Tag Archives: states of matter

STEM Students answer Children’s Questions #7

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When visiting schools and museums our Street Scientists often get asked a variety of questions from curious children. Here are the answers to some of our favourite questions!

This week, we’re answering questions on Chemistry.

What happens when a chemical reaction happens and can it be reversed?

-asked by Charlie, 11, from Burnside Primary School

A chemical reaction is a process of transformation of one set of chemical substances to another. When chemical reaction happens, the atoms of the chemical substances (initially called reactants) are rearranged resulting in different substances we call chemical products. No atoms are lost or gained during the reaction, so the total mass of reactants will be the same as total mass of the products.

There are some reactions that are reversible, meaning the chemical products can go back to become reactants. However, there are also reactions that are irreversible, meaning once the reaction occurs, the products cannot be converted back into reactants again.

An Irreversible reaction is like baking. Once the egg, flour, butter, and milk are mixed and baked into a cake, they cannot go back to their original form. An example of an irreversible reaction is combustion process. Combustion involves burning an organic compound (such as wood) and oxygen to produce carbon dioxide and water. Carbon dioxide and water are stable products and therefore they cannot go back to become wood and oxygen.
-Aurelia, Dentistry Student

Can orange juice be turned into a gas?

-asked by Emily, 11, from Burnside Primary School

Orange juice is a mixture of liquid (water) and solid components (i.e. natural sugars, vitamins and other minerals). If you were to heat orange juice to a temperature above 100°C, the water component of the orange juice would evaporate meaning that the liquid water would change into a gas. However, the sugar, vitamins and minerals would be left behind. 
– Jenny, Dentistry Student

What is the most dangerous chemical?

-asked by Alfie, 11, from Burnside Primary School

There are so many dangerous chemicals out there! The dangerous chemicals are usually toxic, volatile, corrosive, or explosive. One of the most dangerous chemical known to mankind is Chlorine Trifluoride which is also called substance N. This was studied by Nazi scientists during The World War II for military purposes. It is a colourless, poisonous, corrosive and extremely reactive gas. It is so volatile and will react with almost anything! It has been known to set fire to glass, sand, rust, and humans! It is usually transported in the form of condensed liquid which is pale- greenish yellow in colour. The United States tried to transport a huge amount of Chlorine Trifluoride before in a tanker and it was a bad move. The tanker spilled in a warehouse and set fire to it. The whole floor was gone, leaving a few feet of dirts and soil underneath. This compound is now of interest for rocket fuels and nuclear reactor fuel processing.
– Aurelia, Dentistry Student

Why does Carbon Dioxide not turn into water since it’s ice?

-Nathan, 11, from Burnside Primary School

Carbon dioxide does not turn into water even though it is ice because water and carbon dioxide are two different compounds each with their own chemical composition, which means that they have different properties. Carbon dioxide, whether it be in gas or solid (a.k.a. dry ice) form, is made of one carbon and two oxygens. Therefore, it is known as CO2. Whereas water in either gas, liquid or solid state is made of two hydrogens and one oxygen and is therefore known as H2O.

CO2 and H2O are very different from each other and when they are frozen, they form different solids because H2O freezes to form regular ice at 0°C and CO2 gas freezes at -78°C to form dry ice. When regular ice melts, it changes from a solid to a liquid and will leave a puddle. On the other hand, dry ice is so called because when it is exposed to temperatures greater than -78°C it does not melt, it actually sublimes, meaning it changes from a solid straight to a gas and leaves no liquid or puddle behind (hence “dry” ice). 
– Jenny, Dentistry Student

If you have any questions that you would like our team to answer, please leave a comment below!

#TryThisTuesday: Rock Candy

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This weeks Try This Tuesday takes a while, but you end up with a tasty treat!

You will need:

  • A wooden skewer or chopstick
  • Peg
  • 1 cup of water
  • 2-3 cups of sugar
  • A narrow glass or jar

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Clip the wooden skewer into the peg so that it hangs down inside the glass and is a couple of centimetres off the bottom.

Put the water into a pan and bring it to the boil. Pour about a quarter of a cup of the sugar into the boiling water and stir until it dissolves.

Keep adding more and more sugar, each time stirring it until it dissolves, until no more will dissolve. This might take quite a while!

When no more sugar will dissolve remove it from the heat and leave it to cool for about 20 minutes.

Pour the sugar solution into the glass or jar almost to the top. Then put your skewer back into the glass so it hangs down and doesn’t touch the sides.1st

Leave your glass in somewhere it won’t be disturbed. The sugar crystals will grow over 3-7 days. Once these have grown you can eat them!finished-product

The Science

By mixing the sugar and water together when they were really hot, you have created a super saturated solution. This means that the water contains much more sugar than in could in normal circumstances. As the water cools back down the sugar leaves the solution (mixture) and becomes sugar crystals again, forming on the skewer.

Supersaturated solutions are used in real life. In a sealed fizzy drink the drink is saturated (full) with carbon dioxide, as the carbon dioxide is put in using pressure. When you open the drink, the pressure of the carbon dioxide is decreased, which causes your drink to be supersaturated as there is much more carbon dioxide dissolved than there would be at normal pressure. The excess carbon dioxide is given off as bubbles.

#TryThisTuesday: Homemade Ice cream!

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This week we’re making ice cream but instead of using an ice cream machine, we’re going to make it using science!

