Milk turns sour in summer. An iron tawa rusts in humid air. Grapes ferment. Food is cooked, digested and used during respiration. In each case, the nature and identity of the starting substance change. Whenever a chemical change occurs, a chemical reaction has taken place.

Activity 1.1 Burning magnesium ribbon

Caution: This activity needs the teacher's assistance. Wear suitable eyeglasses. Keep the burning ribbon as far away from your eyes as possible.

1. Clean a magnesium ribbon about 3-4 cm long by rubbing it with sandpaper.
2. Hold it with a pair of tongs.
3. Burn it using a spirit lamp or burner.
4. Collect the ash in a watch-glass.
5. Record what you observe.

Observation: Magnesium burns with a dazzling white flame and changes into a white powder, magnesium oxide. Cleaning removes the magnesium oxide coating so that the ribbon can burn readily.

1. 1
2. 2
3. 3
4. 4
5. 5

1. 1Bunsen burner
2. 2Magnesium ribbon
3. 3Tongs
4. 4Watch glass
5. 5Magnesium oxide

Figure 1.1 Burning of a magnesium ribbon in air and collection of magnesium oxide in a watch-glass.

Activity 1.2 Lead nitrate and potassium iodide

1. Take lead nitrate solution in a test tube.
2. Add potassium iodide solution.
3. Observe the colour and physical state of the substance formed.

Observation: A yellow insoluble solid, lead iodide, forms. This solid is called a precipitate.

Figure legend

• Lead nitrate solution
• Potassium iodide solution
• Test tube
• Lead iodide precipitate

Activity 1.3 Zinc and dilute acid

Caution: Handle the acid with care.

1. Take a few zinc granules in a conical flask or test tube.
2. Add dilute hydrochloric acid or dilute sulphuric acid.
3. Observe what happens around the zinc granules.
4. Touch the outside of the container carefully and note any temperature change.

Observation: Hydrogen gas bubbles form and the container becomes warm. The evolution of a gas and rise in temperature show that a chemical reaction has occurred.

1. 1
2. 2
3. 3
4. 4
5. 5
6. 6

1. 1Conical flask
2. 2Cork
3. 3Glass tube
4. 4Zinc granules
5. 5Dilute sulphuric acid
6. 6Hydrogen gas

Figure 1.2 Formation of hydrogen gas by the action of dilute sulphuric acid on zinc.

From Activities 1.1-1.3, these observations help us recognise a chemical reaction:

• change in state
• change in colour
• evolution of a gas
• change in temperature

1.1 Chemical Equations

A long sentence describing a reaction can be shortened to a word-equation:

Magnesium + Oxygen → Magnesium oxide (1.1)

Magnesium and oxygen are the reactants because they undergo chemical change. Magnesium oxide is the product formed by the reaction.

Reactants are written on the left-hand side, products on the right-hand side, and the arrow points towards the products.

1.1.1 Writing a Chemical Equation

Chemical formulae make a word-equation shorter and more useful:

Mg + O₂ → MgO (1.2)

Count the atoms on the two sides. There are two oxygen atoms on the left but only one on the right. Equation (1.2) is therefore an unbalanced or skeletal chemical equation.

Remember: Never change a chemical formula to balance an equation. Changing H₂O into H₂O₂ changes water into a different substance.