# Understand How To Balance Equations

In this worksheet, students will learn how to balance chemical reaction equations including state symbols.

### QUESTION 1 of 10

Balanced equations contain lots of information about chemical reactions. In this activity you will learn how to balance equations and how to read state symbols.

When a chemical reaction happens, we take something, pull its atoms apart, then put them back together in a different pattern. The atoms at the start and at the end are the same; they're just joined together in a different way. This idea gives us something called the Law of Conservation of Mass:

No atoms are made or destroyed during a chemical reaction. The total mass of all the reactants is the same as the total mass of all the products.

(Reactants are the chemicals we have at the start of a chemical reaction; products are the chemicals we produce by the end of the chemical reaction.)

The Law of Conservation of Mass means we have to be careful how we write equations for chemical reactions. We write equations for chemical reactions like this:

reactants → products

(it's a good idea to read the → as "makes" or "becomes", so you have the sense of before and after).

For the Law of Conservation of Mass to work, we have to have the same number of each type of atom on the reactants side of the arrow as on the products side. For example, sodium and chlorine react to make sodium chloride.

2 Na + Cl2 → 2 NaCl (the large 2's in front of Na and NaCl mean that there are two copies of that chemical in the reaction).

To check that the reaction is balanced, count the number of atoms of each type on each side of the reaction.

Na: before the reaction happens, there are 2 Na, and after the reaction happens, there are 2 Na.

Cl: before the reaction happens, there are 2 Cl, and after the reaction happens, there are 2 Cl.

No atoms have been made or destroyed, so the reaction is balanced.

If we need to balance an equation, we can't change the formulas of the chemicals we have, but we can change how many units of that chemical we use. So we were allowed to change NaCl to 2 NaCl, but not to Na2Cl. We can change the large numbers, but not the small ones.

Often, we add state symbols to each chemical, to tell us what the state the chemical is in. There are four symbols you need to know:

Symbol Meaning
(s) solid
(l) liquid
(g) gas
(aq) aqueous (dissolved in water)

Sometimes, the equations are quite complicated, so we need a strategy. Here's what to do:

CaCO3 + HCl → CaCl2 + H2O + CO2

First, find elements which only appear once on each side of the equation, and balance those compounds. Ignore oxygen for now, because that is in several compounds.

There's one Ca in CaCO3, and one Ca in CaCl2 . So their numbers of formula units must be equal.

There's one H in HCl, but 2 H in H2O. So we need two units of HCl for each one unit of H2O.

There's one Cl in HCl, but 2 Cl in CaCl2. So we need two units of HCl for each one unit of CaCl2.

That means that balancing H and Cl needs the equation to be

CaCO3 + 2 HCl → CaCl2 + H2O + CO2

Then, once the easier elements are balanced, check the oxygens. There are three on the reactants side, and three on the products side, so that's OK. If you have elements which appear as pure elements (for example, H2 or O2) leave them until last.

Finally, add state symbols. Calcium carbonate is solid, hydrochloric acid is aqueous, calcium chloride dissolves in water, water is liquid and carbon dioxide is a gas- so we end up with.

CaCO3 (s) + 2 HCl (aq)→ CaCl2 (aq) + H2O (l)+ CO2 (g)

That's it! To balance equations, make sure that there are the same number of atoms of each element on each side of the equation. As long as you work through these gradually and systematically, it will work out fine.

Complete the gaps in the Law of Conservation of Mass.

2 CuO + C → CO2 + 2 Cu. Type numbers into the gaps in the paragraph to describe this reaction. For the final gap, choose "balanced" or "unbalanced".

This equation isn't balanced yet...

Zn + HCl → ZnCl2 + H2

Which of these changes would balance the reaction correctly?

Change "H2" to "H" and ZnCl2 to ZnCl

Change ZnCl2 to ZnCl

Change "HCl" to "2 HCl"

Fill in the blank number to make this reaction balance.

Change "H2" to "H" and ZnCl2 to ZnCl

Change ZnCl2 to ZnCl

Change "HCl" to "2 HCl"

Fill in the blank numbers to make this reaction balance.

Change "H2" to "H" and ZnCl2 to ZnCl

Change ZnCl2 to ZnCl

Change "HCl" to "2 HCl"

Fill in the blank numbers to make this reaction balance.

