[h]Acids, Bases, and Salts][/h]
This chapter is about: Reaction of acids and bases, how acids and bases cancel out each other’s effects and many more interesting things that we use and see in our day-to-day life.
- 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.
How do Acids and Bases React with Metals?
- Acids reacts with metals to give salt and hydrogen gas.
- Acid + Metal → Salt + Hydrogen gas
How do Metal Carbonates and Metal Hydrogencarbonates React with Acids?
- All metal carbonates and hydrogencarbonates react with acids to give a corresponding salt, carbon dioxide and water.
Thus, the reaction can be summarised as –Metal carbonate/Metal hydrogencarbonate + Acid → Salt + Carbon dioxide + Water
- Limestone, chalk and marble are different forms of calcium carbonate.
How do Acids and Bases React with each other?
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
Example: NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
Reaction of Metallic Oxides with Acids
Metallic oxides react with acids to give salts and water, similar to the reaction of a base with an acid, metallic oxides are said to be basic oxides.
Metal oxide + Acid → Salt + Water
Reaction of a Non-metallic Oxide with Base
- This is similar to the reaction between a base and an acid.
- Non-metallic Oxide reacts with Base to give salt and water.
- Since this is similar to the reaction between a base and an acid, we can conclude that non-metallic oxides are acidic in nature.
[h]THINGS ALL ACIDS AND ALL BASES HAVE IN COMMON[/h]
- Acids produce hydrogen ions, H+(aq), in solution, which are responsible for their acidic properties.
- hydrogen seems to be common to all acids.
- acids give H3O+ or H+(aq) ion in water.
[h]What Happens to an Acid or a Base in a Water Solution[/h]
- Acids give H3O+ or H+(aq) ion in water.
- What happens to Base in water ? bases generate hydroxide (OH–) ions in water. Bases which are soluble in water are called alkalis.
- Water soluble bases are called alkalis.
The process of dissolving an acid or a base in water is a highly exothermic one. Care must be taken while mixing concentrated nitric acid or sulphuric acid with water. The acid must always be added slowly to water with constant stirring. If water is added to a concentrated acid, the heat generated may cause the mixture to splash out and cause burns. The glass container may also break due to excessive local heating.
Mixing an acid or base with water results in decrease in the concentration of ions (H3O+/OH–) per unit volume. Such a process is called dilution and the acid or the base is said to be diluted.
[h] Strength of Acid and base solution[/h]
The strength of Acid and base solution can be found using universal indicators .
Universal indicators: universal indicator, is a mixture of several indicators. The universal indicator shows different colours at different concentrations of hydrogen ions in a solution.
pH scale :
- 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 .
- Generally paper impregnated with the universal indicator is used for measuring pH.
- The strength of acids and bases depends on the number of H+ ions and OH– ions produced, respectively.
- Weak acids and Strong acids: 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.
[h]Importance of pH in Everyday Life[/h]
- Our body works within the pH range of 7.0 to 7.8.
- When pH of rain water is less than 5.6, it is called acid rain. When acid rain flows into the rivers, it lowers the pH of the river water. The survival of aquatic life in such rivers becomes difficult.
pH in our digestive system
Our stomach produces hydrochloric acid. It helps in the digestion of food without harming the stomach. During indigestion the stomach produces too much acid and this causes pain and irritation.
- To get rid of this pain, people use bases called antacids.
- These antacids neutralise the excess acid. Magnesium hydroxide (Milk of magnesia), a mild base, is often used for this purpose.
pH change as the cause of tooth decay
Tooth decay starts when the pH of the mouth is lower than 5.5. Tooth enamel, made up of calcium phosphate is the hardest substance in the body. It does not dissolve in water, but is corroded when the pH in the mouth is below 5.5. Bacteria present in the mouth produce acids by degradation of sugar and food particles remaining in the mouth after eating.
Using toothpastes, which are generally basic, for cleaning the teeth can neutralise the excess acid and prevent tooth decay.
Self defence by animals and plants through chemical warfare:
- Bee-sting leaves an acid which causes pain and irritation. Use of a mild base like baking soda on the stung area gives relief.
- Stinging hair of nettle leaves inject methanoic acid causing burning pain.
Chemicals from Common Salt
- Salt formed by the combination of hydrochloric acid and sodium hydroxide solution is called sodium chloride. This is the salt that you use in food.
Formation of sodium hydroxide :
- When electricity is passed through an aqueous solution of sodium chloride (called brine), it decomposes to form sodium hydroxide.
- The process is called the chlor-alkali process because of the products formed– 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. Sodium hydroxide solution is formed near the cathode. The three products produced in this process are all useful. Below you can see the
- Different uses of these products.
Above you have seen that Chlorine is produced during the electrolysis of aqueous sodium chloride (brine).
This chlorine gas is used for the manufacture of bleaching powder
Formation 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
- Ca(OH)2 + Cl2 → CaOCl2 + H2O
Bleaching powder is used –
(i) for bleaching cotton and linen in the textile industry, for bleaching wood pulp in paper factories and for bleaching washed clothes in laundry;
(ii) as an oxidising agent in many chemical industries; and
(iii) for disinfecting drinking water to make it free of germs.
- The soda commonly used in the kitchen for making tasty crispy pakoras is baking soda.
- 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.
Formation of baking soda:
It is produced using sodium chloride as one of the raw materials.
It is a mild non-corrosive basic salt. The following reaction takes place when it is heated during cooking –
Sodium hydrogencarbonate has got various uses in the household.
Use of Sodium hydrogencarbonate :-
(i) 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 –
Carbon dioxide produced during the reaction causes bread or cake to rise making them soft and spongy.
(ii) Sodium hydrogencarbonate is also an ingredient in antacids. Being alkaline, it neutralises excess acid in the stomach and provides relief.
(iii) It is also used in soda-acid fire extinguishers.
Another chemical that can be obtained from sodium chloride is Na2CO3.10H2O (washing soda).
Formation of 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.
Sodium carbonate and sodium hydrogencarbonate are useful chemicals for many industrial processes as well.
Uses of washing soda:-
(i) Sodium carbonate (washing soda) is used in glass, soap and paper industries.
(ii) It is used in the manufacture of sodium compounds such as borax.
(iii) Sodium carbonate can be used as a cleaning agent for domestic purposes.
(iv) It is used for removing permanent hardness of water.
Water of crystallisation
Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt.
Five water molecules are present in one formula unit of copper sulphate. Chemical formula for hydrated copper sulphate is Cu SO4. 5H2O.
One other salt, which possesses water of crystallisation is gypsum. It has two water molecules as water of cyrstallisation. It has the formula CaSO4.2H2O.
Plaster of Paris
Formation if plaster of paris:-
- On heating gypsum at 373 K, it loses water molecules and becomes calcium sulphate hemihydrate.
- This is called Plaster of Paris, the substance which doctors use as plaster for supporting fractured bones in the right position.
- Plaster of Paris is a white powder and on mixing with water, it changes to gypsum once again giving a hard solid mass.
Uses plaster of paris :-
- Plaster of Paris is used for making toys, materials for decoration and
- for making surfaces smooth.
- doctors use it as plaster for supporting fractured bones in the right position.