Mixing, Dissolving & Colour Changes
PrimaryPutting things together
When you stir two things together without them changing into something new, you make a mixture. Mix rice and raisins in a bowl and you can still see and pick out each one — that's a mixture.
Sometimes one thing seems to "disappear" into another. Stir a spoonful of sugar into warm water and, after a moment, you can't see it anymore — but it hasn't vanished, it has dissolved. The sugar has broken into pieces far too small to see, spread evenly through the water. This kind of see-through, evenly-mixed liquid is called a solution. You can prove the sugar is still there — taste the water and it's sweet!
Sometimes mixing two things doesn't just blend them — it makes something brand-new happen, called a chemical reaction. A colour change, bubbles appearing, or a change in temperature are all clues that a reaction has happened.
Disappearing sugar and fizzy surprises
Dissolving isn't magic — it's the tiny bits of sugar being pulled apart by the tiny bits of water and carried around evenly through the whole cup, like glitter shaken all through a bottle of water instead of sitting in one pile. That's why stirring or using warm water helps things dissolve faster: the water's tiny bits are moving more and can pull the sugar apart sooner.
A colour change or fizzing, though, is a different kind of clue. It usually means the two things you mixed weren't just blending — their tiny building blocks rearranged into something new. Mixing vinegar and baking soda makes a rush of bubbles: that gas wasn't in either ingredient before you mixed them — it was made brand new by the mixing.
Try it — the vinegar and baking soda fizz
Ask a grown-up to help with this one, over a sink or tray.
- Pour a little vinegar into a clear cup, about 2 fingers deep.
- Add a spoonful of baking soda and watch closely.
- Notice the bubbles rushing up — that's a new gas being made, a sure sign of a chemical reaction, not just a mixture.
- Now try a second cup: stir a spoonful of sugar into warm water instead, and stir until it disappears. Taste a tiny bit (if a grown-up says it's OK) — sweet! The sugar dissolved but didn't turn into anything new.
- Compare: fizzing = a reaction made something new; disappearing but still-sweet = dissolving, no new substance.
Only taste things a grown-up says are safe to taste, and never taste anything from a real science lab.
If you stir sand into water, does the sand dissolve?
No — sand does not dissolve. It sinks to the bottom and, if you look closely (or wait for it to settle), you can still see the grains. That makes sand-and-water a simple mixture, not a solution.
How can you tell dissolving apart from a chemical reaction?
When something dissolves, it's still the same stuff, just spread out too small to see (like sugar, which you can still taste). In a chemical reaction, something new appears — like bubbles, a colour change, or heat — because the tiny building blocks rearranged into a different substance.
Would sugar dissolve faster in cold water or warm water?
Warm water! The tiny bits in warm water move around faster and pull the sugar apart more quickly, so it dissolves sooner than in cold water.
How the ideas connect
Every key idea in this chapter, branching from the core concept — use it to see the whole picture at a glance.
The key facts, visualised
Worked problems, step by step
Follow each solution line by line, then try to reproduce it on paper before moving on.
Example 1You stir sugar into warm water and it disappears. Where did it go?
- The sugar did not vanish; it broke into bits too small to see.
- The bits spread out evenly through the water.
- This is called dissolving, and the water now tastes sweet.
Example 2You mix baking soda and vinegar and it fizzes. What does the fizz tell you?
- Fizzing means bubbles of gas are being made.
- Making a new gas is a sign of a chemical change.
- A brand new substance (carbon dioxide gas) has formed.
Now you try
Work each one out first, then tap to reveal the worked answer.