Chemical Definition and 1000 Threads

A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that chemical substance cannot be separated into its constituent elements by physical separation methods, i.e., without breaking chemical bonds. Chemical substances can be simple substances, chemical compounds, or alloys. Chemical elements may or may not be included in the definition, depending on expert viewpoint.Chemical substances are often called 'pure' to set them apart from mixtures. A common example of a chemical substance is pure water; it has the same properties and the same ratio of hydrogen to oxygen whether it is isolated from a river or made in a laboratory. Other chemical substances commonly encountered in pure form are diamond (carbon), gold, table salt (sodium chloride) and refined sugar (sucrose). However, in practice, no substance is entirely pure, and chemical purity is specified according to the intended use of the chemical.
Chemical substances exist as solids, liquids, gases, or plasma, and may change between these phases of matter with changes in temperature or pressure and time. Chemical substances may be combined or converted to others by means of chemical reactions.
Forms of energy, such as light and heat, are not matter, and are thus not "substances" in this regard.

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  1. N

    Chemical Engineering Grad Studies at UToronto vs McGill

    Hey all. This really has more to do with uoft. I noticed that the grad program in ChE at UToronto has fewer course requirements, and fewer course options than the other main engineering specializations. I was wondering why that was so. In contrast, I noticed that McGill's grad ChE program...
  2. D

    Overlap integrals in chemical molecules

    In the explicitly-unsolvable Hamiltonian for a chemical system, approximations are made to solve the energy of the system. In particular we see CNDO (ZDO), INDO and MNDO theories. CNDO - complete neglect of differential overlap; two-center electron integrals are zero. INDO - intermediate...
  3. Z

    How Is the Equilibrium Constant Calculated for I2 and I- in KI Solution?

    Homework Statement A saturated solution of iodine in water contains 0.330g of I2/L. More than this can dissolve in a KI solution because of the following equilibrium- I2(aq) +I-(aq) ------> I3- (aq) where 2 and 3 are subscripts and - is negative charge A 0.1M KI solution actually...
  4. Z

    How Do You Calculate CO Concentration at Equilibrium in a Gaseous Reaction?

    Homework Statement At a certain temperature, the equilibrium constant for the gaseous reaction of CO with O2 to produce CO2 is 5000 lit/mole. Calculate [CO] at equilibrium, if 1 mol each of CO and O2 afre placed in a 2 L vessel and allowed to come to equilibrium. Homework Equations...
  5. Z

    Concentration of solids and liquids in Chemical Equilibrium

    Homework Statement Why don't we take into consideration the concentration of pure solids and pure liquids in equilibrium constant expression? I read that their concentration per unit volume does not change. If the concentrations (moles per litre) don't change then it won't undergo any...
  6. M

    Order of atoms in chemical formulas

    what determines what order the atoms in a formula are written? for example, why is it CO2 and not O2C? It doesn't appear to be ordered by atomic weight, place on the peridoic table or alphabetically so what is it?
  7. S

    Industrial engineering to chemical engineering

    Hi, in about 1 year i will graduate with a ba in industrial engineering from a university in Peru. I have really good grades and its supposed to be the best at I.E in the country. I really like science and i have realized that I.E is not really for me since it is more oriented towards...
  8. X

    Chemical Reaction to pulverize Aluminum?

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  9. C

    Debate: Chemical or Materials Engineering as a Career

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  10. 4

    Differential equations chemical solutions problem

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  11. S

    Chemistry? What physical and chemical change occur when a wax candle burns?

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  12. P

    What is the balanced equation for BaO + P2O5?

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  13. J

    What Are the Products of Zinc Sulphate and Barium Nitrate Reaction?

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  14. W

    Chemical Kinetics: Rate Laws, Orders

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  15. R

    Writing out chemical equations

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  16. M

    Creating a Chemical Rocket: Innovative Ideas Using Everyday Items

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  17. S

    How Does LSD-25 Impact the Central Nervous System and Influence Mental States?

    First off, let me make this clear, I am not: promoting the use of drugs or asking how to synthesize LSD-25. I've become curious about the chemical aspects of LSD-25, such as, how does this drug actually affect the central nervouse system, why does it have such dramatic affects on the minds of...
  18. R

    Biology vs Biochemistry vs Chemical Engineering?

    I'm planning on going down the medical path. Will a Bachelor in each of these prepare me for the MCAT and what I'll see in medical school? Less importantly, how "sellable" are these degrees relative to each other (should the medical path not work out)?
  19. B

    Energized Electrons in Chemical Bonding

    In chemistry I learned the bonding rules that I learned for various molecules never mentioned anything about the different energy levels in electrons. Now that I know electrons can have various energy levels from absorption of photons, I would like to know if this changes the behavior of...
  20. E

    Engineering BS in Chemical Engineering, MS in EE

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  21. D

    Is transitioning from chemical to nuclear engineering a good idea?

    I’d like to be a nuclear engineer. I live in South Carolina and I’ve been accepted into Clemson for chemical engineering and also NC State for nuclear. I hear a lot of people have gotten their bachelors in something else and then get their masters in nuclear. Since I plan on getting a...
  22. S

    Chemical Exposure: Worried About Cancer?

