CHEMISTRY / FIRST STEPS

Chemistry

From dissolving sugar to rusting iron, explore the microscopic world of matter.

Start with everyday questions · 2 articles

01 / Dissolving and particles

Has sugar disappeared when it dissolves in water?

Invisible does not mean absent. The sugar has spread through the water.

Start reading ↓02 / Physical and chemical changes

How is rusting iron different from melting ice?

The key question is whether new substances have formed.

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Has sugar disappeared when it dissolves in water?

Invisible does not mean absent. The sugar has spread through the water.

A sugar crystal contains many sucrose molecules. In water, these molecules gradually leave the crystal and spread among water molecules. They are too small to see, so the crystal shrinks and seems to disappear.

Under ordinary conditions, dissolving sucrose in water is mainly a physical change: the sucrose molecules remain sucrose. If nothing spills or evaporates, the solution’s total mass equals the original mass of the water plus the sugar.

At a given temperature, a fixed quantity of water can dissolve only a limited amount of sugar. Once saturated, additional sugar remains at the bottom. Stirring usually speeds up dissolving, but does not indefinitely increase the amount that can dissolve at that temperature.

Think about it: 10 g of sugar dissolves completely in 100 g of water, with no spilling or evaporation. What is the mass of the solution?

110 g. Dissolving changes how the sugar is distributed, not whether its mass exists.

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How is rusting iron different from melting ice?

The key question is whether new substances have formed.

When ice melts, its shape and state change, but it still consists of water molecules. This is a physical change, as is freezing water into ice.

Iron gradually rusts when exposed to water and oxygen. Rust contains substances different from the original iron, so rusting is a chemical change, not simply a change in colour.

A colour change, bubbles or heating can provide clues, but none alone proves a chemical change. Boiling water produces bubbles while remaining water. Always ask whether new substances have formed.

Think about it: which is a chemical change, cutting paper or burning it?

Burning forms new substances, including gases and ash, so it is a chemical change. Cutting changes the shape. This is a conceptual comparison; no fire experiment is needed.

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