الأسبوع 7 · الوحدة الثانية «المادة والتفاعلات الكيميائية» (افتتاحية) والدرس الأول «التفاعلات الكيميائية»: التغير الفيزيائي والكيميائي وماهية التفاعل ودلائل حدوثه وخواص المواد قبل التفاعل وبعده · 3 دروس
3
Evidence that a reaction happened
Science Prep 2 Term 1, Unit 2: pp. 39–48
GoalRecognise signs that a chemical reaction may have happened, and explain why each sign needs careful thinking.
New words
precipitate · a solid that forms in a solution during a reactionevidence · something we observe that supports a conclusiongas bubbles · bubbles of a new gas made in a liquid
Explanation
How do we know a reaction has happened? We look for evidence. Signs include gas bubbles, a change of colour, a precipitate (a solid forming in a liquid), light and heat, a new smell and a change of temperature. But each sign needs thought. Boiling water makes bubbles, but they are vapour of the same water: physical. A cut apple that dries may change colour for another reason. Heat is not a sign of every reaction: some reactions absorb heat and make things cooler. A bottle of water bursting in the freezer is physical. The best proof is a new substance with new properties. Be careful too: we never sniff or taste a reaction to find out. Some classroom reactions use acids, silver salts or burning magnesium, which are unsafe for children; we only give their observations in words.
Sign
Careful thinking
gas bubbles
boiling also makes bubbles
colour change
may be a physical mix or drying
precipitate
a new solid in a solution
heat or light
some reactions absorb heat instead
Worked examples
Milk with a spoon of lemon juice becomes lumpy. A solid has formed in a liquid, so there is evidence of a chemical change: the milk’s proteins clump. Describe what you see; do not taste it.
A pot of water bubbles on a stove. A student says there is a gas, so there is a reaction. Is that right? No. The bubbles are water vapour, so it is a physical change: boiling.
Two nails are put in damp water, and two are kept dry. After several days, the damp ones have red-brown patches and the dry ones are unchanged. The comparison gives good evidence that water is needed for rusting.
Class activity
Evidence hunt, with a grown-up. Put two nails in a damp paper towel in a cup and two in a dry cup, and leave them for a few days. Put a few pieces of cut apple on a plate: brush one with lemon juice, leave one plain. Describe what you see each day and say which sign of reaction each shows. Do not taste or sniff any of the test items.
Practice
1
Name three signs that a chemical reaction may have happened.
For example gas bubbles, a colour change, a precipitate, light or heat, a new smell (any three).
2
What is a precipitate?
A solid that forms in a solution during a reaction.
3
Why are gas bubbles not proof of a reaction on their own?
Boiling also makes bubbles, and that is a physical change.
4
A reaction makes its container colder. Does this mean no reaction happened?
No. Some reactions absorb heat, so a temperature drop can also be a sign of a reaction.
