Class 7 Science Chapter 4 Notes Acids Bases and Salts
→ Acid : Substances that tastes sour taste and capable of turning litmus red in colour is called acid.
→ Acidic : The chemical nature of acid substances are said to be acidic.
→ Base : Substances which are bitter in taste and feel soapy on touching are known as bases.
→ Basic : The chemical nature of base substances are said to be basic.
→ Indicator : Special type of substances are used to test whether a substance is acidic or basic are called indicators.
→ Neutral : The solutions which do not change the colour of either red or blue litmus are known as neutral solutions.
→ Neutralisation : It can be defined as the reaction between an acid and base is known as neutralisation.
→ Salt : In neutralisation reaction a new substance is formed which is called salt.
→ Substances taste sour, bitter, sweet and salty. Substances which taste sour, contain acids. The chemical nature of such substances is acidic. Substances which are bitter in taste and soapy to touch are known as bases. The chemical nature of such substances is basic.
→ Special type of substances, called indicators are used to test the acidic or basic nature of substances. Litmus, turmeric, China rose petals (Gudhal) and red cabbage are some of the natural indicators. Phenolphthalein and methyl orange are synthetic indicators.
→ Litmus is extracted from lichens. It has a mauve (purple) colour in neutral medium.
It turns red in an acidic medium and blue in a basic medium.
→ Substances which do not change the colour of indicators are neutral. These substances are neither acidic nor basic.
→ Colour of some common indicators in acidic and basic solutions :
| Nature of Substance | Indicators | ||||||
| Blue litmus |
Red litmus |
Methyl orange |
Phenol phthalein |
Turmeric | China rose |
Red cabbage |
|
| Acids | Red | No change |
Red | Colourless | No change |
Dark pink |
Red |
| Basic | No change |
Blue | Yellow | Pink | Red | Green | Green |
→ When an acidic solution is mixed with a basic solution, both neutralise the effect of each other and a new substance called salt is formed along with water and heat which raises the temperature of the reaction mixture.
→ Organic acids are the naturally occurring acids, found in plants and animals. They are weak acids. Inorganic (mineral) acids are prepared from the minerals of the earth’s crust. They are strong acids except carbonic acid.
→ Depending on the amount of water present in it, an acid is concentrated or dilute. If small amount of water is present, the acid is concentrated and if large amount of water is present, the acid is dilute. A concentrated acid can be diluted by adding water to it and a dilute acid can be concentrated by evaporating the water from it. Remember, never add water to acid, always add acid to water.
→ Acids which dissociate completely in water and produce more H+ ions are called strong acids. Acids which do not dissociate completely in water and produce much less H+ ions are called weak acids.
→ Bases which dissociate completely in water and produce more OH- ions are called strong bases. Bases which do not dissociate completely in water and produce much less OH- ions are called weak bases.
→ The strength of an acid and a base is measured by a scale called pH scale. pH value of the solution indicates the concentration of hydrogen ions in the solution. The pH values range from 1 to 14. pH paper is used to find out the pH of a solution by matching the change in colour with a universal indicator paper. A universal indicator is a combination of dyes that shows a change of colour over a range of pH.
→ Basicity of an acid is the number of replaceable hydrogen atoms in a molecule of an acid. On the basis of this, acids are monobasic acids e.g., HCl, CH,COOH, etc., dibasic acids e.g., H2SO4, H2CO3, etc., tribasic acids e.g., H3PO4, etc.
→ Acidity of a base is the number of hydroxyl groups present in a molecule of a base. On the basis of this, bases are monoacidic bases e.g., NaOH, KOH, etc., diacidic bases e.g., Mg(OH)2, Ca(OH)2, etc., triacidic bases e.g., Al(OH)3, etc.
→ Normal salt is a salt formed by the complete replacement of the hydrogen atoms of an acid molecule by a metal atom or a positive radical e.g., NaCl, KCl, etc. A salt formed by the neutralisation of a strong acid by a strong base is neutral.
→ Acidic salt is a salt formed by the partial replacement of the hydrogen atoms of a polybasic acid molecule by a metal atom or a positive radical e.g., NaHCO , etc. A salt formed by the neutralisation of a strong acid by a weak base is acidic.
→ Basic salt is a salt formed by the partial replacement of the hydroxyl groups or the incomplete neutralisation of a base e.g., Ca(OH)Cl, Mg(OH)Cl, etc. A salt formed by the neutralisation of a weak acid by a strong base is basic.
→ There are many applications of acids, bases, salts and neutralisation reaction in our daily life.
→ The fixed number of water molecules which remain with the crystalline salt is known as water of crystallisation. Such water molecules give shape and colour to the crystals. Salts with water of crystallisation are called hydrated salts. On heating, hydrated salts lose their shape and colour and change into a powdery substance. These salts are called anhydrous salts.