Chemical Reactions and Quantitative Chemistry

When new substances are made

A chemical reaction is a change that makes one or more new substances. The substances you start with are the reactants, and the new substances formed are the products. During a reaction the atoms are not created or destroyed: they are simply rearranged into new combinations. That single idea explains a great deal, including why the total mass never changes in a sealed container. This module covers what a reaction is, how to spot one, how a chemical change differs from a physical one, and how to use the conservation of mass to work out an unknown mass.

Reaction words

Learn these terms before the reactions are described. They are the words every chemistry answer needs.

Match each term to its reaction meaning

Chemical change against physical change

Not every change is a reaction. Telling a chemical change from a physical one is a skill examiners test often.

What is the product mass?

In a sealed flask, 10 g of one substance reacts completely with 15 g of another. What is the total mass of the products?

• 25 g
• 5 g
• 15 g
• It cannot be worked out

Why mass is conserved

Conservation of mass follows straight from atoms being rearranged rather than made or lost. Every atom in the reactants ends up somewhere in the products, so if you weigh a sealed container before and after, the reading does not change. If a reaction seems to lose mass, a gas has usually escaped; if it seems to gain mass, a gas from the air has joined in. So when you handle a mass calculation, first ask whether the system is closed, then set the total product mass equal to the total reactant mass. That equality is the whole method.

Pick the true reaction facts

Select every true statement about chemical reactions.

• New substances are formed
• Atoms are rearranged, not created or destroyed
• In a closed system the total mass stays the same
• Mass is always lost in every reaction
• No new substances are ever made

Order the steps to find a missing mass

Put the steps for finding an unknown mass using conservation of mass in the correct order.

• Identify the reactants and the products
• Check that the system is closed
• Add up the masses that are known
• Set the total product mass equal to the total reactant mass
• Work out the mass that is missing

Complete the reaction facts

The substances you start with are the _____. The new substances formed are the _____. In a closed container the total mass stays the _____. A change that makes a new substance is a _____ change.

Burning magnesium, weighed carefully

Picture a strip of magnesium burning brightly in oxygen to make a white powder called magnesium oxide. Magnesium and oxygen are the reactants, and magnesium oxide is the product. Suppose the magnesium has a mass of 24 g and it combines with 16 g of oxygen from a sealed supply. Because no atoms are lost, all of that matter ends up in the powder, so the magnesium oxide has a mass of 24 g plus 16 g. If instead you burned the magnesium in the open air and weighed only the ash, it would seem heavier than the metal alone, because oxygen from the air has joined in. Keeping the system closed is what lets the masses add up cleanly.

Spot the signs of a reaction

Tap the TWO observations that are signs a chemical reaction has taken place.

• A gas bubbles off from the mixture
• The mixture changes colour
• The beaker is made of glass
• The bottle has a printed label

Work out the missing mass

In a closed container, 24 g of magnesium combines with 16 g of oxygen to form magnesium oxide. What is the mass of magnesium oxide formed, in grams?

Decide what is happening

Read each situation and choose the best answer.

• Iron and sulfur are heated together and join to make a new substance. What are iron and sulfur in this reaction?
• A reaction is carried out in a sealed jar, and the jar is weighed before and after. What should the readings show?
• A block of ice melts into liquid water. What kind of change is this?

Build a reaction sentence

Choose the word for each gap to complete one clear point about chemical reactions.

Explain reactions and conservation of mass

A friend thinks that mass disappears when things burn. Using what you have learned, explain chemical reactions and the conservation of mass.

• Explain what reactants and products are
• Explain the difference between a chemical and a physical change
• Explain why the total mass stays the same in a closed system
• Explain why a reaction in the open air can seem to change mass