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  23. A

    Engineering Chemistry or Chemical Engineering?

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  24. L

    How Is Chemical Energy Used and Where Can It Be Found?

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  25. I

    How Does Le Chatelier's Principle Explain a Soft Drink Going Flat?

    Homework Statement 1) Use Le Chatelier's prinicile to explain how a shift in the equilibrium H2CO3(aq) ⇌ H2O(l) + CO2(g) causes a soft drink to go flat when its container is left open to the atmosphere. 2) How might the addition of a noble gas to the reaction vessel affect this equilibrium...
  26. L

    Deciding on Which School for Chemical Engineering

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  27. D

    Is the Fugacity Equation the Key to Proving Chemical Thermodynamics?

    Hey. I am currently studying chemical thermodynamics and have reached the section on solution thermodynamics. For a pure liquid: \left(\frac{\partial g}{\partial P}\right)_T=RT \left(\frac{\partial ln(f)}{\partial P}\right)_T Where g is the molar gibbs energy P is pressure T is...
  28. D

    Mechanical or Chemical Engineer?

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  29. E

    Exploring the Concept of Zero Chemical Potential in Photons: A Clear Explanation

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  30. A

    Understanding Chemical Kinetics: Troubleshooting Common Questions in the Lab

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  31. A

    In search of a Unique Chemical

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  32. Sirsh

    Explaining How Temperature Change Affects Chemical Equilibrium

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  33. D

    Voltaric Cell/Cell Chemical Reaction

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  34. J

    Mass lost in chemical reactions?

    Homework Statement In chemical reactions, when energy is released, mass is lost according to Einstein’s formula just as it is for nuclear energy (though the amounts lost are so small that for centuries scientists thought that mass was conserved). When a kilogram of coal is burned, it...
  35. K

    Chemical vs Physical change/property

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  36. C

    Chemical Engineering, but want to do physics too

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  37. C

    What Does (I) Stand for After a Chemical Compound?

    I was wondering what (I) means when it is written after a chemical compound, for example, H2SO4 (I). Thanks!
  38. D

    Entropy, free energy and chemical potential of mixtures

    Hi, Homework Statement Consider a mixture of different gases with N_i molecules each (i=1...k denotes the species). For ideal gases the following relation yields: S(T,V,N_1,...N_k)=\sum_{i=1}^k S_i(T,V,N_i) a)Give explicit expressions for the entropy, the internal energy, Helmholtz free...
  39. M

    Engineering Chemical engineering not as popular as other 3 big engineering branches?

    Chemical engineering not as popular as other 3 "big" engineering branches? It seems to me chemical engineering is not as popular as the other 3 "big" engineering branches (civil, mechanical, electrical). If true, what's the reason for this? Lower demand? More challenging? Less glamorous?
  40. C

    Atom's Chemical Fingerprint: Banding Pattern Explained

    What determines an atom's chemical fingerprint (banding pattern)?
  41. A

    I with chemical equilibrium problems

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  42. J

    Chemical Kinetics (Reaction Mechanisms)

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  43. Char. Limit

    How Do You Write Chemical Equations for These Reactions?

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  44. r-soy

    Blance the chemical by algebric method ?

    Hi all i wan check my answer Blance the chemical by algebric method ?
  45. T

    Can a Catalyzer Also Be an Intermediary in Chemical Reactions?

    Can a catalyzer in a reaction be an intermediary as well? Im not sure, i don't think because it would have to consume it self...
  46. D

    How Does the Reaction Between Chemicals A and B to Form C Progress Over Time?

    Two chemicals, A and B, are combined, forming chemical C. The rate of the reaction is jointly proportional to the amounts of A and B not yet converted to C. Initially, there are 50 grams of A and 80 grams of B, and, during the reaction, for each two grams of A used up in the conversion, there...
  47. C

    How Do You Calculate the Initial Rate of H2O2 Disappearance?

    Homework Statement In the reaction between H2O2 and I-, 15 mL of 0.30M KI, 75. mL of 0.10M H2O2 and 21 mL of 0.020M Na2S2O3 were mixed in a flask containing starch and buffer and the total volume was made up to 500mL with distilled water. The solution turned blue (indicating the presence of...
  48. F

    Understand the Mystery of MgO: Magnesium + Oxygen Chemistry

    So I'm taking chemistry again after a five hear hiatus from the topic and I would really appreciate an explanation on a certain formula. magnesium +oxygen forms magnesium oxide Mg+O2=MgO My question is, what happened to the 1 of the oxygen atoms? Shouldn't it be MgO2?
  49. T

    Chemical Kinetics: Reaction Mechanism (My working attempt shown)

    Homework Statement 1. According to the mechanism below, what is the correct expression for the rate of consumption of A? In the mechanism, M is a molecule that collides with A and A* is a highly-energized form of A. Step 1 (reversible): A + M <---> A* + M rate constants k1 and k-1 Step...
  50. T

    Chemical Kinetics: Reaction Mechanism

    Homework Statement 1. According to the mechanism below, what is the correct expression for the rate of consumption of A? In the mechanism, M is a molecule that collides with A and A* is a highly-energized form of A. Step 1 (reversible): A + M <---> A* + M rate constants k1 and k-1 Step...
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