A RAW MATERIAL FOR CHEMICALS - The common salt thus obtained is an important raw material for chemicals of daily use, such as sodium hydroxide, baking soda, soda, bleaching powder and many more. It is used for making all these different substances. #### Sodium hydroxide - When electricity is passed through an aqueous solution of sodium chloride (called brine), it decomposes to form sodium hydroxide and chlorine gas. This process is called the chlor-alkali process because of the products- chlor for chlorine and alkali for sodium hydroxide. - 2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g) - Chlorine gas is given off at the anode, and hydrogen gas at the cathode. The sodium hydroxide solution is formed near the cathode. The products produced in this process are all useful. Figure 2.8 shows different uses of these products. #### Bleaching powder - You have already come to know that chlorine is produced during electrolysis of aqueous sodium chloride (brine). This chlorine gas is used for the manufacture of bleaching powder. Bleaching powder is produced by the action of chlorine on dry slaked lime [Ca(OH)₂]. Bleaching powder is represented as CaOCl₂, though the actual composition is quite complex. - Ca(OH)₂ + Cl₂→ CaOCl₂ + H₂O - Bleaching powder is used: - for bleaching cotton and linen in the textile industry, for bleaching wood pulp in paper factories and for bleaching washed clothes in laundry - as an oxidising agent in may chemical industries - to make drinking water free from germs. #### Baking soda - The baking soda is commonly used in the kitchen for making tasty crispy cakes, cookies, breads etc. Sometimes it is added for faster cooking. The chemical name of the compound is sodium hydrogencarbonate (NaHCO₃). It is produced using sodium chloride as one of the raw materials. - NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃ - Did you check the pH of sodium hydrogencarbonate in Activity 2.14? Can you correlate why it can be used to neutralise an acid? It is a mild non-corrosive basic salt. The following reaction takes place when it is heated during cooking. - 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂ - Sodium hydrogencarbonate has got various uses in the household. - For making baking powder, which is a mixture of baking soda (sodium hydrogencarbonate) 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 can cause bread or cake to rise making them soft and spongy. - Sodium hydrogencarbonate is also an ingredient in antacids. Being alkaline, it neutralises excess acid in the stomach and provides relief. - It is also used in soda-acid fire extinguishers. #### Washing soda - Another chemical that can be obtained from sodium chloride is Na₂CO₃.10H₂O (washing soda). You have seen above that sodium carbonate can be obtained by heating baking soda; recrystallisation of sodium carbonate gives washing soda. It is also a basic salt. - Na₂CO₃ + 10H₂O -> Na₂CO₃.10H₂O