Chemical reactions and word equations
By the end of this chapter you will be able to
- Say how a chemical reaction differs from a physical change
- Name the reactants and the products of a simple reaction
- Write a word equation for a simple reaction, with the arrow and the plus sign
- Give the signs that show a chemical reaction has happened
- Say whether a reaction is exothermic or endothermic from the temperature change
You should already know
- Physical changes and reversible changes of state (Chapter 4 of this book)
- Elements and compounds, and symbols (Chapters 6 and 7 of this book)
Where this leads
- Conservation of mass in chemical reactions (Chapter 10)
National Curriculum for England, Key Stage 3 science, Year 7. Content: C7 chemical reactions produce new substances; reactants and products, C7 word equations for simple reactions; signs of a chemical reaction (colour, gas, temperature, precipitate); energy changes (basic only).
Exercise P Do you remember?
Method. Recall the difference.
- Recall. Melting makes no new substance, so it is a physical change.physical
- Write the answer. physical
Check. The change can be reversed by freezing. ✓
Method. Recall the word.
- Recall. A substance made of two or more elements chemically bonded is a compound.compound
- Write the answer. compound
Check. Water is a compound. ✓
Method. Recall the symbol.
- Recall. Hydrogen has the symbol H.H
- Write the answer. H
Check. One capital letter. ✓
Method. Subtract.
- Subtract. Take 8 from 25.25 - 8 = 17
- Write the answer. 17
Check. Check: 17 + 8 = 25. ✓
Method. Recall the gas.
- Recall. Living things need oxygen from the air for respiration.oxygen
- Write the answer. oxygen
Check. Carbon dioxide is the gas we breathe out. ✓
Chemical reactions make new substances
A chemical reaction is a change in which one or more new substances are made. The starting substances are called the reactants. The new substances formed are the products.
A chemical change is usually difficult or impossible to reverse. A physical change makes no new substance, for example melting or dissolving.
Key ideas
- A chemical reaction makes one or more new substances.
- Reactants are the starting substances; products are the new substances.
- A physical change makes no new substance.
- Chemical changes are usually hard or impossible to reverse.
Method. Find the new substance.
- Find the new substance. The magnesium and oxygen react to make a new white solid.new white solid
- Name it. The product is magnesium oxide.magnesium oxide
- Write the answer. magnesium oxide
Check. Magnesium and oxygen are the reactants. ✓
Method. Check whether a new substance forms.
- Check. Ice and water are the same substance, water, in a different state.same substance
- Decide. No new substance forms, so it is a physical change, and the answer is no.no
- Write the answer. no
Check. Melting is physical, and can be reversed by freezing. ✓
Method. Find the starting substances.
- Find the starting substances. The reactants are the substances that go into the reaction.go in
- Name them. The reactants are iron and oxygen.iron and oxygen
- Write the answer. iron and oxygen
Check. Iron oxide is the product. ✓
Watch out
Watch out: a change that looks different is not always a chemical change. Melting, dissolving and boiling are physical changes.
Exam tip
Exam tip. To show a chemical reaction, say that a new substance is made, and name it.
Try it (with your teacher, or a grown-up at home). Look at a labelled picture of a burning candle, or a rusting nail. Name the reactants and the product, if you can. Explain why this is a chemical change, not a physical one. Do not burn anything yourself.
Exercise 9A Chemical reactions make new substances
Method. Recall the word.
- Recall. The starting substances of a reaction are the reactants.reactants
- Write the answer. reactants
Check. Reactants react to form products. ✓
Method. Recall the word.
- Recall. The new substances formed in a reaction are the products.products
- Write the answer. products
Check. Products are made by the reaction. ✓
Method. Recall the fact.
- Recall. A chemical change usually makes new substances that cannot be turned back easily.cannot be turned back
- Decide. So the answer is no.no
- Write the answer. no
Check. Burning wood cannot be undone. ✓
Method. Check for a new substance.
