Class 10 Science — Chemical reactions and equations
Balance a skeletal equation by counting atoms, name the five kinds of reaction the chapter sets out, and say which substance has been oxidised and which reduced.
Name: ________________________
1.Bags of crisps are flushed with nitrogen before they are sealed. What is that for?
- a) It keeps the crisps dry
- b) It cools the crisps down
- c) It stops the bag from being crushed
- d) It keeps the fat in them from being oxidised
2.Iron nails are left in blue copper sulphate solution for twenty minutes. What do you see when they come out?
- a) The nails have turned black and the solution is clear
- b) The nails are unchanged and the blue is deeper
- c) The nails have turned brownish and the blue has faded
- d) The nails have dissolved and the solution is green
3.Limestone is heated strongly in a kiln and gives quick lime and carbon dioxide. What kind of reaction is that?
- a) A precipitation reaction
- b) A combination reaction
- c) A displacement reaction
- d) Thermal decomposition
4.Zinc granules are dropped into dilute sulphuric acid. Bubbles rise round them and the tube grows warm. Which two of the chapter's signs of a reaction is that?
- a) The colour changes, and a gas is given off
- b) The temperature changes, and the colour changes
- c) The state changes, and the colour changes
- d) A gas is given off, and the temperature changes
5.Match each everyday sight to what it is called.
- An iron gate turning reddish brown
- A silver spoon going black
- Butter left out until it smells off
- A white solid appearing as two clear solutions meet
- Rusting
- Corrosion
- Precipitation
- Rancidity
6.Put the chapter's balancing procedure back into the order it gives, starting with "Draw a box round each formula, and change nothing inside a box".
- Draw a box round each formula, and change nothing inside a box
- Balance the elements that are still unequal, one at a time
- Count the atoms on both sides again to check the equation
- Start with the compound that has the most atoms, and balance its commonest element
- Write in the physical states of the reactants and products
- List how many atoms of each element sit on the two sides
7.In 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂, how many oxygen atoms stand on the right-hand side? Answer with a number.
8.Sort each of these by whether energy comes out of the reaction or goes into it.
Groups: Heat given out · Energy taken in
- Natural gas burning
- Respiration in our cells
- Quick lime added to water
- Silver chloride left in sunlight
- Water split by an electric current
- Green crystals heated until they give off a sharp smell
Answer key — Class 10 Science — Chemical reactions and equations
- 1. d) It keeps the fat in them from being oxidised
- 2. c) The nails have turned brownish and the blue has faded
- 3. d) Thermal decomposition
- 4. d) A gas is given off, and the temperature changes
- 5. An iron gate turning reddish brown → Rusting; A silver spoon going black → Corrosion; Butter left out until it smells off → Rancidity; A white solid appearing as two clear solutions meet → Precipitation
- 6. 1. Draw a box round each formula, and change nothing inside a box 2. List how many atoms of each element sit on the two sides 3. Start with the compound that has the most atoms, and balance its commonest element 4. Balance the elements that are still unequal, one at a time 5. Count the atoms on both sides again to check the equation 6. Write in the physical states of the reactants and products
- 7. 12
- 8. Quick lime added to water → Heat given out; Respiration in our cells → Heat given out; Natural gas burning → Heat given out; Silver chloride left in sunlight → Energy taken in; Water split by an electric current → Energy taken in; Green crystals heated until they give off a sharp smell → Energy taken in