Acids, Bases, Salts, Physical and Chemical Change Short Notes (NCERT Science)Acids, Base and Salts Curd, lemon juice, orange juice and vinegar taste sour. These substances taste sour because they contain acids.The chemical nature of such substances is acidic.Citrus fruits like lemons and oranges have citric acid in their juice. This is what makes lemons taste so sour. Substances like these which are bitter in taste and feel soapy on touching are known as bases. The nature of such substances is said to be basic. Ex: Baking Soda The solutions which do not change the colour of either red or blue litmus are known as neutral Blue litmus paper turns red under acidic conditions and red litmus paper turns blue under basic or alkaline conditions, with the color change occurring over the pH range 4.5-8.3 at 25 C (77 F). Neutral litmus paper is purple. Litmus can also be prepared as an aqueous solution that functions similarly. If it has a lot of hydrogen ions, then it is an acid. If it has a lot of hydroxide ions, then it is a base. From 0 to 7 are acids, with 0 being the strongest. From 7 to 14 are bases with 14 being the strongest base. If a liquid has a pH of 7, its neutral. This would be something like distilled water. Indigestion : Our stomach contains hydrochloric acid. It helps us to digest food, But too much of acid in the stomach causes indigestion. Sometimes indigestion is painful. To relieve indigestion, we take an antacid such as milk of magnesia, which contains magnesium hydroxide. It neutralises the effect of excessive acid. Ant bite: When an ant bites, it injects the acidic liquid (formic acid) into the skin. The effect of the acid can be neutralised by rubbing moist baking soda (sodium hydrogen carbonate) or calamine solution, which contains zinc carbonate. Soil treatment: Excessive use of chemical fertilisers makes the soil acidic. Plants do not grow well when the soil is either too acidic or too basic. When the soil is too acidic, it is treated with bases like quick lime (calcium oxide) or slaked lime (calcium hydroxide). If the soil is basic, organic matter is added to it. Organic matter releases acids which neutralises the basic nature of the soil. Factory wastes: The wastes of many factories contain acids. If they are allowed to flow into the water bodies, the acids will kill fish and other organisms. The factory wastes are, therefore, neutralised by adding basic substances. Our stomachs use hydrochloric acid to help digest foods. This strong acid also kills bacteria and helps to keep us from getting sick. Our muscles produce lactic acid when we exercise. Also, our pancreas uses a base called an alkali to help with digestion.

Physical and Chemical Changes Properties such as shape, size, colour and state of a substance are called its physical properties. A change in which a substance undergoes a change in its physical properties is called a physical change. A physical change is generally reversible. In such a change no new substance is formed. A change in which one or more new substances are formed is called a chemical change. A chemical change is also called a chemical reaction. The process of depositing a layer of zinc on iron is called galvanisation. Crystallization is an example of a physical change. Acids and bases can help neutralize each other. Acids turn litmus paper red, bases turn it blue. Strong bases can be slippery and slimy feeling. Acids taste sour, bases taste bitter. Proteins are made up of amino acids. Vitamin C is also an acid called ascorbic acid. Ammonia is a base chemicalSome important questions to be solved:-Qus 1: What do you mean by universal indicator and its use ?Qus 2: If we react magnesium with HCL what gas will be produced ?Qus 3: Nitric acid + sodium hydroxide ?Qus 4: Carbonic acid is used in which human consumables ?Qus 5: Antacid contains which two bases ?Qus 6: Chemical name of vitamin C is ?Qus 7: Name the gas used in making of TNT and urea ?Qus 8: which acid is known as Oil of Vitriol ?Qus 9: Contents of Royal Water