- Check. The sugar is still sugar, spread through the water.still sugar
- Decide. So no new substance forms, and the answer is no.no
- Write the answer. no
Check. Dissolving is a physical change. ✓
Method. Look at the reaction.
- Look. Magnesium oxide is formed in the reaction, so it is new.formed
- Decide. So magnesium oxide is a product.product
- Write the answer. product
Check. Magnesium and oxygen are the reactants. ✓
Method. Check each change.
- Check. Burning and rusting both make new substances.new substances
- Decide. Melting makes no new substance, so it is physical.melting
- Write the answer. melting
Check. Melting can be reversed by freezing. ✓
Method. Check the substance.
- Check. The powder is magnesium oxide, a new substance made from magnesium and oxygen.new substance
- Decide. So Ali is not right.no
- Write the answer. no
Check. Oxygen is part of the product. ✓
Method. Check the roles.
- Check. The magnesium and oxygen go into the reaction, so they are the reactants.reactants
- Decide. So Ruby is not right.no
- Write the answer. no
Check. Products come out of the reaction. ✓
Method. Think about evidence.
- Think. Looking different is not enough; a new substance is the real evidence.new substance
- Name one. A gas being produced, or a temperature change, is also evidence.gas
- Write the answer. Accept any one: a new substance, a gas being produced, or a temperature change.
Check. Evidence must show a new substance has formed. ✓
Method. Think about the new substance.
- Think about the reactants. Magnesium and oxygen react together.react together
- Think about the product. A new substance, magnesium oxide, is formed.magnesium oxide
- Write the answer. Accept any sentence that says magnesium reacts with oxygen to make a new substance, magnesium oxide.
Check. A new substance is the sign of a chemical reaction. ✓
Method. Think about the new substance.
- Think about the rust. Rust is a new substance, iron oxide.iron oxide
- Think about reversing. Cooling does not turn a new substance back into its reactants.does not turn back
- Write the answer. Accept any sentence that says the rust is a new substance, so it cannot be turned back into iron and oxygen by cooling.
Check. New substances are not undone by cooling. ✓
Word equations
A word equation shows the reactants and the products in words. The reactants are written before the arrow, and the products are written after it. A plus sign (+) separates each substance from the next.
For example: magnesium + oxygen → magnesium oxide.
The table gives the word equations used in this chapter. Some are teacher demonstrations only.
| Reactants | Products | Note |
|---|---|---|
| magnesium + oxygen | magnesium oxide | teacher demonstration only |
| iron + oxygen | iron oxide | the slow reaction that makes rust |
| calcium carbonate + hydrochloric acid | calcium chloride + water + carbon dioxide | teacher demonstration only |
| sodium hydrogencarbonate + ethanoic acid | sodium ethanoate + water + carbon dioxide | the fizzing of bicarbonate of soda and vinegar at home |
| magnesium + hydrochloric acid | magnesium chloride + hydrogen | teacher demonstration only |
Key ideas
- Reactants go before the arrow; products go after it.
- A plus sign separates the substances on each side.
- Word equations use names, not symbols.
- Count the products by counting the substances after the arrow.
Method. Put the reactants on the left, and the product on the right.
- Left side. The reactants are iron and oxygen, separated by a plus sign.iron + oxygen
- Right side. The product is iron oxide, after the arrow.iron oxide
- Write the answer. iron + oxygen → iron oxide
Check. Two reactants and one product. ✓
Method. Count the substances after the arrow.
- Read the products. The products are calcium chloride, water and carbon dioxide.three names
- Count them. Each product is separated by a plus sign, so there are three.3
- Write the answer. 3
Check. There are two plus signs after the arrow, so three products. ✓
Method. Recall what is on each side.
- Recall. The reactants are on the left of the arrow, and the products on the right.left
- Decide. Magnesium is on the left, so it is a reactant.reactant
- Write the answer. reactant
Check. Magnesium oxide, on the right, is the product. ✓
Watch out
Watch out: the arrow means “reacts to form”, not “equals”. Reactants go before it and products go after it.
