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Hydrogen burns and oxygen helps things burn — yet together they make water, which puts fires out! How can a compound behave so differently from the elements inside it? And does any mass go missing when things react?
- Water comes from many sources — are all samples chemically identical?
- Oxygen is written as O and sometimes O₂ — what is the difference?
- Why does dissolved salt conduct electricity but sugar does not?
In this chapter we will learn about the Law of Conservation of Mass and the Law of Constant Proportions, Dalton’s atomic theory, how atoms combine through covalent and ionic bonds, how to write chemical formulae, and how to find molecular mass and formula unit mass.
Before we state any law, let us investigate what happens to the total mass during a physical change and during a chemical change.
In this Activity, we will investigate whether the total mass changes when common salt dissolves in water — a physical change.
- Place a clean, dry 100 mL beaker on a digital weighing balance.
- Set the balance to zero using the tare/reset button.
- Pour about 50 mL of water into the beaker.
- Add a spatula full of common salt to the water (Fig. 9.1a).
- Record the reading on the balance.
- Swirl until the salt dissolves and record the reading again (Fig. 9.1b).
In this Activity, we will find out whether the total mass stays the same during a chemical change — the reaction of vinegar with baking soda.
- Set-up 1 (open): pour ~20 mL vinegar into a conical flask; put ~2 g baking soda in a balloon; record the initial mass of flask + balloon (Fig. 9.2a).
- Transfer the baking soda from the balloon into the vinegar (Fig. 9.2b) and record the final reading (Fig. 9.2c).
- Set-up 2 (closed): fix the balloon on the mouth of the flask with a thread so the gas cannot escape; weigh, then let the baking soda fall in and react (Fig. 9.3).
- Record the final reading in the closed set-up.
Vinegar + Baking soda (sodium hydrogencarbonate) → Carbon dioxide + Other substances. You saw this fizzing reaction earlier — here we track its mass .
- Keep the conical flask and the balloon on the balance.
- Small traces of baking soda may stay stuck to the balloon.
- Keeping both together prevents this error in the reading.
