Chemical Reactions and EquationsIntroduction
It is a hot afternoon in May at Riya's house. A glass of milk forgotten on the kitchen counter has turned sour. The iron tawa, left wet in the sink overnight, has grown a reddish-brown crust. And the grapes Nani put away in a jar are fizzing and smell of something new.
Riya's day is full of changes like these, even ones she cannot see:
- milk going sour in the summer heat,
- an iron tawa, pan or nail rusting in damp air,
- grapes fermenting in a jar,
- dal being cooked for dinner,
- food being digested inside her body,
- and every breath she takes — respiration.
In each case, the substance she started with is no longer the same substance: something new has formed. In earlier classes you learnt to tell a physical change (only the shape or state changes, as when ice melts) from a chemical change (a new substance forms). Whenever a chemical change takes place, we say that a chemical reaction has taken place.
Quick check. Which of these is not a chemical change?
Right: melted ice is still water, so that is only a physical change. But it leaves two questions. What exactly is a chemical reaction, and how can you tell that one has happened? Let's find out at the lab bench.
The Ribbon That Burns White
Activity 1.1. Take a strip of magnesium ribbon about 3–4 cm long and clean it by rubbing it with sandpaper. Hold it with a pair of tongs and burn it in the flame of a spirit lamp, collecting whatever forms in a watch-glass below. Keep the burning ribbon as far away from your eyes as you can.
Caution
Do this activity only with your teacher's help, and wear suitable safety glasses — burning magnesium is bright enough to hurt your eyes.
Predict first. What will happen when the clean ribbon meets the flame?
Now run it. Rub the ribbon with sandpaper, then hold it in the flame.
The ribbon burns with a dazzling white flame and changes into a white powder. That powder is magnesium oxide, a brand-new substance. It forms because magnesium reacts with the oxygen in the air. And notice that your ribbon would not catch fire until you had cleaned it — keep that in mind for later.
Four Tell-Tale Signs
Two more experiments, and this time watch closely for what changes.
Activity 1.2. Pour some lead nitrate solution into a test tube and add potassium iodide solution to it. What do you observe?
Activity 1.3. Put a few zinc granules in a conical flask or a test tube and add dilute sulphuric acid to them (dilute hydrochloric acid works too). Is anything happening around the zinc granules? Touch the flask: has its temperature changed?
Caution
Handle the acid with care. Never touch it, and wash it off with plenty of water if any spills on you.
Run each experiment, then tap every sign of a reaction that you saw in it.
How to tell that a reaction has taken place
Any one of these observations is a sign of a chemical reaction:
- a change in state,
- a change in colour,
- a gas being given off,
- a change in temperature.
Look around and you will spot these signs everywhere — a huge variety of chemical reactions is going on around you all the time. In this chapter you will learn how chemists write reactions down in a short, symbolic form, and how to sort them into a few main types.