Exam tip
Exam tip. Write a word equation with names, not symbols, and check every substance is on the correct side of the arrow.
Try it (with your teacher, or a grown-up at home). Write a word equation for the reaction of bicarbonate of soda and vinegar, using the name sodium hydrogencarbonate for bicarbonate of soda. Write the reactants before the arrow, and the products after. Check your equation with a partner.
Exercise 9B Word equations
Method. Recall the rule.
- Recall. The products are written after the arrow.products
- Write the answer. products
Check. Reactants are written before the arrow. ✓
Method. Recall the meaning.
- Recall. The plus sign separates the substances, meaning one substance and another.and
- Write the answer. and
Check. The plus sign joins two substances on the same side. ✓
Method. Recall the meaning.
- Recall. The arrow means the reactants react to form the products.reacts to form
- Write the answer. reacts to form
Check. An arrow is not an equals sign. ✓
Method. Read the equation.
- Read the equation. The product after the arrow is magnesium oxide.magnesium oxide
- Write the answer. magnesium oxide
Check. Magnesium oxide is a white solid. ✓
Method. Count the substances before the arrow.
- Count. Magnesium and oxygen are two reactants.two
- Write the answer. 2
Check. Two substances on the left. ✓
Method. Count the substances after the arrow.
- Count. Only magnesium oxide is after the arrow.one
- Write the answer. 1
Check. One product. ✓
Method. Read the left side.
- Read the left side. The reactants are on the left, and include iron.left side
- Write the answer. iron
Check. Oxygen is also a reactant. ✓
Method. Read the right side.
- Read the right side. The product after the arrow is iron oxide.iron oxide
- Write the answer. iron oxide
Check. Iron oxide is the new substance. ✓
Method. Read the left side.
- Read the left side. The reactants are on the left: calcium carbonate and hydrochloric acid.calcium carbonate
- Write the answer. calcium carbonate and hydrochloric acid
Check. These are joined by a plus sign. ✓
Method. Count the products after the arrow.
- Count. The products are sodium ethanoate, water and carbon dioxide.three
- Write the number. So there are three products.3
- Write the answer. 3
Check. Three products, separated by plus signs. ✓
Method. Check the sides.
- Check the sides. Magnesium oxide is the product, but it is written as a reactant.reactant
- Decide. So the equation is written the wrong way round, and the answer is no.no
- Write the answer. no
Check. Reactants go before the arrow. ✓
Method. Use the reactants and product.
- Reactants. Iron and oxygen are the reactants.iron + oxygen
- Product. Iron oxide is the product, after the arrow.iron oxide
- Write the answer. Accept: iron + oxygen → iron oxide.
Check. Check the sides of the arrow. ✓
Method. Use the reactants and the products from the table.
- Reactants. Magnesium and hydrochloric acid are the reactants.magnesium + hydrochloric acid
- Products. The products are magnesium chloride and hydrogen.magnesium chloride + hydrogen
- Write the answer. magnesium + hydrochloric acid → magnesium chloride + hydrogen
Check. Two reactants and two products. ✓
Signs of a chemical reaction
There are several signs that a chemical reaction has taken place. Each one is evidence that a new substance has probably formed:
| Sign | What you would see |
|---|---|
| a colour change | iron turns brown as rust forms when it reacts with air and water over time |
| a gas is produced | fizzing or bubbles, as carbon dioxide forms when vinegar meets bicarbonate of soda |
| a temperature change | the mixture gets warmer or colder |
| a solid forms in a liquid | a precipitate, a solid made by mixing two solutions |
| light or a flame | the reaction gives out light, as when magnesium burns (teacher demonstration) |
A precipitate is a solid formed when two solutions are mixed. It falls to the bottom of the container. Seeing one sign is good evidence, but a fair test checks more than one observation.
Key ideas
- A colour change may show a new substance.