You will need:

  • Two Ziploc bags – one small, one large
  • 100ml double cream
  • 50ml milk
  • 40g sugar
  • Vanilla extract
  • Ice
  • Salt

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  1. Measure out the milk, cream and sugar and place them into the smaller Ziploc bag.
  2. Add a dash of vanilla extract  then zip up the bag.
  3. Fill the larger bag 2/3 full with ice.
  4. Pour a generous amount of salt onto the ice.p1020738-3
  5. Making sure the small bag is tightly zipped up, place it inside the bigger bag with the salt and ice.
  6. Gently shake the bag for 5-10 minutes, be careful not to rip the bag!
  7. Leave the ice cream to sit inside the ice and salt bag for another 10 minutes
  8. Open up your bag and enjoy!

p1020740-2Try making different flavours of ice cream by swapping the vanilla extract for strawberry or mint extract or even cocoa powder for chocolate ice cream. You could also try adding chocolate chips.

 

 

 

 

How does this work?

Water, as I’m sure you know, freezes to make ice at 0oC. But your freezer at home is around -18oC, so how are we making the ice cold enough to freeze your creamy mixture? The secret is in the salt.

Ice is in a constant state of melting and refreezing and melting and refreezing. When we add salt, the salt particles block the path of the melted ice, stopping it from freezing back on to the rest of the ice but ice can still melt. Therefore more ice is melting that freezing.

Now you may be thinking that surely if the ice is melting that means it is getting warmer? It’s actually the opposite. For ice to melt it needs to break the bonds that are formed between the H2O molecules. This breaking requires energy which it gets in the form of heat. When a molecule melts away a bond is broken, taking heat away from the surrounding, causing the temperature to drop.

This is also the reason that salt is put on icy roads – it stops water forming ice.

#TryThisTuesday: Oil and Water

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For this experiment all you will need is a clear bottle or jar with a lid, water, cooking oil and some washing up liquid.

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Fill the water bottle half full with water.

Pour about 100ml of oil in to the bottle and observe what happens.

The oil should float on the water. Try and mix them together or challenge other people to mix them! It is impossible, the oil and water always separate out again.

Add a squeeze of washing up liquid to the bottle and shake. The oil and water now mix together.

The Science

Oil is less dense than water so floats on top. Oil and water don’t mix together as the water molecules are more attracted to each other than the oil molecules. Oil molecules are hydrophobic or ‘water-fearing’.

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Washing up liquid molecules are attracted to both water and oil. When you add a squirt in, one end of the washing up liquid molecule attaches to a water molecule and the other end attaches to an oil molecule. This creates a mix of water with oil droplets spread throughout it. This is because one end of the washing up liquid molecule is hydrophobic (water fearing) and one is hydrophilic (water loving).

The washing up liquid acts as a stabiliser and creates an emulsion. This is a mixture of two liquids that wouldn’t normally mix.

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Real Life Applications

We use washing up liquid when we are washing up as it attaches to the oil on the dirty dishes and lifts it off into the water.

Animals that live in the ocean also stay warm by producing an oily substance on their fur or feathers which keeps the cold water away from their skin.

#TryThisTuesday: Making Snow

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We’re feeling very festive this Tuesday so we thought it was the perfect time to make snow with science. All you need for this one is some shaving foam and bicarbonate of soda.

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Simply mix the bicarbonate of soda and shaving foam together in a bowl until you get a powdery consistency.

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Pick it up and have a play – you might notice that your fake snow actually feels cold too. This is due to the reaction between the bicarbonate of soda and the shaving foam. The reaction is endothermic meaning that it requires heat to occur, it takes this from the environment and so decreases the temperature around it.

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The Science of Shaving Foam

Do you think shaving foam is a liquid or a solid? It’s actually a colloid. A colloid is a substance which has droplets of one state surrounded by another state. There are lots of different types of colloids with different combinations of states making up the droplets and the surrounding. In the case of shaving foam, the droplets are gas and the surrounding is liquid making it a foam colloid.

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#TryThisTuesday: Slime!

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With Halloween coming up, what better time to make some of your very own slime?

It’s super easy and quick to make – you just need to mix water and cornflour! Start with a little bit of both, if it seems too runny you can add more cornflour and if it becomes a solid then add more water.

You can also add food colouring and glitter if you want to add some sparkle to your slime.

The slime should become a consistency that appears to be a liquid but if you hit it or try to stir it quickly it becomes a solid – so which is it?

Liquid or Solid?

Slime isn’t actually a solid or a liquid – it is a non-Newtonian fluid, this is a fluid that changes its properties when a stress or force is applied.

The slime we’ve made is a particular non-Newtonian fluid called oobleck (yes it’s a funny sounding word – that’s because it is derived from a Dr. Seuss book). The particles of cornflour don’t dissolve in the water, they become suspended in the water and repel each other. Mechanical stress, such as stirring quickly provides energy that overwhelms the repulsive forces, causing the particles of cornflour to temporarily stick together. When the stress is removed, the repulsion returns and the slime becomes liquidy again.

More Non-Newtonian Fluids

1. Custard behaves just like oobleck, in fact if you filled an entire swimming pool with custard, you would be able to walk across it!

2. Ketchup is almost the opposite of oobleck – it become thinner and runnier under impact, that’s why it helps to bang the end of a ketchup bottle when you’re struggling the get some out.

3. Whipping cream acts differently when under a constant and prolonged stress, such as whipping. If you whip cream for long enough it will appear to go from liquid to solid as it becomes whipped cream.

4. Honey similarly needs prolonged stress to change it’s properties. When you stir honey, it will become more like a liquid than a solid.

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