الأسبوع 13 · المحور الثاني «المادة والطاقة»، الوحدة الثانية «حركة الجسيمات»، المفهوم 2.3 «مقارنة تغيرات المادة» (ج): الكتلة والتغيرات الفيزيائية والكيميائية · 3 دروس
2
A walk through the market
Discovery Education Science P5: Term 1, Unit 2, pp. 197–200
GoalShow that the parts of a mixture keep their properties, and sort everyday changes into physical changes and changes that make something new.
New words
physical change · a change in size, shape or state where no new substance formsingredient · one of the things that are put together to make a food or a mixturenew substance · a material with different properties from the ones you started with
Explanation
Imagine a walk through a busy street market. A tailor shortens a pair of trousers by cutting the cloth. A seller crushes sugarcane to make juice. Under a fish stall the ice melts. A customer stirs sugar into hot hibiscus tea. All of these are physical changes. The size, shape or state changed, but the same substances are still there: the cloth is still cloth, the ice is still water, and the sugar is still sweet. Mixtures keep the properties of their parts, so we can often separate them again. Other changes make a new substance with new properties. A seller roasts a sweet potato in an oven, and it turns soft, brown and sweet, and it cannot be made raw again. An iron shop shutter left in damp air turns red-brown and flaky. A good question to ask is: is there a new substance, and can I get the old one back?
At the market
What changed?
New substance?
cutting cloth
size and shape
no: physical
ice melting
state
no: physical
roasting a sweet potato
colour, softness, taste
yes: new substance
Worked examples
Mina watches the juice seller. Crushing the sugarcane only breaks it into pieces and squeezes out juice. It is a physical change because the sugarcane juice is still sugarcane juice.
Before
After roasting
pale, firm
brown, soft
raw
cannot be made raw again
Marwan’s mother roasts sweet potatoes in the oven. The pale, firm potato becomes soft and brown and smells different, and nothing can turn it back into a raw potato. A new substance has formed.
A shop shutter made of iron is left outside in damp air for months. Iron, oxygen from the air and water join to make rust, a reddish, flaky new substance. Rust is not the same as shiny iron.
Class activity
Market detective (a grown-up helps). On a walk or in the kitchen, find six changes, such as peeling an orange, melting butter, cutting bread, a rusty gate, a banana going brown and tea being stirred. Draw a table with two columns, “physical change” and “new substance”. Put each change in a column and write your evidence.
Practice
1
Is cutting a tomato a physical change? Give a reason.
Yes. Only the size and shape change. The pieces are still tomato.
2
Sugar is stirred into tea and seems to vanish. How could you get the sugar back?
Let the water evaporate. The sugar stays behind, because stirring was only a physical change.
3
Which pictures show a physical change? Say the letters.
A and B. Cutting cloth and melting ice give no new substance. Baking a cake (C) makes a new substance.
4
Give one reason why rust is not the same substance as shiny iron.
Rust is red-brown and flaky, but iron is grey and shiny. Their properties are different, so rust is a new substance.
