GoalState the law of conservation of mass and use it to find a missing mass in a reaction.
New words
law of conservation of mass · in a closed system the total mass of the reactants equals the total mass of the productsclosed system · a system in which no matter enters or leavesLavoisier · a French chemist who named oxygen and measured masses carefully
Explanation
Antoine Lavoisier, a French chemist, named oxygen and insisted on weighing everything carefully. His work led to the law of conservation of mass: in a closed system, the total mass of the reactants equals the total mass of the products. The reason is simple: atoms are only rearranged, so none are made or lost, and every atom has the same mass before and after. For example, 20 g of calcium reacts with 8 g of oxygen to give 28 g of calcium oxide, because 20 + 8 = 28. Why does a burning log seem to lose mass? Because the gases, such as carbon dioxide and water vapour, escape into the air, together with some ash; if we could weigh everything the mass would be the same. So the law needs a closed system: no matter may enter or leave. A balanced equation, in which every element has equal atom numbers on the two sides, shows this law. The picture of a balance with two pans is an analogy.
Worked examples
In a closed reaction, 12 g of carbon joins with 32 g of oxygen to make carbon dioxide only. The mass of carbon dioxide is 12 + 32 = 44 g.
A closed bag holds baking soda and vinegar kept apart. It is tilted to mix them, and gas inflates the bag. The mass on a kitchen scale stays the same while the bag is closed, because no matter left. If a scale is too coarse to show a change, we say so honestly.
Count atoms on both sides of 2Ca + O₂ → 2CaO. Left: 2 Ca and 2 O. Right: 2 Ca and 2 O. The atoms match, so the equation is balanced and mass is conserved.
Class activity
Bead atoms. Use beads or paper clips of two colours for H and O. Build two molecules of H₂O₂ (each is H–O–O–H). Break the bonds and rearrange the beads into two H₂O and one O₂. Count the beads of each colour before and after: they match, and so would their masses. Write one sentence about why a log seems to lose mass.
Practice
1
State the law of conservation of mass.
In a closed system the total mass of the reactants equals the total mass of the products.
2
6 g of carbon burns completely with 16 g of oxygen in a closed flask. What mass of carbon dioxide forms?
6 + 16 = 22 g.
3
Wood ash weighs much less than the wood that burned. Where did the rest of the mass go?
Into the air as gases such as carbon dioxide and water vapour; the mass was not destroyed.
4
100 g of reactants make some products in a closed flask. What is the total mass of the products?
