What Chemistry Is and Why It Matters

1A question to start

Fresh milk left in a covered pot for two days thickens and turns sour: it has become ikivuguto. Juice squeezed from ripe bananas and mixed with a little roasted sorghum flour bubbles for a few days and becomes urwagwa. A shiny new iron roof sheet slowly goes brown. A lump of charcoal in an imbabura glows, gives out heat and leaves a little grey ash.

In every case one kind of stuff turns into another. What are these substances made of, why do they change, and can we control the change? Those are the questions chemistry asks.

2Activity: a gas from the kitchen cupboard

You will need a conical flask (or a clean plastic bottle), a balloon, a spoon, vinegar and baking soda (sodium hydrogencarbonate, sold in shops for baking).

1. Pour vinegar into the flask to a depth of about 2 cm.
2. Use the spoon to put two spoonfuls of baking soda inside the balloon.
3. Stretch the neck of the balloon over the mouth of the flask, letting the balloon hang down so that no powder falls in yet (Figure 1.1 A).
4. Lift the balloon so that the powder drops into the vinegar. Watch the liquid and the balloon (Figure 1.1 B).

Figure 1.1: The balloon experiment. The fizzing in B shows that a gas is being made; the gas collects in the balloon.

Table 1.1: Observations in the balloon experiment

3What happened?

The powder and the vinegar did not simply mix. The bubbles and the full balloon show that a new substance, a gas, was made that was not there before. Chemists write this change as a word equation:

or, using chemical formulas,

$$\mathrm{N}\mathrm{a}\mathrm{H}\mathrm{C}{\mathrm{O}}_3+\mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}\to \mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{N}\mathrm{a}+{\mathrm{H}}_2\mathrm{O}+\mathrm{C}{\mathrm{O}}_2\backslash mathrm\{NaHCO\_3\; +\; CH\_3COOH\; \backslash rightarrow\; CH\_3COONa\; +\; H\_2O\; +\; CO\_2\}$$

You do not need to learn this equation yet. Notice only its shape: the substances you start with (the reactants) are on the left, the new substances (the products) are on the right, and the arrow means "turn into". A change that makes new substances is a chemical change, or chemical reaction. You will study these in Unit 3.

4Chemistry: the central science

People practised chemistry long before they had a name for it. They learned to control fire to cook food and to fire clay into pots. In the Great Lakes region, iron-workers smelted iron ore in clay furnaces to make hoes and knives. Brewers fermented bananas and sorghum. Others made soap by boiling fats with ash. Each of these changes one substance into a more useful one.

Today chemistry is called the central science because almost every other science and industry depends on it (Figure 1.2). Doctors need medicines, farmers need fertilisers, builders need cement, and water engineers need chemicals that kill germs.

Figure 1.2: Chemistry at the centre of daily life, with a Rwandan example for each area.

Figure 1.3: Water seen three ways. One molecule of water is one oxygen atom joined to two hydrogen atoms, so its formula is ${\mathrm{H}}_2\mathrm{O}\backslash mathrm\{H\_2O\}$.

5How chemists find things out

Chemists do not guess and stop. They follow the scientific method (Figure 1.4). Suppose Mugisha notices that milk sours faster in the hot season. He asks: does temperature change how fast milk sours? His hypothesis is that warmer milk sours faster. He tests it with two identical cups of milk, one in a cool place and one in a warm place, and checks both every six hours. If the warm cup sours first, the results support his hypothesis; if not, he must think again.

Figure 1.4: The scientific method is a cycle. Every conclusion raises new questions.

6The branches of chemistry

Table 1.2: The main branches of chemistry

7Chemistry in Rwanda: benefits and problems

Chemistry touches every Rwandan household. Treated tap water is made safe with chlorine. Maize and potato farmers raise their harvests with fertilisers. Health centres give oral rehydration salts (ORS) to children with diarrhoea. Cement from Rusizi builds houses and roads, and methane gas from Lake Kivu generates electricity.

Chemistry can also cause harm when it is misused. Pesticides washed off fields can poison rivers and fish. Burning plastic releases poisonous smoke. For that reason Rwanda banned polythene carrier bags in 2008, and today shops use paper and cloth bags instead. A citizen who knows some chemistry can weigh the benefits of a substance against its risks and use it wisely.