- A gas being produced shows a reaction, such as fizzing.
- A temperature change shows a reaction, and is an energy change.
- A precipitate is a solid formed when two solutions are mixed.
Method. Match the observation to a sign.
- Match. A solid formed in a liquid is a precipitate.precipitate
- Name the sign. So the sign is a precipitate.precipitate
- Write the answer. a precipitate
Check. A precipitate is a new solid. ✓
Method. Match the observation to a sign.
- Match. Bubbles and fizzing show that a gas is being produced.gas
- Name the sign. So the sign is a gas being produced.a gas is produced
- Write the answer. a gas is produced
Check. The gas is carbon dioxide. ✓
Method. Match the observation to a sign.
- Match. The beaker getting warmer is a temperature change.warmer
- Name the sign. So the sign is a temperature change.a temperature change
- Write the answer. a temperature change
Check. Warmer means energy is given out. ✓
Watch out
Watch out: a change in colour, or a solid forming, is not proof on its own. Look for a new substance, which needs more evidence.
Exam tip
Exam tip. Name the sign you see, and say what it tells you: a gas is produced, a precipitate forms, or the temperature changes.
Try it (with your teacher, or a grown-up at home). With a grown-up, look at a glass of cold tap water and a glass of warm tap water, each with a drop of food colouring. Is there a sign of a chemical reaction? Explain why this is a physical change, not a chemical one.
Exercise 9C Signs of a chemical reaction
Method. Recall the word.
- Recall. A solid formed when two solutions are mixed is a precipitate.precipitate
- Write the answer. precipitate
Check. It settles at the bottom. ✓
Method. Recall the sign.
- Recall. Bubbles and fizzing show that a gas is being produced.bubbles
- Write the answer. bubbles or fizzing
Check. Gas bubbles come out of the liquid. ✓
Method. Recall the sign.
- Recall. If the mixture gets warmer, the temperature rises.warmer
- Write the answer. temperature rises
Check. Warmer means energy is given out. ✓
Method. Think about the evidence.
- Think. Some colour changes are physical, such as a drink being mixed.physical
- Decide. So a colour change alone is not always proof, and the answer is no.no
- Write the answer. no
Check. Look for more than one sign. ✓
Method. Match the observation.
- Match. A solid appearing when two solutions are mixed is a precipitate.precipitate
- Write the answer. precipitate
Check. A precipitate is a sign of a reaction. ✓
Method. Count the signs.
- Count. The two signs are a gas being produced, and the temperature rising.two signs
- Write the number. So there are 2 signs.2
- Write the answer. 2
Check. Two signs give better evidence than one. ✓
Method. Check the evidence.
- Check. Colour alone may come from mixing, not from a new substance.mixing
- Decide. So Ruby needs more evidence, and the answer is no.no
- Write the answer. no
Check. Look for more than one sign. ✓
Method. Check the definition.
- Check. A precipitate is a solid formed when two solutions are mixed.solid formed
- Decide. So Ben is right, if the solid formed from two solutions.yes
- Write the answer. yes
Check. A solid forming in a liquid is the sign. ✓
Method. Think about what you can see.
- Think. A gas produced in a reaction makes bubbles, or inflates a balloon.inflates
- Name one. So look for bubbles or a balloon that inflates.bubbles
- Write the answer. Accept any one: bubbles, fizzing, or a balloon that inflates.
Check. Observations must be seen, not guessed. ✓
Method. Think about physical changes.
- Think about physical changes. Some physical changes also show a colour change or a temperature change.also
- Think about evidence. So several signs give better evidence that a new substance has formed.better evidence
- Write the answer. Accept any sentence that says one sign can come from a physical change, so several signs give better evidence of a new substance.
Check. Several signs give better evidence. ✓
Method. Think about the precipitate.
- Think about the solid. The solid is a new substance that was not in either solution.new substance
- Think about the reaction. Its formation shows that a reaction has taken place.reaction
- Write the answer. Accept any sentence that says the precipitate is a new, insoluble substance formed from the two solutions, which is a new substance.
