الأسبوع 13 · المحور الثاني «المادة والطاقة»، الوحدة الثانية «حركة الجسيمات»، المفهوم 2.3 «مقارنة تغيرات المادة» (ج): الكتلة والتغيرات الفيزيائية والكيميائية · 3 دروس
3
A new substance appears
Discovery Education Science P5: Term 1, Unit 2, pp. 201–205
GoalExplain what a chemical change is, name its signs, and sort changes as physical or chemical.
New words
chemical change · a change that makes a new substance with new propertiesrust · the reddish, flaky substance made when iron joins with oxygen and watersign · something you can see that gives evidence of a change
Explanation
In a chemical change, a new substance forms, and it has properties that the starting materials did not have. Iron, oxygen and water make rust. Cake batter becomes a spongy cake in the oven. An egg turns solid when it is cooked. Milk left out for too long turns sour. Wood burning makes ash, heat and light, which we only talk about and never do in class. Signs of a chemical change are bubbles of gas, a new colour, heat or light, or a new smell, and we never sniff a test on purpose. Chemical changes are usually hard to reverse: rust does not turn back into a nail, and a cake does not turn back into batter. A physical change, such as melting or dissolving, is usually easy to reverse. When vinegar meets baking soda, gas bubbles form. If the cup is open the gas escapes, and a scale would show the mass drop. That is why we do not use a fizzing pair for the mass test.
Change
Sign
Type
ice melting
same substance
physical
iron nail rusting
red-brown flakes
chemical
baking a cake
new colour, firm cake
chemical
salt dissolving
nothing new
physical
Worked examples
Day
Damp nail
Dry nail
1
shiny
shiny
5
a few spots
shiny
10
red-brown rust
shiny
Tarek puts a clean iron nail in a damp cloth and keeps another nail dry. After 10 days the damp nail has red-brown rust and the dry nail is still shiny. Iron needs water and air to rust.
A grown-up pours a little vinegar onto baking soda in an open cup on a tray, and the children wear goggles. Bubbles of gas fill the cup. The gas is a new substance, so this is a chemical change. It is never done in a closed bottle.
Sorting by asking: is there a new substance? Melting ice, cutting paper and dissolving salt stay the same substance. A cooked egg, a rusting nail and a baked cake are new substances.
Class activity
Fizzing and rusting (a grown-up helps, goggles on). On a tray, put two spoonfuls of baking soda in an open cup. The grown-up adds a little vinegar. Write what you see and which sign shows a new substance. Never close the cup. Also put one clean iron nail in a damp cloth and one on a dry plate, and look at both every day for a week.
Practice
1
What is a chemical change?
A change in which a new substance forms, with properties different from the starting materials.
2
Name two signs that a chemical change may have happened.
Any two: bubbles of gas, a new colour, heat or light, a new smell.
3
Physical or chemical? (a) tearing paper into pieces, (b) an apple slice turning brown, (c) stirring sugar into tea, (d) a shiny iron gate turning red-brown.
