What do the state symbols (s), (l), (g) and (aq) mean?

Solid, liquid, gas and aqueous (dissolved in water).

Which seven elements are diatomic?

H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂.

What must be true of a balanced symbol equation?

It has the same number of each type of atom on both sides.

Which numbers can you change when balancing an equation?

Only the large numbers in front of formulae, never the small numbers inside them.

Balance: CH₄ + O₂ → CO₂ + H₂O

CH₄ + 2O₂ → CO₂ + 2H₂O

State the law of conservation of mass.

The total mass of the products equals the total mass of the reactants, because atoms are not created or destroyed.

Why are atoms not lost in a reaction?

They are only rearranged to form new substances.

Why does the mass fall when marble chips react with acid in an open flask?

The carbon dioxide gas made escapes into the air.

Why does magnesium gain mass when it burns in air?

It combines with oxygen from the air to make magnesium oxide.

What is a closed system?

A system where nothing can enter or leave, so the total mass cannot change.

Writing and balancing equationsIB MYP Chemistry: Flashcards

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What does a word equation show?

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What does a word equation show?: The names of the reactants on the left and the products on the right, joined by an arrow.
What do the state symbols (s), (l), (g) and (aq) mean?: Solid, liquid, gas and aqueous (dissolved in water).
Which seven elements are diatomic?: H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂.
What must be true of a balanced symbol equation?: It has the same number of each type of atom on both sides.
Which numbers can you change when balancing an equation?: Only the large numbers in front of formulae, never the small numbers inside them.
Balance: Mg + O₂ → MgO: 2Mg + O₂ → 2MgO
Balance: H₂ + O₂ → H₂O: 2H₂ + O₂ → 2H₂O
Balance: CH₄ + O₂ → CO₂ + H₂O: CH₄ + 2O₂ → CO₂ + 2H₂O
State the law of conservation of mass.: The total mass of the products equals the total mass of the reactants, because atoms are not created or destroyed.
Why are atoms not lost in a reaction?: They are only rearranged to form new substances.
Why does the mass fall when marble chips react with acid in an open flask?: The carbon dioxide gas made escapes into the air.
Why does magnesium gain mass when it burns in air?: It combines with oxygen from the air to make magnesium oxide.
What is a closed system?: A system where nothing can enter or leave, so the total mass cannot change.

Exam questions on Writing and balancing equations

In a school laboratory in Dubai, students add small pieces of zinc to dilute hydrochloric acid in a test tube. Bubbles of hydrogen gas form and the zinc slowly disappears, leaving a colourless solution of zinc chloride.
Explain why the number 2 is written in front of HCl in the balanced equation.2 marks
A student in Lagos places a conical flask containing marble chips (calcium carbonate) and dilute hydrochloric acid on a balance. Cotton wool is pushed into the neck of the flask. As the reaction takes place the balance reading falls by 0.64 g, even though no solid or liquid has left the flask.
A different student does the same reaction in a strong sealed container. Explain why the total mass would not change.2 marks
A student investigates whether mass changes during a reaction. She places a small test tube of sodium carbonate solution inside a conical flask containing calcium chloride solution, seals the flask with a stopper and weighs it. She tips the flask so that the two solutions mix and a white precipitate of calcium carbonate forms. She then weighs the sealed flask again. Her balance reads to 0.01 g. The mass of the sealed flask before and after mixing was: trial 1, 215.32 g then 215.32 g; trial 2, 187.45 g then 187.45 g; trial 3, 201.18 g then 201.17 g.
State a testable hypothesis for this investigation, give a scientific reason for it, and identify the dependent variable.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).