Check. A new substance is the key evidence. ✓
Energy changes in reactions
An exothermic reaction gives energy out to the surroundings, so the temperature rises. A hand warmer is an example.
An endothermic reaction takes energy in from the surroundings, so the temperature falls. Some cold packs use this.
The table gives the start and end temperatures of two reactions, and the change.
| Reaction | Start (degrees C) | End (degrees C) | Change (degrees C) | Type |
|---|---|---|---|---|
| reaction A: a hand warmer (iron reacting with air) | 20 | 27 | +7 | exothermic |
| reaction B: citric acid and sodium hydrogencarbonate in water | 20 | 12 | -8 | endothermic |
Key ideas
- Exothermic: energy is given out, and the temperature rises.
- Endothermic: energy is taken in, and the temperature falls.
- Change in temperature = end temperature minus start temperature.
- A rise is positive, and a fall is negative.
Method. Look at the direction of the change.
- Look. The temperature rises, so energy is given out.rises
- Decide. A reaction that gives energy out is exothermic.exothermic
- Write the answer. exothermic
Check. The temperature went up by 7 degrees. ✓
Method. Subtract the start from the end.
- Read the temperatures. The start is 20 degrees and the end is 12 degrees.20 and 12
- Subtract. Change = end minus start = 12 minus 20.12 - 20 = -8
- Write the answer. -8 degrees C
Check. 20 + (-8) = 12. ✓
Method. Use the sign of the change.
- Look. The temperature falls, so energy is taken in.falls
- Decide. A reaction that takes energy in is endothermic.endothermic
- Write the answer. endothermic
Check. Cold packs use an endothermic reaction. ✓
Watch out
Watch out: exothermic does not mean the reaction is hot at the start. It means the energy given out raises the temperature of the surroundings.
Exam tip
Exam tip. For exothermic, say “energy given out, temperature rises”. For endothermic, say “energy taken in, temperature falls”.
Try it (with your teacher, or a grown-up at home). Look at a labelled picture of a hand warmer and a cold pack. For each one, say whether the reaction is exothermic or endothermic, and say what happens to the temperature. Do not open or squeeze any real packs.
Exercise 9D Energy changes in reactions
Method. Recall the word.
- Recall. A reaction that gives energy out is exothermic.exothermic
- Write the answer. exothermic
Check. The temperature rises. ✓
Method. Recall the word.
- Recall. A reaction that takes energy in is endothermic.endothermic
- Write the answer. endothermic
Check. The temperature falls. ✓
Method. Recall the effect.
- Recall. Energy given out raises the temperature of the surroundings.raises
- Write the answer. rise
Check. A hand warmer gets hot. ✓
Method. Recall the effect.
- Recall. Energy taken in lowers the temperature of the surroundings.lowers
- Write the answer. fall
Check. A cold pack gets cold. ✓
Method. Recall the example.
- Recall. A hand warmer gets hot, so energy is given out.gets hot
- Decide. So it is exothermic, and the answer is yes.yes
- Write the answer. yes
Check. Iron reacts with air and gives out heat. ✓
Method. Subtract the start from the end.
- Subtract. The change is 27 minus 20.27 - 20 = 7
- Write the answer. 7
Check. 20 + 7 = 27. ✓
Method. Subtract the start from the end.
- Subtract. The change is 12 minus 20.12 - 20 = -8
- Write the answer. -8
Check. 20 - 8 = 12. ✓
Method. Look at the direction.
- Look. The temperature rose from 15 to 22 degrees.rose
- Decide. A rise means energy is given out, so the reaction is exothermic.exothermic
- Write the answer. exothermic
Check. A rise of 7 degrees. ✓
Method. Look at the direction.
- Look. The temperature fell from 25 to 19 degrees.fell
- Decide. A fall means energy is taken in, so the answer is yes.yes
- Write the answer. yes
Check. A fall of 6 degrees. ✓
Method. Recall the definition.
