7th Class Science Acids, Bases and Salts / अम्ल, क्षार एवं लवण Acids, Bases and Salts

Acids, Bases and Salts

Acids

Acids are present in things that we eat such as lemon and orange contains citric acid, tomato contains oxalic acid, vinegar contains acetic acid, yoghurt contains lactic acid and frizzy drinks contain carbonic acid. Acids found in foods are mild. Acids such as hydrochloric acid (HCI), sulphuric acid \[({{H}_{2}}S{{O}_{4}})\]and nitric acid \[(HN{{O}_{3}})\]are called mineral acids. They are strong acids. Touching strong acids can cause acid burns.

Acids are formed by dissolving oxides of non-metals such as carbon, sulphur and nitrogen in water. The oxides of non-metals are acidic because when dissolved in water they form acids. For example:

\[C{{O}_{2}}+{{H}_{2}}O\xrightarrow{{}}{{H}_{2}}C{{O}_{3}}\]

\[S{{O}_{3}}+{{H}_{2}}O\xrightarrow{{}}{{H}_{2}}S{{O}_{4}}\]

\[3N{{O}_{2}}+{{H}_{2}}O\xrightarrow[{}]{}NO+2HN{{O}_{3}}\]

Characteristics of Acidic Substances

The acidic substances show the following behaviour:

• Taste sour.
• Turn blue litmus paper into red.
• Corrode metals. Acids react with metals to form a compound known as salts. For example, if iron reacts with sulphuric acid, the following reaction takes place:

       \[Fe+{{H}_{2}}SO\xrightarrow[{}]{}FeS{{O}_{4}}+{{H}_{2}}\]

• Acidic substances react with carbonates and hydrogen carbonates and liberate \[C{{O}_{2}}\] with effervescence:

      \[N{{a}_{2}}C{{O}_{3}}+2HCI\xrightarrow[{}]{}2NaCI+{{H}_{2}}O+C{{O}_{2}}\]

Bases

Bases are generally the oxides and hydroxides of metals. For example, sodium oxide\[(N{{a}_{2}}O)\], calcium oxide\[(CaO)\], sodium hydroxide\[(NaOH)\], etc.

Soaps, toothpastes, solution of washing soda, soap solution, slaked lime and whitewash are some examples of basic substances.

The bases that are soluble in water are called alkalis. For example, sodium hydroxide\[(NaOH)\], potassium hydroxide\[(KOH)\], calcium hydroxide \[(Ca{{(OH)}_{2}}))\]and ammonium hydroxide\[(N{{H}_{4}}OH)\]. Sodium hydroxide and potassium hydroxide are called caustic alkalis because they burn the skin.

Formation of Bases

Bases can be formed in the following ways:

• When the metal combines with oxygen, metallic oxide is formed.

      \[2Mg+{{O}_{2}}\xrightarrow[{}]{}2MgO\]

• When metals react with water or steam, metallic hydroxide is formed, which is basic in nature.

      \[2Na+2{{H}_{2}}O\xrightarrow[{}]{}2NaOH+{{H}_{2}}\]

Characteristics of Basic Substances

The basic substances show the following behaviour:

• Taste bitter.
• Turn red litmus into blue.
• Bases neutralise acids.
• Caustic alkalis corrode glass and some metals such as aluminium, zinc, tin and lead.

Salts

Salts are neutral substances found in abundance in the earth's crust. Salts are also dissolved in water and found in lakes, rivers and sea. Sea water contains salts such as chlorides, bromides, iodides and sulphates of sodium, potassium, magnesium and calcium. Salts are also present in fruits and vegetables.

Salts are formed in the following ways:

• Salts are formed by the direct combination of elements:

 \[2Na+C{{I}_{2}}\xrightarrow[{}]{}2NaCI\]

• When acid reacts with metal\[Mg+2HCI\xrightarrow[{}]{}MgC{{I}_{2}}+{{H}_{2}}\]