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Acids, Bases and pH Indicators

Grade 8 Science Worksheets

Acids and Bases are groups of chemical compounds, which react with each other to form salts and water. The reaction is known as a Neutralization process – in other words, acids and bases neutralize one another.

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Acids

Acid solutions have a sour taste, they produce a burning sensation in contact with skin, and they are capable of dissolving many metals. Acids can be useful and also dangerous and poisonous.

Hydrochloric Acid(HCl) in the human stomach helps in the process of digestion. Vinegar is Acetic Acid (CH3COOH), which is used in salad dressings and for food preservation, and we obtain Citric Acid(C6H8O7) from oranges and lemons. Many types of cola contain Phosphoric Acid (H3PO4).

Sulfuric Acid (H2SO4), which is extremely harmful and corrosive to skin, is used in car batteries while Nitric Acid (HNO3) is used in explosives and fertilizers.

Acids are classified as Organic if they contain carbon atoms, else they are termed Inorganic. Organic acids are used in beverages, cosmetics, detergents, foods, drugs, soaps, and plastics. Inorganic acids are generally much stronger and are used to manufacture paints, explosives, fertilizers, and other chemicals.

Bases or Alkalis

Bases are substances that react with an acid to decrease or neutralize its acidic properties. Bases dissolved in water are called Alkalis or Alkaline Solutions. Bases have a slippery texture and taste bitter.

Bases are also very useful chemicals. Sodium Hydroxide [NaOH] is used in the manufacture of soap, paper and synthetic materials. Calcium hydroxide [Ca(OH)2] is used in bleaching powder and to neutralize soil acidity. Magnesium hydroxide [Mg(OH)2] is a common laxative.

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Acids and Bases in Chemistry

Acids dissolve in water to produce hydrogen ions. A hydrogen ion is a proton formed when a hydrogen atom loses its electron. Acid is also defined as a Proton Donor – it readily gives up a proton (hydrogen ion) to another substance. A base is able to accept a hydrogen ion from an acid – it is a Proton Acceptor.

However, all acids may not be able to release hydrogen and may not always be proton donors. In broader terms, therefore, all acids are defined as Electron-Pair Acceptors while bases are defined as Electron-Pair Donors – that is, acids can accept a pair of electrons from another atom or molecule while bases can share electron pairs with acids.

There are Strong Acids and Weak Acids, Strong Bases, and Weak Bases. Weak acids and bases do not ionize completely when dissolved in water, while strong acids are fully ionized. Sulfurous Acid (H2SO3) and Nitrous Acid(HNO2) are weak acids but sulfuric acid and nitric acid are strong. Weak acids like vinegar can be consumed without any problem but strong acids like hydrochloric acid will burn skin in touch.

The pH of Acids and Bases

pH, which stands for Potential(or Power) of Hydrogen, is a number that indicates the concentration of hydrogen ions in a solution. The concentration is expressed in moles of hydrogen ions per liter of solution.

pH ranges from 0 to 14. A pH below 7 indicates that a solution is acidic while above 7 indicates that it’s basic (alkaline). A pH of 7 indicates a perfectly neutral solution – neither acidic nor basis – pure water, for example.

pH Indicators

Indicators are compounds that change color when they come in contact with acids and bases. They are therefore useful in detecting acids or acidity of solutions.

The most commonly used natural indicator is a dye named Litmus obtained from lichens – slow-growing plants on rocks, walls, and trees. Litmus has a mauve color in pure water. It turns red in an acidic solution and blue in alkaline solutions. Another natural indicator is Turmeric, a yellow-colored common kitchen spice, which remains yellow when dipped in acid but turns red in alkalis.

Many fruits and vegetables containing a group of pigments called Anthocyanins change color in response to pH. Red cabbage, grape juice, beet, tomato, and skins of peach, pear, plum, radish, and turnip are just a few. Pigments extracted from these can be made into juice or paper to conduct pH tests in homes and labs.

Chemical compounds like Phenolphthalein, Methyl Red, Methyl Orange, Thymol Blue, and many others also change color depending on whether they are added to acidic or basic solutions. They are frequently used in lab experiments.

Fun Fact: Hydrangea or snowball flowers are blue when grown in acidic soils and pink or red in basic soils.

Check Point

  1. Which is an organic acid?
    1. Acetic acid (CH3COOH)
    2. Phosphoric Acid (H3PO4)
    3. Nitric Acid (HNO3)
  2. Bases dissolved in water are called _______ solutions.
  3. Acids are Electron-Pair _______ while bases are defined as Electron-Pair _______.
  4. Which of these is alkaline?
    1. Grapefruit juice
    2. Ammonia solution
    3. Saliva
    4. Black coffee
  5. Litmus turns ______ in an acidic solution.

Answer Key

  1. a) Acetic acid (CH3COOH)
  2. Alkaline
  3. Acceptors, Donors
  4. b) Ammonia solution
  5. Red

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