- Recall. An exothermic reaction gives energy out, so the temperature rises.rises
- Decide. So Ali is not right.no
- Write the answer. no
Check. Exothermic and endothermic are opposites. ✓
Method. Think about the evidence.
- Think. A cold flask means the temperature fell, which suggests endothermic.fell
- Decide. So the conclusion is not reasonable, and the answer is no.no
- Write the answer. no
Check. Check the direction of the temperature change. ✓
Method. Think about the energy.
- Think about the energy. The reaction takes energy in from its surroundings.takes energy in
- Think about the temperature. So the pack and its surroundings get colder.colder
- Write the answer. Accept any sentence that says the reaction takes energy in from the surroundings, so the temperature of the pack falls.
Check. Energy taken in makes the temperature fall. ✓
Method. Think about the sign.
- Think about the rise. In an exothermic reaction the end temperature is higher than the start.higher
- Think about the fall. In an endothermic reaction the end temperature is lower, so the change is negative.lower
- Write the answer. Accept any sentence that says a rise in temperature gives a positive change, and a fall gives a negative change.
Check. The sign shows the direction of the change. ✓
Acids, carbonates and carbon dioxide
Many carbonates react with acids. The products are a salt, water and carbon dioxide. For example:
calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide
Carbon dioxide can be identified with limewater. When carbon dioxide is bubbled through limewater, the limewater turns milky. This test is a teacher demonstration.
Key ideas
- A carbonate and an acid make a salt, water and carbon dioxide.
- Carbon dioxide turns limewater milky.
- Fizzing or bubbles in an acid and carbonate mixture show carbon dioxide is produced.
- Acids are dangerous; pupils do not handle them, except safe kitchen vinegar with a grown-up.
Method. Recall the products.
- Recall. The products are a salt, water and carbon dioxide.carbon dioxide
- Name the gas. The gas is carbon dioxide.carbon dioxide
- Write the answer. carbon dioxide
Check. Carbon dioxide makes limewater milky. ✓
Method. Recall the test.
- Recall. Carbon dioxide turns limewater milky.milky
- Decide. So the limewater turns milky.turns milky
- Write the answer. it turns milky
Check. This is the test for carbon dioxide. ✓
Method. Read the equation.
- Read the right side. The products are calcium chloride, water and carbon dioxide.three products
- Name them. They are calcium chloride, water and carbon dioxide.calcium chloride, water and carbon dioxide
- Write the answer. calcium chloride, water and carbon dioxide
Check. Three products on the right of the arrow. ✓
Watch out
Watch out: limewater turns milky with carbon dioxide, not with any gas. Only carbon dioxide gives this test.
Exam tip
Exam tip. For the product of a carbonate and an acid, name the salt, water and carbon dioxide.
Try it (with your teacher, or a grown-up at home). With a grown-up, put a spoonful of bicarbonate of soda into a clean cup, then add a little vinegar. Watch the fizzing, and say which gas is produced. Do not taste or smell the mixture closely. Wash your hands afterwards.
Exercise 9E Acids, carbonates and carbon dioxide
Method. Recall the product.
- Recall. Carbon dioxide is one of the products.carbon dioxide
- Write the answer. carbon dioxide
Check. The fizzing is the carbon dioxide. ✓
Method. Recall the test.
- Recall. Carbon dioxide turns limewater milky.milky
- Write the answer. milky
Check. This is the test for carbon dioxide. ✓
Method. Read the equation.
- Read the equation. The salt on the right of the arrow is calcium chloride.calcium chloride
- Write the answer. calcium chloride
Check. Salts are made in these reactions. ✓
Method. Read the equation.
- Read the equation. The third product after the arrow is water.water
- Write the answer. water
Check. Water is always a product here. ✓
Method. Read the equation.