Change "H2" to "H" and ZnCl2 to ZnCl

Change ZnCl2 to ZnCl

Change "HCl" to "2 HCl"

Match the state symbols with their meanings

## Column B

(s)
aqueous
(l)
gas
(g)
solid
(aq)
liquid

What would be the symbol for "sodium chloride dissolved in water"?

## Column B

(s)
aqueous
(l)
gas
(g)
solid
(aq)
liquid

Sulfuric acid is H2SO4, and dissolves in water. Sodium sulfate is Na2SO4, and dissolves in water.

Write a balanced symbol equation (with state symbols) for sodium + sulfuric acid making sodium sulfate + hydrogen

## Column B

(s)
aqueous
(l)
gas
(g)
solid
(aq)
liquid

If we heat solid zinc oxide with solid carbon, we produce zinc metal plus carbon dioxide. Fill in the gaps to write a balanced symbol equation with state symbols.

## Column B

(s)
aqueous
(l)
gas
(g)
solid
(aq)
liquid
• Question 1

Complete the gaps in the Law of Conservation of Mass.

EDDIE SAYS
This is a key phrase to memorise. It explains a lot of how equation balancing needs to work, and you might be asked to quote it.
• Question 2

2 CuO + C → CO2 + 2 Cu. Type numbers into the gaps in the paragraph to describe this reaction. For the final gap, choose "balanced" or "unbalanced".

EDDIE SAYS
2 CuO means \"2 units of CuO\", so it includes 2 Cu ions and 2 O ions. If you\\\'re not sure about the difference (and similarity) between atoms and ions, have a look at the activities about ionic bonding.
• Question 3

This equation isn't balanced yet...

Zn + HCl → ZnCl2 + H2

Which of these changes would balance the reaction correctly?

Change "HCl" to "2 HCl"
EDDIE SAYS
The important thing is that we can't change the formula of any of the chemicals, so changing ZnCl2 to ZnCl isn't allowed.
• Question 4

Fill in the blank number to make this reaction balance.

EDDIE SAYS
To balance the Na, we need two units of NaOH for every one of Na2SO4. Once you have done that, there are four hydrogens on the reactant side, so you need to make two water molecules to use up all the hydrogen. Once you have two water molecules, the oxygens also balance.
• Question 5

Fill in the blank numbers to make this reaction balance.

EDDIE SAYS
Deal with the hydrogen and carbon first, because we can flex the number of O2 molecules to balance oxygen. To use all four hydrogen atoms in C2H4, make 2 H2O. To use both carbons in C2H4, make 2 CO. Then there are four oxygen atoms on the product side, so we need 2 O2.
• Question 6

Fill in the blank numbers to make this reaction balance.

EDDIE SAYS
The hydrogen and carbon balances work in the same was as the last question; the difference is that we are now making carbon dioxide, not carbon monoxide. That means there are six oxygen atoms on the products side, so we need 3 O2 molecules to provide those atoms on the reactants side.
• Question 7

Match the state symbols with their meanings

## Column B

(s)
solid
(l)
liquid
(g)
gas
(aq)
aqueous
EDDIE SAYS
Most of these symbols are simple to remember. \"Aqueous\" is a bit less obvious. \"Aqua\" means water- think of words like aquarium.
• Question 8

What would be the symbol for "sodium chloride dissolved in water"?

EDDIE SAYS
"(d)" isn't a state symbol. "(l)" would be liquid (molten) sodium chloride; "(aq)" is dissolved in water.
• Question 9

Sulfuric acid is H2SO4, and dissolves in water. Sodium sulfate is Na2SO4, and dissolves in water.

Write a balanced symbol equation (with state symbols) for sodium + sulfuric acid making sodium sulfate + hydrogen

EDDIE SAYS
Balancing this equation only needs the sodium atoms to be balanced; mostly this question is about the state symbols.
• Question 10

If we heat solid zinc oxide with solid carbon, we produce zinc metal plus carbon dioxide. Fill in the gaps to write a balanced symbol equation with state symbols.

EDDIE SAYS
Start with oxygen- you need two oxygen atoms to make CO2. That means you need to start with two units of ZnO, so you end up with 2 Zn on the products side.
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