- Read the equation. Calcium chloride is a salt, and it is a product.salt
- Decide. So the answer is yes.yes
- Write the answer. yes
Check. Salts come from acids reacting with carbonates. ✓
Method. Recall the test.
- Recall. Carbon dioxide turns limewater milky.carbon dioxide
- Write the answer. carbon dioxide
Check. The other products do not do this. ✓
Method. Use the reaction.
- Use the reaction. Bicarbonate of soda is a carbonate, so carbon dioxide is produced.carbonate
- Decide. So the fizzing gas is carbon dioxide.carbon dioxide
- Write the answer. carbon dioxide
Check. The gas comes out as bubbles. ✓
Method. Read the products.
- Read the products. The products are a salt, water and carbon dioxide.salt, water and carbon dioxide
- Decide. Oxygen is not one of them, so Ali is not right.no
- Write the answer. no
Check. Learn the three products. ✓
Method. Recall the test.
- Recall. Carbon dioxide turns limewater milky, not clear.milky
- Decide. So Ruby is not right.no
- Write the answer. no
Check. Milky means cloudy. ✓
Method. Think about the test.
- Name the test. Bubble the gas through limewater.limewater
- Name the result. If the limewater turns milky, the gas is carbon dioxide.milky
- Write the answer. limewater; it turns milky
Check. A clear result gives the answer. ✓
Method. Think about the sign.
- Think about the sign. The milky colour is a clear, visible change.visible
- Think about the reliability. Only carbon dioxide gives this change, so it is a reliable test.reliable
- Write the answer. Accept any sentence that says the milky colour is a clear sign that carbon dioxide is present, which is a reliable test.
Check. A specific test gives a reliable answer. ✓
Method. Think about the gas.
- Think about the gas. The fizzing is carbon dioxide gas being produced.gas
- Think about the substance. A new substance is produced, so a chemical reaction has happened.new substance
- Write the answer. Accept any sentence that says the fizzing is a gas being produced, which is a new substance, so a chemical reaction has happened.
Check. A gas is a new substance formed. ✓
Chapter summary
| Skill | Method |
|---|---|
| Chemical reaction | New substances (products) are made from reactants. Hard to reverse. |
| Word equation | Reactants before the arrow, products after. Plus signs separate the substances. |
| Signs | Colour change, gas produced, temperature change, precipitate, or light. |
| Energy | Exothermic: energy out, temperature rises. Endothermic: energy in, temperature falls. |
| Carbonate and acid | Salt + water + carbon dioxide. Carbon dioxide turns limewater milky. |
Can you…?
- I can say how a chemical reaction differs from a physical change
- I can name the reactants and products of a simple reaction
- I can write a word equation with the arrow and the plus signs
- I can name the signs that show a chemical reaction has happened
- I can say whether a reaction is exothermic or endothermic from the temperature change
Key terms
- chemical reaction
- a change in which one or more new substances are made
- reactant
- a substance that is used up in a chemical reaction
- product
- a new substance made in a chemical reaction
- word equation
- a way of showing a reaction in words, with the reactants before the arrow
- precipitate
- a solid formed when two solutions are mixed
- exothermic
- a reaction that gives energy out, so the temperature rises
- endothermic
- a reaction that takes energy in, so the temperature falls
- carbonate
- a compound containing carbon and oxygen, that reacts with acids to make carbon dioxide
- limewater
- a solution that turns milky when carbon dioxide is bubbled through it
Exercise 9Z Check-up
Method. Recall the word.
- Recall. The starting substances are the reactants.reactants
- Write the answer. reactants
Check. Products come after the arrow. ✓
Method. Read the reaction.
- Read the reaction. Magnesium and oxygen react to make magnesium oxide.magnesium oxide
- Write the answer. magnesium oxide
Check. Magnesium oxide is the product. ✓
Method. Recall the meaning.
- Recall. The arrow means the reactants react to form the products.reacts to form
- Write the answer. reacts to form
Check. The arrow is not an equals sign. ✓
Method. Recall the word.
- Recall. A solid formed in a liquid is a precipitate.precipitate
- Write the answer. precipitate
Check. The precipitate is a new solid. ✓
Method. Recall the definition.
- Recall. A rise in temperature means energy is given out, which is exothermic.exothermic
- Write the answer. exothermic
Check. A hand warmer is exothermic. ✓
Method. Subtract the start from the end.
- Subtract. The change is 12 minus 20.12 - 20 = -8
- Write the answer. -8
Check. 20 - 8 = 12. ✓
Method. Recall the product.
- Recall. Carbon dioxide is a product of a carbonate and an acid.carbon dioxide
- Write the answer. carbon dioxide
Check. It makes limewater milky. ✓
Method. Recall the test.
- Recall. Bubbling the gas through limewater tests for carbon dioxide.limewater
- Write the answer. limewater
Check. The limewater turns milky. ✓
Method. Check for a new substance.
- Check. Melting makes no new substance.no new substance
- Decide. So the answer is no.no
- Write the answer. no
Check. Melting is a physical change. ✓
Method. Think about the energy.
- Think about the energy. The reaction is exothermic, so it gives energy out.gives energy out
- Think about the temperature. So the hand warmer gets hot.gets hot
- Write the answer. Accept any sentence that says the reaction gives energy out to the surroundings, so the temperature rises.
Check. Exothermic reactions warm their surroundings. ✓
Total 17 marks. Allow about 17 minutes.
Exercise 9X Practice test questions
Method. Put the reactants before the arrow and the product after.
- Reactants. Iron and oxygen are the reactants.iron + oxygen
- Product. Iron oxide is the product.iron oxide
- Write the answer. iron + oxygen → iron oxide
Check. Two reactants, one product. ✓
Method. Read the reaction.
- Read the reaction. The product is magnesium oxide.magnesium oxide
- Write the answer. magnesium oxide
Check. It is a new white solid. ✓
Method. Look at the direction.
- Look. The temperature rises, so energy is given out.rises
- Decide. So the reaction is exothermic.exothermic
- Write the answer. exothermic
Check. The change is 25 minus 18, which is 7 degrees. ✓
Method. Subtract the start from the end.
- Subtract. The change is 11 minus 18.11 - 18 = -7
- Write the answer. -7
Check. 18 - 7 = 11. ✓
Method. Recall the products.
- Recall. The products are a salt, water and carbon dioxide.a salt, water and carbon dioxide
- Write the answer. a salt, water and carbon dioxide
Check. These three are always products. ✓
Method. Recall the test.
- Recall. Carbon dioxide turns limewater milky.carbon dioxide
- Write the answer. carbon dioxide
Check. This is a specific test. ✓
Method. Recall the sign.
- Recall. A solid forming in a liquid is a precipitate.precipitate
- Write the answer. precipitate
Check. The precipitate is a new solid. ✓
Method. Check for a new substance.
- Check. Melting makes no new substance; the water is still water.still water
- Decide. So Ruby is not right, and melting is a physical change.no; physical
- Write the answer. no; physical
Check. Melting can be reversed by freezing. ✓
Method. Think about the new substance.
- Think about the reaction. Magnesium reacts with oxygen.reacts
- Think about the product. A new substance, magnesium oxide, is formed.magnesium oxide
- Write the answer. Accept any sentence that says magnesium reacts with oxygen to make a new substance, magnesium oxide.
Check. A new substance means a chemical reaction. ✓
Method. Recall the sign.
- Recall. Bubbles and fizzing show a gas is produced.bubbles
- Write the answer. Accept any one: bubbles, fizzing, or a balloon inflating.
Check. A gas is a new substance. ✓
Method. Think about the energy.
- Think about the energy. The reaction takes energy in.takes energy in
- Think about the temperature. So the temperature falls.falls
- Write the answer. Accept any sentence that says the reaction is endothermic, taking energy in from its surroundings, so the temperature falls.
Check. Energy taken in makes things colder. ✓